Archive for May, 2010

Evolution of Animal Fighting Behaviour

It is recommended that you read ‘An Introduction to the Evolution of Animal Fighting Behaviour‘ before you read this, as there are some concepts explained in the earlier article which are used without explanation in this article.

Introduction

Success in fighting behaviour is frequency dependent i.e. as the population size increases fighting success decreases. Fighting behaviour amongst a species is often explained in terms of roles, for example we have previously looked at the fairly simplistic Hawk/Dove model which gives animals the role of either hawk or dove.

When determining which strategy will be dominant amongst a population we check it against the ‘standard conditions’ of Maynard Smith. If a strategy is adopted amongst the majority of a population we call it an evolutionarily stable strategy (ESS). An ESS is a strategy which, if adopted by most members of a population, cannot be invaded by a mutant strategy which is initially rare. The standard conditions to determine if a strategy (e.g. strategy I) is an ESS are:

Either:

1 – E (I, I) > E (J, I)

Or

2 – If E (I, I) = E (J, I) then E (I, J) > E (J, J)

E (I, I) is the payoff of strategy I against strategy I. Therefore by the conditions of 1, strategy I is an ESS if the payoff of I vs. I is greater than the payoff J receives from fighting I i.e. E (J, I).

The Hawk/Dove/Retaliator Model

The Hawk/Dove model makes certain assumptions:

  • Animals fight with equal ability
  • Animals fight in pairs
  • Animals can only display, escalate or retreat
  • Encounters are random

Due to the limitations of the Hawk/Dove model an additional strategy was added – retaliator

  • Hawks (H)– Escalate immediately, retreat if injured
  • Doves (D)– Display immediately, retreat if opponent escalates
  • Retaliators (R)– Immediately display, escalate if opponent escalates and retreat if injured

The payoff matrix looks like this:

Vs. > H D R
H (V-C)/2 V (V-C)/2
D 0 V/2 (V/2)x 0.9
R (V-C)/2 (V/2)x 1.1 V/2
  • V = Value of resource.
  • C = Cost paid attempting to gain resource.
  • The reason for the x1.1 is that retaliators do slightly better than doves (10%) as they sometime escalate so their payoff is increased.
  • The reason for the x0.9 is the same as above, but because the doves sometimes lose, their payoff is reduced by 10%.

When V>C: Hawk is not an ESS, doves are unable to invade and therefore retaliator is an ESS.

When V<C: Hawk is not an ESS, doves are unable to invade and therefore retaliator is an ESS.

The War of Attrition

When 2 animals meet in a contest for a resource, the amount of energy they are willing to invest to win that resource are predetermined. The animal will therefore display until this time/energy is up. This value is not modified during the display. In the contest, the animal which wins is therefore the one who invested the most predetermined energy. We can model this:

  • Rate of cost accumulation: c
  • Contest length: T
  • Cost of contest: cT
  • Resource value: V
  • Animal A persistence time: TA
  • Animal B persistence time: TB

In this example TA > TB

Payoff to animal A = V – cTB (Animal A wins the resource V but still has to pay the cost, c for the length of the contest, the length of the contest would therefore be TB as the contest ended when animal B gave in.)

Payoff to animal B = – cTB (Animal B does not win any resource, but must still pay the energy that was used during the contest)

A population does not evolve a constant persistence time, however:

  • If cT < V it is worth persisting longer for a resource as the payoff is greater than the cost.
  • If cT > V then a persistence time of 0 spreads amongst the population as to engage in contest for the resource will mean a loss of energy (even if winning). It is therefore better to not engage and lose nothing.

The length animals choose as their persistence time follows a negative exponential distribution, i.e. many choose short times and a very limited number choose long times. The length of contests will therefore also follow this distribution. The log of the number of contests plotted against the length of the contests will give a straight line.

Examples:

  • Damselfly larvae compete for perching space. Intruders encroaching on perching space will be warned by a ritualised display of the abdomen. The intruder may either leave or contest against the perch space owner. The contests are slow, but their duration follows the negative exponential predicted earlier, however 70-80% of the contests are won by the original occupant and not the 50% you would expect.
  • The fighting of male dungfly over female dungfly (which can be considered a resource) follows the same negative exponential pattern.

The assumption that all contests are fought symmetrically (equal chances of winning) is false, we can assume asymmetry because:

  • Resource Value – Resources are worth more or less to different animals, e.g. a piece of food may be worth more to a hungry animal than a recently fed animal.
  • Resource Holding Power (RHP) – The fighting ability of the animal, this will vary amongst the population, those with a higher RHP will be more likely to win a contest.
  • Uncorrelated Asymmetry – This is any asymmetry which is not correlated to the value of the resource.

Resource Holding Power (RHP):

  • RHP is the fighting ability of an animal, therefore the animal with the greatest RHP is going to keep the resource and the animal with the lower RHP will retreat.
  • Animals must therefore find a way to assess the RHP of others.
  • If RHPs are of similar value, this is when a fight will escalate.
  • For example:
    • The roar contest in red deer helps to determine the RHP. The deer with the lower roaring rate retreats as it is very likely to have a lower RHP. This type of contest is a true signal of RHP (unlike size for example).
    • Croaking in toads when trying to find/compete for a female allows the toads to determine RHP and thus whether or not to attack. Larger males produce deeper croaks and are determined to have a higher RHP.

