There are very few mammals with venom- why?
Poison seems like a rather effective defensive trait- many animals such as snakes and spiders can bring down animals much bigger and stronger than them.
I would guess that it is a founder effect (although I am open to correction). Some distant ancestor had success with venom and that trait has passed down through a lot of generations as the various new species branched out. (Aside from insects, where I am not that familiar with all the various species and genera, most of the venomous critters tend to be relegated to snakes–and only particular genera within snakes. Th Gila monster and a couple of other lizards do have venomous bites, but they are rather rare. (Komodo dragons, for example, do not have venom–they have really rotten (bacteria filled) saliva that tend to bring down their prey through infection.)
Similarly, another large group of cold blooded critters have rather few venomous species, although they do have quite a few poisonous ones. I can’t think of a single fish, off hand, with a venomous bite, although there are several species with poisonous spines to ward off predators.
Similarly, I can think of quite a few poionous amphibians, (notably poison arrow frogs), but no venomous ones. (This is not to say that there are no venomous amphibians, but there do not seem to be so many that any come readily to mind.)
This ordering along lines of descent suggests to me that the traits arose only a few times and have since been passed down through successive species.
(Venomous: animal injects toxins by biting or through some other aggressive action.
Poisonous: animal has toxins in its own body that are ingested when another animal strikes or bites the first.)
Mammals seem to have evolved several other traits that are more effective than venom.
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growing big. Mammals, especially the more successful ones, are much bigger than the venomous cold-blooded reptiles.
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moving fast. Mammals in general can move faster than venomous animals, and have the endurance to keep up that fast movement for much longer.
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the teeth, claws, even hooves wielded by mammals seem to be pretty effective.
P.S. I don’t think venom is a “defensive” trait. It’s mainly designed to kill (and even partly digest) their prey. It can function as a defense, but that’s really a last resort. Often “the” last resort – the venomous animal may poison it’s attacker, but by the time the venom takes effect, they are long dead. Like a snake can strike a horse, and inject enough venom to eventually kill the horse. But long before that, the snake will have been trampled to death by the hooves of the herbivorous horse.
There are a couple of poisonous mammals, I’m sure. One is the platypus.
T-bonham, the first two properties of mammals that you cite (size and speed) are advantages held by mammals by dint of their being mammals. That is, warm-bloodedness would seem to have advantages for size and speed. Venom does not seem to be so connected to an animal’s mammalian / reptilian nature, at least not in a way I can see.
I basically agree with tomndebb on this one. It’s not really a case of warm blooded versus cold blooded, it’s a case of whether the trait evolved at all and how successful it was when it did. As he points out it’s no more common in reptiles generally than it is in mammals generally if you discount one branch of the snakes. Basically it appears successful because it is so very useful and successful for a predatory ‘lizard’ with no legs to be able to use prey. Venom is nowhere near as successful for animals that have the option of remaining more robust than there prey, which is the case for most other tetrapods.
Spiders were also somewhat ‘destined’ to evolve venom simply because they eat by injecting their prey with digestive enzymes. It wasn’t a very big leap for those enzymes to be modified from a digestive to an offensive form. Since vertebrates generally don’t inject their prey with enzymes it wasn’t quite so easy for the trait to evolve.
Excluding humans the most successful mammals in terms of geographic spread or numbers are very much smaller than the venomous cold-blooded reptiles. The single most successful mammal for example is the house mouse, even more successful than humans in terms of numbers and very close to being as successful in range. The other successful mammals also tend to be rodents, bats, rabbits and the like that are also smaller than or only very slightly bigger than the venomous reptiles found in the same environments.
I have to disagree. Why else does a snake need venom so potent that one bite could kill hundreds of mice? It only needs to kill one. There are some extremely poisonous spiders whose venom is pretty much overkill. A cone shell is another example- this critter eats little fish, yet the same amount of venom in a person will kill them in a matter of minutes!
Animals can recognize other animals that are poisonous, and avoid them. Mimicry is commonly used to mimic animals which are poisonous. So I would say an animal that had venom/poison has a defensive advantage, whether it is for that indiviual, or that species.
Yes, but it needs to make sure it’s very dead, very quickly. Trying to shove a still-struggling meal down one’s gullet is risky, and it only takes one unfortunate incident to find one’s career as a predator over.
This is true of those which use defensive toxins, not so much for those which use it offensively. One doesn’t find brightly-colored rattlesnakes, for example. The distinction noted by tom is not trivial: venomous animals don’t go for the mimicry routine, poisonous ones do. Those who use toxins defensively - or those who mimic them - prefer to advertise the fact (all the better to avoid confrontation in the first place), whereas a predator who uses venom to subdue its prey has no advantage in making such advertisements.
What? Bacterial infection would take at least hours before the bitten creature was affected, wouldn’t it? Do Komodo dragons bite their prey, then back off and stalk it for hours/days, waiting for it to weaken?
