What's the purpose of the ones without the venom?
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Snakes have a variety of ways of catching prey. Venom offers an advantage in that the snake doesn't have to expend energy to fight with its prey. Live prey is also much more difficult to swallow. Besides using venom some snakes kill their prey by constriction or the power of the intial strike. Some snakes do have a diet that requires food to be killed before injesting such as insects, worms, and smaller vertebrates. Venom favors snakes that take down large prey or hunt by sitting and waiting rather than foraging.
The evolution of snake venom is still somewhat of a debate. Some aspects of venom could have developed on its own while other aspects can be traced back to a common ancestor. Not all snakes are venomous and while families tend to have a nonvenomous or venomuos tendency their are most always exceptions.
Colubrids, such as garter snakes, have a few rear fanged venomous species belonging to the family. Others contain a mild venom originating from the Douvernoy's Gland which is believed to be an step in the evolution of snake venom. Garter snakes and hognose snakes are two common examples of snakes with this gland. In fact some people consider calling the hognose a mildly venomous species as a bite from its rear fangs can cause a rash.
So to wrap things up and answer your question. Some snakes do not have venom because they belong to a link in which venom is still developing. Others have found other ways of catching/killing prey and do not need the assistance of venom. If a snakes dietary food is not known to fight back than one of the main advantages for venom becomes irrelevent. Some snakes that were previously venomous may have found alternative solutions to the problems of life, without venom acting as an advantage it can be lost over time.
The evolution of snake venom is still somewhat of a debate. Some aspects of venom could have developed on its own while other aspects can be traced back to a common ancestor. Not all snakes are venomous and while families tend to have a nonvenomous or venomuos tendency their are most always exceptions.
Colubrids, such as garter snakes, have a few rear fanged venomous species belonging to the family. Others contain a mild venom originating from the Douvernoy's Gland which is believed to be an step in the evolution of snake venom. Garter snakes and hognose snakes are two common examples of snakes with this gland. In fact some people consider calling the hognose a mildly venomous species as a bite from its rear fangs can cause a rash.
So to wrap things up and answer your question. Some snakes do not have venom because they belong to a link in which venom is still developing. Others have found other ways of catching/killing prey and do not need the assistance of venom. If a snakes dietary food is not known to fight back than one of the main advantages for venom becomes irrelevent. Some snakes that were previously venomous may have found alternative solutions to the problems of life, without venom acting as an advantage it can be lost over time.
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They use other ways to kill their prey. Constrictors.
Others have no need for venom or constricting because they eat insects.
Others have no need for venom or constricting because they eat insects.