Bourgeois Strategy

The Bourgeois strategy is a method used to determine the payoff values for competing for resources. It is substituted into the Hawk/Dove model. The Bourgeois strategy is:

  • If it is the owner of a resource, it plays hawk
  • If it is the intruder, it plays dove
  • The assumption is 50% of the time; the Bourgeois is the owner of the resource.

When we put this into a payoff matrix we get the results:

  • If V>C – The hawk strategy is an ESS
  • If V<C – The Bourgeois strategy is an ESS

e.g. The speckled wood butterfly protect areas of sunlight as they are looking for a mate. When an intruder approaches a short spiral fight occurs. The owner of the path of sunlight always wins the spiral fight. If there is confusion over the ownership of the sunlight patch then the spiral fight lasts much longer.

There also exists an Anti-Bourgeois strategy where the intruder always wins the resource for example:

  • In certain spider species, intruders always displace the owner of a web funnel.
  • Seagulls on a flag post always give up the space immediately to invaders.
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Animal Communication

Introduction

Communication is the process of transferring information from one animal to another; there are typically three types of roles involved with communication. These are:

  • Signaller – Initiates the communication, the signaller must benefit from making the signal
  • Receiver – Receives the signals sent by the signaller, these signals are often of benefit to the receiver as well
  • Eavesdropper – Takes advantage of the signals generated by the signaller

There are also different forms of communication two of which are:

  • Cooperative signalling – Signalling that benefits both parties involved, e.g. a birding singing to attract a mate
  • Non-cooperative signalling – Only one party benefits, e.g. the ‘broken wing display’ by birds (The bird pretends to have a broken wing to make it appear an easy target, thus distracting the predator from its young).

Types of Communication

Examples of communication types:

Type of Communication Example Species Function
Pheromones Moth Pheromones are short chain hydrocarbons which are easily diffused and are detected by vomeronasal organs. Pheromones and their receptors are species specific and so undetectable by predators. It only takes one molecule of pheromone to elicit a response – unlike smell.
Stridulation Grasshopper Stridulation is the rubbing together of body parts to produce a sound. Stridulation is used when auditory signals are of more benefit than pheromones (e.g. in long grass pheromones are diffused and harder to detect). Different species produce different sounds and calls via stridulation.
Bioluminescence Fireflies A specialised pigment – luciferin releases energy in the form of light by oxidised by the enzyme luciferinase. The light produced is aposematic, this warns predators that they are distasteful. Different species produce different patterns of light.
Alarm Calls Birds – Bird song Similar amongst all species, alarm calls alert other members of the group that a predator is nearby. Alarm calls are generated in a way that makes it difficult to pinpoint the exact source.

Bird Song

Bird song differs between habits, differing mainly by frequency. The birds have adapted to produce a song which is designed to travel as far as possible given the habitat.

In open areas, with wind, a higher frequency and rapid songs are best as they allow the message to travel far without being disrupted by the wind.

In more closed habitats such as woodland, purer tones are used. This helps to prevent degradation of the song by obstructions e.g. trees.

Dance Language of Honeybees

The primary use of the honeybee dance language is to communicate the location of food sources to colony mates. The dance involves 2 components:

  1. A straight run whilst waggling the abdomen
  2. Return back to starting point and repeat but in reverse

This dance conveys information about distance from a food source as well as its direction. The angle of the waggle tells the colony mates the angle of the flower from the sun. The direction of the straight run shows the direction of the flower and the distance of the straight run is related to the distance to the flower. The bees also buzz during the ‘dance’ which also helps to convey the message.

The honey bees are still able to communicate the whereabouts of a food source, even without the sun. On a cloudy day they use their ability to detect polarised light and relate this to the direction of the food source.

The honey bees are able to roughly estimate the distance to the flower by measuring the rate of optic flow. They can roughly estimate their speed by using objects in the distance at and how quickly they pass by, from this they can estimate distance as they know roughly how long they have been travelling.

Experiments have been conducted to show that the dance is an important tool for honey bees. One such experiment measured this by measuring the reproductive success of the honey bees under two conditions:

  1. Diffuse light (non-directional) which lead to the bees giving a disorientated dance
  2. Orientated light (natural e.g. sun) which allowed the bees to give dances as normal

The bees with the orientated light source had much better reproductive success. The dance becomes very important during winter when food sources run low.

Intrasexual Selection

Introduction

Intrasexual selection is when members of the same sex (within a species) compete with each other in order to gain opportunities to mate with others, e.g. the male against male competition for females. Because intrasexual selection often involves fighting, species or individuals well adapt for intrasexual selection will have developed better armourments (weapons) than their competition.

On the other hand, there is intersexual selection. Often known as female choice, it is the process where the female choses the male based on certain ornaments e.g. a peacock’s tail. The ornament is not usually beneficial to the male (e.g. bright colours make it an attractive target for predators) but the female prefers the larger ornaments as it signals the male’s is able  to cope with the hindrance – and therefore a better genetic make-up which will be passed on to her offspring. The reason the females choose is to prevent wasting invested time and energy on offspring which are of poor genetic merit.

Competition

There are two main types of competition over females, scramble and contest competition.

  • Scramble: Typically whoever gets to the female first. An example with dung flies; brightly coloured male dung flies are attracted to a dung pat. Shortly after females will arrive at the dung pat. These females are quickly grabbed by the males. Very shortly after female arrival rate decreases and the number of both males and females around the pat decreases

In a similar scenario, male damselflies also grab females as soon as they arrive. However male body size also contributes to reproductive success. Larger males live for longer and hence have more possible days or reproductive ability but smaller males have a higher daily mating rate as they are more agile and able to grab the females faster. It is therefore beneficial for reproduction to be of intermediate size.