I don’t see this. True, no rattlesnake is brilliantly colored, but some venomous snakes are–coral snakes, for example. The venomous lizards are also conspicuously colored. The extremely poisonous (defensively) scorpion fish, on the other hand, is highly camouflaged.
That’s exactly what they do.
You are confusing venom and poison. Different things. Poison is defensive, venom is offensive. No two ways about it. The ‘overkill’ venoms are designed to kill prey quickly, to make sure that the prey dies, fast. Not to mention that venomous snakes use minute quantities of their venom on a per strike basis, usually only enough to kill one or two prey items. It is relatively taxing to produce venom. Venom gives little or no defensive advantage to an animal; otherwise there would be relatively few predators of spiders and venomous snakes. Poisons, combined with aposematic coloration are what provide a defensive advantage. This is seen throughout the animal kingdom, from monarch butterflies to ringneck snakes of North America and Poison Dart Frogs of South America.
The purpose of the bright coloration of the coral snakes and their supposed “mimics” is still very much under debate. Additionally, compared to their surroundings, Gila Monsters and Mexican Beaded Lizards are not conspicuously colored.
Except for the rattle. That’s pretty conspicuous advertisement.
But that’s because scorpion fishes an ambush predator, not because it’s necessarily hiding form prey. Lions and tigers are equally heavily camouflaged despite having few predators themselves. Anything that hunts from ambush can’t afford to be brightly coloured no matter how much more effective that may make its defences.
And yes there are brightly coloured snakes, but you can count them on the fingers of one hand. Compared that to the vast numbers of venomous snakes that are well camouflaged. In fact all the most toxic snakes in the world are camouflaged and are regularly prey for the local predators, which in itself shows that being venomous is of little protective value. Venomous snakes are no less preyed upon than comparable sized lizard or mammals in the same environment.
The vast majority of venomous animals are not, however, conspicuously colored. You can see from looking through the various pictures here that while venomous snakes may come in a variety colors, these colors and patterns often serve more as camouflage than warnings. Not surprising since snakes are ambush predators.
The only venomous lizards are the Gila Monster and the Bearded Lizard. The Gila Monster is not what I would call conspicuously colored. Same goes for the Bearded Lizard.
Of a pissed-off snake, perhaps. The rattle does not specifically advertise, “I am venomous, fear me!”
No, not necessarily. tomndebb has the distinction right - Venomous animals inject the toxic substances, poisonous ones have toxins in their own body.
Gila Monsters and Mexican Beaded Lizards are venomous, but use their venom almost exclusively for defense. They feed mostly on eggs, nestlings, and baby mammals, not really prey that takes a big jolt of venom to subdue.
Likewise honeybees use their venom exclusively defensively, not to secure prey.
It is true that the majority of venomous snakes are cryptic, rather than aposematic. Some, however, such as rattlesnakes and cobras have pretty intimidating warning displays, and I think it’s absurd to say that venom gives them “little or no defensive advantage.” While some specialized predators may be able to prey on them, they would be much more vulnerable without their venom.
The major exception to crypsis in venomous snakes is coral snakes. Given the fact that they are innately recognized and avoided by birds, I have little doubt that their venom and their aposematic coloration serves a defensive purpose, as well as being used to subdue their prey.
Rattlesnakes are preyed on by a variety of animals, including possums, skunks, badgers, coyotes, raccoons, wolves, eagles, hawks, owls, alligators and other snakes such as king snakes and racers. Rattlesnakes have many natural predators. Their main defenses are their speed and their ability to put up an impressive show, letting the predators know that they won’t go down without a fight. The venom isn’t a factor in this display; it is seen in many snakes, including the rat snakes, the bull or gopher snakes, the hognose snakes, the boids of the New and Old Worlds and the natricines. Intimidating warning displays are not the sole property of venomous snakes. A skunk may not be willing to invest the energy in subduing a pissed off rattlesnake, but their venom is not a primary defense system as they would be just as deterred by an angry cornsnake. Among the smaller species such as the massassaugas and pigmy rattlesnakes, neither an impressive show nor a potent venom is available.
The same goes for cobras: they are preyed on by land mammals such as mongeese (mongooses?) and birds. I think the defensive merit of their venom demands further study. Any defensive advantage is conferred only during a face-to-face encounter; the majority of the predators of venomous snakes, like the predators of almost all snakes, strike quickly and without warning. If venom were an inherently effective defense mechanism, why would such elaborate displays be necessary? When a rattlesnake goes into a defensive display, one of two things happens: the snake either rears up, rattles, begins hissing fiercely and scoots backwards until it reaches bushes or other cover to scoot away under, or it curls up into a tight ball and ceases all motion until the predator loses interest. Even if strikes are made, the majority of defensive bites contain no venom, at least among the crotalid snakes.
