Yesterday at Magnolia Gardens’ Audubon Swamp Garden I saw a great egret manage to swallow (with much difficulty) a large water moccasin (cottonmouth) and wondered how it avoided the fangs. One possibility is that it is immune to the toxin. Another possibility is that it just knew to avoid them before killing the creature… which brings me to some other questions in this area.
Predators avoid poisonous prey (such as birds avoiding Monarch butterflies), but how do they know to avoid them? They have only one chance. Is this knowledge somehow passed on, or is that the prey has a bad taste (due to the alkaloids from the milkweed plant which make them poisonous) and they’ve learned from the bad taste to avoid them? It may be that such poisonous prey have a bad taste, but what about prey that have toxins, such as poisonous snakes? We’ve learned what snakes are poisonous, but predators don’t appear to have the means to pass this knowledge on.
Natural selection selects against traits that make dangerous things attractive. Namely, if you have a freak mutation making you nauseated when you see the color yellow, and there’s a lot of seemingly edible things around you that are yellow AND toxic, you are at a genetic advantage to individuals with no aversion to yellow. Just a n artificial example though.
No snake is poisonous. You can eat any snake with no ill effects.
Several types of snakes are venomous. There is a difference.
Snake venoms are modified digestive fluids / saliva. They need to be introduced into the bloodstream of the prey animal to have effect. The venom of the cottonmouth breaks down the walls of the prey’s blood vessels, causing death that way. (Cobras and coral snakes have a neurotoxin.) The venom starts the digestive process, since snakes cannot chew. Once swallowed the venom is quickly broken down by the digestive juices of the ingester.
Monarch butterflies are not so poisonous as to kill a predator. One or two will give the bird an upset stomach, and birds can learn fast. When your metabolism is set so fast as to require frequent meals, you learn quickly what can be eaten and what cannot.
Birds have experimentally been shown to have innate, non-learned aversion to at least some poisonous snakes, in particular coral snakes, which have bright warning colors. This has presumably been as the long-term result of selection.
I know that, but this question was how did the egret manage to avoid the snake’s fangs? **Colibri ** noted that birds innate aversion to some poisonous snakes, such as the coral, but this egret had no aversion to that cottonmouth. Yet it managed not to be bitten. Just lucky, careful? I assume now that it is not immune to the toxin (or venom, as you put it).
I imagine that you’d have to ask an ornithologist to find out if egrets are in the habit of eating cottonmouths, or if this one just got lucky. If they eat them all the time, you might be able to find the answer. Otherwise, probably not.
Since you brought up Monarch butterflies, just for the record, they are not instant death for predators. But they do taste so unpleasant that most birds will drop a Monarch after one bite, and after a couple encounters will tend to avoid taking Monarchs or similarly marked butterflies. I don’t believe it’s been shown that there is a genetic aversion in any species to orange and black butterflies.
And the one bite is quite often not fatal to the Monarch either, demonstrating that there is benefit to an individual butterfly to be toxic.
In the end venom is just another complex protein. If the mucus lining of the egret’s stomach is durable enough (and based on it’s diet I imagine it’s pretty rugged) , the acids in the digestive fluids would quickly denature those proteins in short order.
Re the avoidance of fangs issue cottonmouths are not that fast as striking snakes go. If Steve Irwin can keep his distance from faster snakes while flipping them around like pancakes, I don’t imagine that a staying away from a cottonmouth strike is that big a deal for a hungry, experienced egret.
Also bear in mind that egrets have very long legs that are little more than skin and bone and don’t provide a good target for the snake. A striking moccasin likely couldn’t reach the bird’s fleshy parts, while the egret could peck quickly at the snake from above.
Offhand, I am not aware of any birds with a substantial immunity to snake venom, but it’s possible. A number of snake-eating mammals, such as the mongoose, are more tolerant of snake venom than most animals, although they are not completely immune.
Egrets and other snake-eating birds such as Roadrunners have extremely fast reaction times. This probably permits them to kill venomous snakes with little fear. I am not sure which kinds of birds have been shown to have innate fear of snakes, but it’s possible that they were species that are not snake predators but instead species that were more likely to be snake victims.