  • Contest: Contest competition is a more typical form of competition where the male with the best fighting technique, largest body size or the largest weapons will win the female. Although not always guaranteed to win, they have a much higher chance than inferior males. This has however; inevitably lead to the production of larger male offspring by reproductive selection – as the larger males are more likely to reproduce and pass on their genes.
  • Alternative Mating Techniques: Smaller males would therefore seem to be at a disadvantage during contest competitions. Fortunately there are species where the smaller males have developed alternative mating tactics to ensure reproductive success. For example:
  • Red deer – Smaller males with small antlers are much less likely to win in a contest competition. Instead they wait near a female deer and when the large male intending to copulate with her engages in a contest with a competitor, the smaller deer sneaks in and copulates with the female.
  • Sunfish – Males defend their territory and wait for females to come and lay their eggs. When a female arrives at the nest she will lay her eggs as the male fertilises them. However subordinate males may quickly dart in-between the male and female. The subordinate male mimics the female as not to alarm the dominant male and both males deposit sperm, this gives the subordinate male a chance to fertilise the eggs of the female.
  • Coho Salmon – There are two forms of male Coho salmon, the larger males known as Hooknoses and the smaller males known as Jacks. Females and Hooknoses spend 3 years at sea before returning to reproduce; Jacks spend only 2 years, meaning a larger proportion return – a lower mortality rate. As is typical with other species, the larger males compete for females by fighting, whilst the smaller males sneak to mate with the females. When comparing Jacks to Hooknoses, both have the same level of reproductive fitness (resulting in a mixed evolutionarily stable strategy).

Sperm Competition

Once a male has mated with a female, it is still possible for the sperm of another male to fertilise the female. Some species have therefore developed methods to prevent this. The basic methods are pre/post copulation guarding. Prior to copulation the male will guard the female until she is sexually receptive and after copulation the male will guard the female until she has laid her eggs.

There is also the basic sperm competition, where the sperm ‘compete’ against the sperm of other males within the female reproductive tract. Two examples of more dedicated sperm competition are:

  • Scrapers – Males who compete by this method use bodily structures to remove the sperm of other males from the female reproductive tract
  • Mating Plugs – Males which use the mating plug method, copulate with a female and when they disengage a ‘plug’ is left within the female. This plug prevents further males from mating with the female.

Intersexual Selection

Introduction

Intersexual selection, often known as female choice, is the process where the female choses the male based on certain ornaments e.g. a peacock’s tail. The ornament is not usually beneficial to the male (e.g. bright colours make it an attractive target for predators) but the female prefers the larger ornaments as it signals the male’s is able  to cope with the hindrance – and therefore a better genetic make-up which will be passed on to her offspring. The reason the females choose is to prevent wasting invested time and energy on offspring which are of poor genetic merit. A study which monitored female choice in peacocks, found that 19 out of 22 times the female mated with the male which had the largest tail.

Fisher’s Runaway Process

Fisher’s runaway process is a method which explains the reasoning for selection and development of male ornaments.

  • Males have a gene which determines the ornament trait e.g. tail length
  • Females have a gene which makes them find the male ornament appealing
  • Initially females will base their choice on what is best for their offspring. For example the utilitarian optimum is the optimum tail length for flight in birds; females will therefore select males who have a tail length closest to this.
  • Continuing with the example, the female will produce male offspring that have a longer tail length and closer to the optimum for flight.
  • The male offspring are more likely to be selected and reproduce, meaning even more offspring produced with longer tails.
  • Natural variation and selection for the largest tail length will eventually lead to males that have tail lengths exceeding the optimum, yet the females continue to prefer the males with the longer tails.
  • A trade-off is produced; a longer than optimum tail length leads to decreased flight ability but increased reproduction success.
  • The two traits eventually reach equilibrium, as males with too long a tail are unable to survive.

Zahavi’s Handicap Principle

Zahavi’s Handicap Principle says that although exaggerated ornaments are selected for by females, it does not actually benefit the female directly (however, you could argue that it helps to spread her genes because her offspring have the exaggerated feature, which in turn leads them to greater reproductive success). The exaggerated features are not beneficial for the male; they act as a disability e.g. the large tail that prevents flight, bright colours which alert predators or larger ornaments which make escape difficult.

The reason the female still chooses the male with the disabling trait, is because it shows that despite the disability, the male is still able to survive. This in turn must mean that the male has good genes, which is why the female choses him.

A distinction is often made between indicator genes, which indicate that beneficial genes will be passed to the offspring (‘good genes’) for example a colourful plumage, and genes which will simply make the male offspring sexually favoured when searching for a mate (‘pure Fisherian’).

The Hamilton & Zuk Hypothesis

This hypothesis states that female swill be more likely to choose healthy male, i.e. those with a resistance to parasites.

This is often seen in birds, a way to determine whether the male is healthy or not is to observe the colour of its plumage. The more colourful the male, the better its resistance against parasites and the more likely it is to be sexually selected by the female (as she will want to pass on those parasitic resistant genes to her offspring). This is because a bird burdened with a parasite will be unable to meet the metabolic rate required to produce a colourful plumage whilst trying to remove the parasite.

However, this is not apparent in all birds only those where parasitic burden is likely to occur. Birds where incidence of parasitism is low do not tend to display bright colours as there is no need for the female to base her mate choice on parasitic burden. If parasite presence is high amongst a species, then that species is more likely to display bright colours as a way to show they are not burdened with the parasite – thus increasing their reproductive success.