The primary instinct of any snake, venomous or otherwise, is to escape immediately, as the snake will lose any encounter ninety-nine times out of one hundred. The odds greatly favor the predator in such a scenario. Otherwise, the snake would stand its ground and try to kill the attacker. The Gilas and honeybees use their venom as a defense because they have no other defenses. Whether or not that is a secondary advantage of the venom or the primary reason for its evolution is debatable. However, the use of the honeybee’s venom means death for the bee anyway.
If venom conferred any defensive advantage, then we would expect the rates of predation of venomous snakes to be significantly lower than the rates of nonvenomous snakes by the same predators. This hasn’t been demonstrated, AFAIK. Add in the fact that the majority of defensive bites, at least among crotalids contain no venom, and there is little support for the hypothesis that venom confers any significant defensive advantage to snakes.
Venom doesn’t give spiders much of a defensive advantage, either.
I believe that is just Baytesian mimickry though. Do you have any evidence that non-venomous snakes living in regions with no venomous snakes using such displays bother to use it? IF not then it’s almost certainly mimickry. The non-venomous snakes are bluffingt hat they are venomous and the predators don’t know the difference.
Not true. The predators of most snakes, whether it be a skink, a secretary bird or a kookaburra take quite along time to kill the snake and invariably make many feints before they inflict any serious damage.
Reference? That is an extraordinarily high number. Even rabbits aren’t downed by predators that often.
That is exactly what we would never expect. Since snakes in the same environment will be almost perfect competitors for the same food such a situation would lead to the rapid extinction of non-venomous snakes if it were ever to occur. Assuming that venom does confer an advantage of that type we would expect any non-venomous snakes that have survived to not be any more prone to predation that the venomous simply by way of the fact that they have not become extinct.
To use an analogy, springboks are no more prone to predation by lions that are wildebeest. That’s not because neither form has any defensive advantages but because the advantages have balanced out. What one loses in bulk it make sup for in speed and behaviour. If it didn’t it would be hunted to extinction. In exactly the same way what a non-venomous snake lacks in deterrence it must make up for somewhere else if it still exists.
The fact that venomous snakes are no les preyed upon can’t be used to support either position.
Batesian mimicry? Defensive displays are found in most snakes throughout the world. To suggest that such displays are the result of Batesian mimcry borders on absurd. That is akin to suggesting that a cornered possum is mimicking a cornered bobcat.
More accurate would be to say that some predators do that. Many predators, most of the birds and some of the mammals such as the coyotes simply run up or swoop down and grab the snake by the spine, killing it quickly.
I was referring to situations where a predator has actually physically taken a snake. When a Coyote has a snake in it’s teeth, the snake has little chance of survival. An exaggeration, but very little of one.
I don’t understand what you are trying to imply here. Venomous snakes are sympatric with nonvenomous snakes analalogous to them in size and prey preference throughout the world. In the southeast, for exampe, the Cottonmouth (Agkistrodon piscivorous is found very commonly with many species of the water snakes (genus Nerodia) they are of similar size (cottonmouths grow slightly larger) and have similar habits, habitats and diets. They are also preyed upon equally by alligators, snapping turtles, hawks, skunks, possums, etc.
I don’t understand your point.
[QOUTE]To use an analogy, springboks are no more prone to predation by lions that are wildebeest. That’s not because neither form has any defensive advantages but because the advantages have balanced out. What one loses in bulk it make sup for in speed and behaviour. If it didn’t it would be hunted to extinction. In exactly the same way what a non-venomous snake lacks in deterrence it must make up for somewhere else if it still exists.
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A better analogy would be that springboks and dik-diks have no predation differential. The difference between springboks and wildebeest is in no way akin to the difference between water snakes and cottonmouths. You seem to have some misconceptions about predator-prey interactions. The evolution of predation defenses is an “evolutionary arms race” between the predator and the prey to continually “out-evolve” the other. This balances in the end due to the fact that neither ever gets to far ahead of the other.
Yes, it can.
Boy, that post was a mess. I meant to hit preview.
In addition, [URL=http://www.bio.davidson.edu/people/midorcas/research/herppub-pres/dorcas-pdfs/GibbonsandDorcas2002.pdf]here[/ULR] is a paper about the use of defensive displays versus actual strikes by cottonmouths that were threatened in several different manners, suggesting that venom is not often used as a defensive measure and is not very well suited to be used as one.
Grrrr… Link to Gibbons and Dorcas, 2002
A better analogy would use springboks and dik-diks, or some other similar, deer like animal. They are similarly sized and have similar predation rates by larger predators. They also use similar defences (speed and agility), as do such snakes as Cottonmouths and their non-venomous congeners, i.e. fleeing; remaining motionless; coiling up, flattening out and hissing; gaping and striking, in that order.