An example of this has been shown with sticklebacks. The males display a bright red colour on their stomach; females choose males with the brightest stomach. To prove this was the case, scientists bathed the experiment tank with green light to remove the red colouring from the stomach. The result was random female choice. Then some of the males were infected with a parasite, they proceeded to lose colour an when females were given the choice of which mate they chose the more colourful, non-infected males.

Intersexual Role Reversal

Intersexual selection is not always a female’s choice however. There are examples where males are the ones who invest more time in the upbringing of offspring. There are some species of bird where the females lay their eggs in many nests leaving the males to raise the offspring.

A more specific example is that of bush crickets. Bush crickets only feed on the pollen of one specific plant, early in the season this plant is numerous and pollen is high. However later in the season, the plant number decreases and therefore so does the pollen. The mating process of bush crickets sees the males transfer a sack of sperm during mating, when the female is ready to lay her eggs she is able to eat a protein rich sack from her own body to give her enough energy.

Early in the season when pollen is high males outnumber females so intersexual selection acts as normal. However later in the season when pollen begins to run low, the females are able to consume their protein rich sack to ensure they have enough energy. Males do not have a structure similar to this and so decline in number. This leads to females outnumbering males, and the occurrence of a sexual role reversal – therefore males are now the ones able to choose which females they mate with.

Opioid Analgesics (Morphine) & Equine Colic (Butorphanol)

Introduction

The primary effect of opioids is to temporarily remove pain when used at therapeutic levels; this is done by binding to opioid receptors found primarily in the central nervous system (some receptors are found in the gastrointestinal tract). When larger doses are given, opioids can induce beneficial and non-beneficial pharmacological effects such as sedation, respiratory depression or constipation. The term given to non-synthetic opioids is opiates; opiates are derived from the naturally occurring opium alkaloids found in the resin of the opium poppy.

The main uses of opioids include:

  • Treatment of acute pain (e.g. post-operative)
  • Palliative care to alleviate serve chronic pain (e.g. cancer)
  • Surgical premedication regimes (due to their calming, sedative action – also reduces the amount of post pain relief required)
  • Neuroleptanalgesia (a state of quiescence, altered awareness, and analgesia produced by a combination of an opioid analgesic and a neuroleptic – a tranquilliser). And neuroleptanaesthesia (a form of anaesthesia achieved by the administration of a neuroleptic agent, a narcotic analgesic, and nitrous oxide with oxygen. Induction of anaesthesia is slow, but consciousness returns quickly after the inhalation of nitrous oxide is stopped)
  • Restraint
  • Antitussive (the alleviating or suppressing coughing)

Pharmacology

The body naturally releases endogenous opioid peptides or endorphins which bind to opioid receptors in the body. There are three primary receptor types, each with different functional responses and these are:

  • μ (mu) – Responsible for supraspinal (above the spine) analgesia, respiratory depression, euphoria and physical dependence of opioids (misuse and abuse of opioids)
  • κ (kappa) – Responsible for spinal analgesia, miosis (pupil constriction of the eye) and sedation
  • δ (delta) – Responsible for hallucinations and dysphoria (agitation and anxiety)

Exogenous opioids (synthetic or natural) mimic the body’s own endogenous opioids and are therefore able to bind to the above receptors – resulting in a response specific to the receptor they bound.  Opioids are able to either stimulate or depress the receptors, meaning opioid drugs can be classes as; agonists, antagonists or both – agonists-antagonists. Agonists bind to receptors and induce pharmacological responses whereas antagonists bind to receptors and do not produce a response, this makes them able to counteract the effect of other drugs or endogenous compounds.

Agonists are used for the primary reasons listed earlier, mainly analgesia. Examples of opioid agonists are:

  • Morphine
  • Pethidine
  • Methadone
  • Fentanyl
  • Etorphine.

Antagonists are primarily used to reverse the effects of agonists i.e. analgesia. They do this by binding to μ and κ opioid receptors, which together are responsible for analgesia. Examples of opioid antagonists include:

  • Naloxone (Narcan)

Agonists-Antagonists have both agonistic and antagonistic properties. This means they are able to antagonise the pure agonists (e.g. morphine) at μ and κ opioid receptors but they also have their own milder agonistic effects. The agonist effect is sufficient enough to be used as analgesics. Examples of opioid agonists-antagonists include:

  • Butorphanol
  • Pentazocine (Fortral)
  • Nalorphine
  • Diprenorphine (Revivon)
  • Buprenorphine (Temgesic)

The principal usage of opioids in medication is for analgesia. Analgesia is the loss of pain perception. Opioids effect both the physical and psychological perception of pain, physically blocking or raising the threshold of pain stimulation and removing the association of pain with fear. Associated with analgesia is sedation which is not considered hazardous, respiratory depression (which can also be associated with opioid analgesia) however can be a distressing side effect. A list of unwanted opioid effects includes:

  • Sedation
  • Excitement
  • Respiratory depression
  • Cough suppression
  • Nausea
  • Vomiting
  • Constipation

Opioid Selection

There are many opioids available for use, each with different properties, when selecting an opioid it is important to consider its potency, how quickly it acts (speed of onset) and how long it lasts (duration). The best analgesics are those which have a mild potency, rapid onset and a long duration of effect. When combining an opioid with a neuroleptic for neuroleptanalgesia, the desired properties of the opioid are slightly different; strong potency, rapid onset and brief period of duration.

As opioids can have an effect on the gastrointestinal system, (as opioid receptors are also found in the gastrointestinal tract) if they are to be given orally then they must have low lipid solubility.

Another point to consider is whether to use an agonist or an agonist-antagonist as both are able to produce analgesia. The main consideration is that pure agonists are more reliable and predictable than agonist-antagonists, but the agonist-antagonists produce fewer side effects such as vomiting, sedation and respiratory depression. Also as agonist-antagonists have antagonistic effects, any further use of analgesics may be compromised.

Below is a comparison of the potency of certain opioids relative to morphine, the most potent being Etorphine. Etorphine (or Immobilon) is extremely powerful and typically only used to immobilise large mammals (e.g. elephants). Due to its potency it can prove lethal to man.

Drug Relative Potency
Meperidine 0.1
Morphine 1
Butorphanol 1-2
Hydromorphone 10
Alfentanil 10-25
Fentanyl 75-125
Remifentanil 250
Sufentanil 500-1,000
Etorphine 1,000-3,000

Opioids are often used as part of a pre-medication routine i.e. before surgery as a pre-emptive form of analgesia. This is because once pain has been established (i.e. during surgery) pain relief drugs prove less effective. As a result larger doses would be needed to prevent the pain which increases the onset of associated side effects e.g. respiratory depression.

Examples of Opioids and their Properties:

Morphine

Morphine (agonist) is considered the standard opioid with all other forms of analgesia being compared against it. It is the most potent natural analgesic, more potent derivatives have been artificial synthesised. Morphine produces a mixture of stimulant and depressant actions depending on the size of the dose as well as the species and absence or presence of pain.

Differences between the species can be observed e.g. in the dog, the cortex is depressed and little excitement is produced. In the cat, very small doses are able to induce excitement and in the horse morphine will not produce excitement if no pain is present (effect is less predictable in horses however). Despite this morphine is safe to use in all species as long as the correct dosage is used, the presence of excitement tends to increase with dose.

The duration of morphine is about 4 hours in all species, it is eventually metabolised by the liver. It is normally injected subcutaneously at a dose of around 0.1mg/Kg (in dogs and cats).

Use of morphine can either stimulate the medulla (which is followed by depression of the medulla) or directly depress the medulla.

Morphine has a number of effects on the gastrointestinal tract. Initially it may invoke vomiting and defaecation which is followed by constipation. Constipation is due to local effects on the small/large intestinal opioid receptors. Segmental tone of the intestines increases, along with sphincter tone but the action peristalsis decreases. This increases the time taken for intestinal contents to pass.

Other areas affected by morphine include:

  • The chemoreceptor trigger zone (CTZ) of the medulla is stimulated by morphine – this induced vomiting.
  • The occulomotor centre is stimulated which is responsible for producing miosis.
  • The cough centre is depressed – reducing coughing but making post-operation mucus accumulation a possible problem.
  • The vagal centre is stimulated, which increases gastrointestinal activity and is responsible for the initial defaecation. If a large dose is administered bradycardia may be induced (slowed heart rate <60bpm) by myocardium depression.
  • The respiratory centre is easily depressed by morphine, even with a low dose. This is due to a reduced response to elevated CO2 levels. The mechanisms involved in the regulation of respiratory rhythm are also affected – contributing to the overall depression of the respiratory centre.

Butorphanol

Butorphanol (agonist-antagonist) is a widely used sedative and analgesic in dogs, cats and horses – combined with tranquillisers for sedation. It is around 1-2 times as potent as morphine, but it does have a slightly shorter duration of action at around 2-3 hours (Morphine – 4 hours). It has a much less profound effect on the respiratory system as the dose increases compared to morphine.

One major use of Butorphanol is for intravenous administration in the horse (0.1mg/Kg) to alleviate abdominal pain associated with torsion, impaction, intussusception (intestinal prolapse) and spasmodic colic.

Equine Colic

The term colic can encompass all forms of gastrointestinal conditions which cause pain as well as other causes of abdominal pain not involving the gastrointestinal tract. Some examples are:

  • Spasmodic colic – Increased peristaltic contraction
  • Impactive colic – Caused by irritation to the lining of the bowel or ileum due to diet or ingestion of large amounts of sand/ dirt
  • Obstructive colic – Obstruction of the bowel by large food masses
  • Flatulent colic – Build of intestinal gases causing distension and pain
  • Parasitic colic – Intestinal pain from parasites such as roundworm or tapeworm
  • Idiopathic colic – From another cause which remains unknown

There are also many diagnostic tests for equine colic:

  • Increased heart rate with decreased circulating volume
  • Distinctive behavioural signs
  • Auscultation – Listening to internal body sounds
  • Abdominocentesis – The extraction of fluid from the peritoneum which can be useful in assessing the state of the intestines
  • Nasogastric Intubation – Insertion of a tube from the nose to the stomach which can be used to drain excess liquid from the stomach – for therapeutic reasons and for diagnosis
  • Rectal/Faecal examination

There are also many drugs/treatments available to treat the symptoms:

  • Analgesics
  • Spasmolytics
  • Lubricants/laxatives
  • Antizymotics – Used against disease producing organisms e.g. bacteria
  • Anthelmintics – Used against parasites
  • Fluid therapy

The major analgesics used against colic are α2-agonists (xylazine, romifidine and detomidine), opioids (butorphanol) and NSAIDs (flunixin).

Butorphanol is usually used alongside small doses of xylazine, romifidine and detomidine. This is because it has minimal effects on the cardiovascular system (which is not true for xylazine, romifidine and detomidine). Both butorphanol and the α2-agonists have a duration of around 2-3 hours and they both reduce intestinal motility/activity.

Comparative Nitrogen Excretion

Introduction

In animals all waste products must be excreted from the body in some manner. The urinary system is involved in the excretion of nitrogenous waste products such as urea, uric acid or ammonia. The excretion of nitrogenous waste from animals is important because if it is allowed to build up, it can prove toxic. We obtain nitrogen from protein in the diet, when protein is metabolised it produces amino groups which are able to form the highly toxic ammonia. Different classes and species of animals deal with toxic ammonia in different ways. A summary:

Compound Chemical Formula Toxicity Water Solubility Animals
Ammonia NH3 High High Aquatic Animals
Urea (NH2)2CO Moderate Moderate Terrestrial Animals
Uric Acid C5H4N4O3 Low Low Reptiles and Birds

Method of Excretion by Species:

Fish

Fish excrete nitrogenous waste as ammonia; this is unusual because ammonia is highly toxic therefore storage in the body can pose a risk. However, fish are able to cope due to their environment – the large volumes of water they reside in allow them to continuously excrete ammonia (without the need for storage) directly into the water, diluting the ammonia to non-toxic levels.

Fish deaminate (remove the amine group) amino acids (obtained in the diet by protein consumption) in the gills. The pure ammonia diffuses into the respiratory water which is leaving via the gills; it is therefore excreted at the site of production with minimal time in the body to reduce toxicity.

There are two extremes of environment which fish have to adapt to however – marine and freshwater. The difference in concentration of salt between these two environments has led to slightly different methods of dealing with nitrogenous waste. Osmoregulation of fish living in fresh water requires them to excrete large volumes of dilute urine. This is due to osmosis, which allows large volume of water to enter body fluid from the surrounding hypotonic freshwater.

The converse is true for marine fish, the hypertonic water means water leaves bodily fluid by passive osmosis, in an attempt to conserve water small amounts of concentrated urine are released.

Elasmobranches, such as sharks, primarily live in seawater and are able to produce urea as well as ammonia. This is important for osmoregulation, as said earlier the hypertonic seawater means water is able to leave bodily fluids by osmosis. By producing urea (which is less toxic than ammonia and therefore storable), elasmobranches are able to increase their osmolarity higher than the seawater and therefore take up water from the sea like freshwater species. This is due to the urea being retained in the blood and therefore increasing the osmolarity of the blood.

Fish still have kidneys however, despite nitrogen excretion being handled by the gills. Instead the kidneys are involved in the excretion of excess water and divalent ions, e.g. Mg2+ and SO42-.

Lungfish, which are able to live out of water, have another adaption. They excrete ammonia in the usual way when in the water, but during times of drought when the lungfish buries itself in the mud the production of toxic ammonia could prove fatal. So instead, when buried in the mud, the lungfish produces urea which can be accumulated and excreted into the environment at less toxic levels.

Amphibians

As with fish the type of nitrogenous excretion depends entirely on environment. Typically tadpoles use the same method as fish – ammonia excretion via the gills. However adult amphibians produce urea as they do not remain constantly in the water. The urea is stored, diluted in the bladder, but as amphibians may be prone to desiccation they have adapted the ability to reabsorb water from the urine when needed. This requires them to have relatively large bladders for storage.

Amphibians do not drink to obtain water; instead they are able to absorb water through a region of the skin called the ventral pelvic area.

Birds

All birds excrete a paste like substance called uric acid (which contains the waste nitrogen) independent of habitat. It initially begins as watery urine which travels down the ureters, from the kidney to the cloaca. The cloaca is a unified exit for the gastrointestinal, urinary and reproductive systems.

In the cloaca, the urine mixes with faecal matter from the digestive tract and as much water is removed as possible to produce a dry, crystalline paste containing uric acid. A small amount of watery urine may be excreted as well.

Reptiles

The method of nitrogen excretion depends on the habitat; aquatic reptiles (e.g. turtles) will mainly excrete ammonia (possibly urea) as with fish. Whilst reptiles living in drier conditions (e.g. lizards, snakes & tortoises) excrete uric acid – the low toxicity, crystalline paste. This helps to conserve water and during development in the egg, a build-up is not fatal. Uric acid, as with birds, is excreted from the cloaca.

The kidneys of reptiles do not have loops of Henle and so they are unable to produce concentrated urine also, whilst a large bladder is present in chelonians (turtles) snakes and some lizards do not have a bladder at all.

Mammals

All mammals produce primarily urea (sometime ammonia) which is excreted in urine. Mammals are able to osmoregulate and maximise water conservation by varying the concentration of urine depending on the hydration of the body. Mammals living in environments with plentiful access to fresh water will excrete large amounts of dilute urine. Mammals living in dry or marine environments will excrete small amounts of concentrated urine.

Osmoregulation in mammals is mainly controlled by the hormone ADH, its release (when a mammal is dehydrated) will result in the reduction of water loss and increased reabsorption of water in the kidneys by the loop of Henle.

Osmoregulation

Osmoregulation has been mentioned above in some cases, it can be associated with the excretion of nitrogenous waste because without correct osmoregulation, levels of ammonia can build within the body. A well osmoregulated animal will be taking on water and balancing ions well enough to ensure that nitrogen can be excreted and not build up.

The primary problem facing osmoregulation is the build-up of salt ions, this can occur with animals living in marine habitats.

  • Fish can actively excrete NaCl via gills
  • Some cartilaginous fish such as sharks have specialised salt-excreting glands in their rectum
  • Birds and reptiles have specialised salt-excreting glands on their heads
  • Marine mammals are able to excrete salt in urine efficiently

Comparative Digestion

Introduction

The method of digestion which an animal uses depends on its diet i.e. carnivore, herbivore or omnivore. For example not all mammals are carnivores (e.g. dogs and cats), herbivorous mammals include rabbits, chinchillas, guinea pigs etc.

Generally, carnivores consume animal tissue which is similar to their own; therefore all the body needs to do is break down the tissue and absorb the different components which can then be used in the carnivores own body. Omnivores have very similar digestive systems to carnivores with the addition of a caecum.

Herbivores consume plant matter which is more difficult to break down than tissue. Therefore herbivores have evolved fermentation systems which contain specific microflora, the microflora breakdown the plant material releasing useful nutrients which the herbivore utilises.

Digestion by Diet:

Carnivores

Because meat is easily digested, the gastric system of carnivores is typically short and simple. They are monogastric meaning they have only one stomach (unlike a ruminants’ stomach which has four chambers). Due to the ease at which components required for growth are obtained from food, some carnivores have lost the ability to synthesis them (e.g. cats are unable to synthesis taurine).

The teeth of carnivores are sharp and strong, this makes it easy to rip and tear meat from bones of prey. When possible, the meat is broken down further by the teeth to ensure maximum surface area for digestion by enzymes in the stomach and small intestines. True carnivores do not have digestive enzymes in their saliva.

Due to the lack of salivary enzymes, food spends little time in the mouth of a carnivore, it is shortly swallowed and travels down the oesophagus. The oesophagus is a tube which runs from the pharynx (back of the oral cavity) to the stomach. The walls of the oesophagus are protected from damage by food by stratified squamous epithelium arranged in longitudinal folds, this also allows for expansion as the food travels down to the stomach. Food is passed down the oesophagus by peristalsis which is the contraction and relaxation of longitudinal and circular muscles, pushing food down to the stomach in wave like motion.

The next stop is the stomach, the stomach has multiple roles in digestion, including:

  • A reservoir for food
  • A sterilising chamber, due to the low pH (high acid content – HCL)
  • A churning chamber to mix food with digestive gastric juices a
  • The initial site of protein digestion, primarily by pepsin – secreted by the epithelial lining of the stomach

Food is moved to the next site of digestion, the small intestine, by peristalsis. The small intestine is a long and narrow ‘tube’ with a structure and epithelium that maximises surface area. This is important because the small intestine is the primary site of digestion by enzymes. Food continues to travel along the small intestine by peristalsis. The small intestine can be divided into the duodenum, jejunum and the ileum. The pancreatic duct connects the pancreas to the duodenum – the majority of the digestive enzymes enter the small intestine by this duct. To aid in lipid digestion, bile is secreted by the liver (stored in the gallbladder). Bile emulsifies lipids which gives them a larger surface area, increasing enzyme efficiency.

The small intestine joins to the large intestine, which consists of the caecum, colon and rectum. In carnivores the caecum has no function (as it is used in herbivores/omnivores as a site of bacterial fermentation of plant matter). The colon absorbs minimal nutrients from the ingested food; instead its primary role is the reabsorption of water, vitamins and electrolytes from the mixture of food, saliva and gastric & pancreatic juices passing through. This prevents excessive water loss and therefore dehydration. The remnants are excreted via the rectum and anal sphincters.

Herbivores

Herbivores only consume plant material which is very difficult to digest. No vertebrates make an enzyme capable of breaking down cellulose, the tough sugar that makes up plant cell walls which is unfortunate as its digestion yields glucose. As the diet includes large amounts of fibre the digestive tract of herbivores is comparatively much longer than carnivores, due to fibre being much more difficult to digest.

To overcome this herbivores have developed a symbiotic relationship with a population of microflora that inhabit a specialised region of the gut for fermentation e.g. the caecum or rumen of ruminants. The microflora population of the gut is able to breakdown cellulose and use the glucose for its own metabolic needs. As a waste product of this process, the microflora population releases volatile fatty acids (e.g. acetate, butyrate & propionate) which the herbivore utilises for energy. The production of these fatty acids is known as fermentation (fermentation also produces heat which keeps the animal warm).

There are two types of fermenting herbivores, those which ferment in the foregut and those which ferment in the hindgut. The difference between them is the site of fermentation and the organ used for fermentation; the attributes of the fermentation chamber remain the same however – Anaerobic, plenty of fluid, regulated pH, steady nitrogen supply and the correct temperature.

  • Foregut Fermentation – The majority of foregut fermenters are ruminants (including cow, sheep, goat, ox and deer) who ferment their food before it reaches the ‘true’ stomach. The stomach of a ruminant exists as four chambers which are the rumen, reticulum, omasum and abomasum (true stomach). Non-ruminant foregut fermenters (e.g. camels, llamas and whales) do not have the four distinct chambers; instead they simply have modifications to the gut before the true stomach which allows them to ferment. Ruminants digest food more efficiently than hindgut fermenters as they are able to consume food into the rumen – the site of fermentation, allow microbial digestion and then regurgitate the ‘cud’ and chew it some more. This means by the time the ingested food reaches the abomasum, all the extractable nutrients have been metabolised (some microflora from the rumen may also be digested in the abomasum which increases nutrient intake).
  • Hindgut Fermentation – Hindgut fermenters (e.g. e.g. elephant, horse, guinea pig, rabbit, herbivorous reptiles, e.g. tortoise and herbivorous birds) have a digestive system very similar to carnivores, except due to the large amounts of fibre and other difficult-to-digest components of the diet, the complete digestive tract is much longer. Hindgut fermenters also have a working, enlarged caecum which is the site of bacterial fermentation. The process of fermentation is the same as that of foregut fermenters, however as the caecum is located after the stomach and small intestine, the majority of food reaches the caecum undigested. Bacterial fermentation occurs in the caecum and colon allowing some volatile fatty acids to be absorbed, but then the digested food is excreted (along with the microflora). This is why some hindgut fermenters are seen eating their faeces – the food making up the faeces has been digested by the microflora making it of nutritional value. The ingestion of the faeces allows the restoration of the microflora population.

The foregut fermenter herbivores are a lot more efficient as the food is digested on the first pass through the digestive system. Unfortunately for hindgut fermenters digestion is more difficult; however they do have the ability to expel their microflora population which is useful during times such as hibernation.

Omnivores

Omnivores consume both meat and plant matter; they have a digestive system very similar to carnivores but also have a working caecum (not as well adapted as in herbivores). Due to this flexibility they are able to consume a wide diet, which has also prevented them losing the ability to synthesise certain products in the body (as in carnivores).

The process of digestion is extremely similar to carnivores, except a few minor adaptations which allow them to digested plant matter – although not as efficiently as herbivores.

Digestion by Species:

Many species have digestive systems very similar to those shown above; however there may be slight tweaks to the systems between the species, below are some examples.

Birds

Birds do not have teeth and so cannot chew; they are able to break up food however by using their beak. Only some species of bird (e.g. sparrow) are able to produce saliva with the amylase enzyme (for digestion of carbohydrates prior to the stomach).

When a bird swallows food, it passes down the oesophagus into a structure called the crop. The crop is primarily a storage area for food consumed by the bird, differing in size between species however certain adaptions in some species allow it to produce ‘crop milk’ which is rich in protein and fat and fed to the young during their first few days of life. Another adaptation found in one species so far, is that the crop acts as a foregut fermentation chamber.

The stomach of a bird exists as two parts, the proventriculus and the gizzard (ventriculus). As the bird is unable to chew, powerful muscles in the gizzard allow it to grind food up, the presence of grit in the gizzard aids this process.

From this point, the digestive system is similar to other species – small intestine, large intestine, etc.  until it reaches the end of the tract. At the end of the large intestine, the digestive tract opens into the cloaca which is simply a common exit, shared with the urinary and reproductive tracts. Shortly before the cloaca there is a pair of caecum or caeca, unlike the single caecum found in other species. (The caeca are rudimentary or absent in species such as hawks and parrots).

Rabbits

Rabbits are herbivorous hindgut fermenters able to rapidly pass food through their digestive system and quickly eliminate fibre. Due to this ability, he rabbit has remained small and agile, able to quickly escape predators.

Rabbits have a very typical herbivorous digestive system, the only differences are they are unable to vomit and have a large caecum.

One variation, which rabbits are well known for is that they consume their own faeces – coprophagia. This is due to them being hindgut fermenters and losing a fair amount of nutrients and vitamins in the faeces. The rabbit produces two forms of faeces:

  1. Hard, fibrous –No nutritional value
  2. Soft, caecotrophs – High protein content as well as vitamins B & K and volatile fatty acids. The caecotrophs are not digested/damaged in the stomach acid as a layer of mucus surrounds them and protects them, this allows all the nutrients and vitamins to be absorbed in the small intestine

Myomorphs (Rodents)

Myomorphs are omnivorous and have a very typical digestive system. Due to a structure between the oesophagus and cardiac region of the stomach however, it is almost impossible for them to regurgitate food.

Most of the rodents in this group lack a caecum or similar specific organ involved in the fermentation of cellulose, hamster however do have a foregut – similar to ruminants, which has a high pH and a large microflora population.

All myomorphs, like rabbits show some degree of coprophagia for the same reasons – to consume the vitamins and nutrients lost in the faecal pellets.

Sciuromorphs (Chipmunks)

Also an omnivore and has a very similar digestive system to myomorphs.

Hystricomorph (Guinea Pigs/Chinchilla)

The guinea pig is an herbivorous hindgut fermenter, which a large caecum for its body size – containing up to 65% of the total contents of the digestive tract at one time. As will myomorphs, guinea pigs exhibit coprophagia.

Chinchillas are very similar to guinea pigs, they have evolved however to survive on the nutritionally poor yet highly fibrous grasses of the Andes, this means over indulgence of highly nutritious treats (captive kept chinchillas) can cause fatal constipation or diarrhoea.

Chelonians (Turtles, Tortoises & Terrapins)

Chelonians lack teeth so have to use their horny, beak like structure to cut up food. The small intestine of chelonians is relatively short when compared to mammals. Like birds, reptiles have a common exit from the body – the cloaca (urinary, digestive and reproductive systems exit from here).

Snakes

Snakes are carnivorous and possess many teeth which are regularly replaced. Due to the shape of the snake (elongated) the digestive system remains the same, but all the organs are also elongated appropriately.

Lizards

The diets of lizards vary greatly, so the digestive system adapts accordingly, from herbivorous to insectivorous. Variations occur in the efficiency of the caecum (herbivorous/omnivorous lizards).

Fish

Predatory, carnivorous fish have ‘throat teeth’ located just before their oesophagus used for catching and holding prey. Structure of the digestive system are more tube shaped than in other species and can vary in length greatly, depending on the diet of the fish.