How viper* behavior increases their effect on prey populations
*Vipers are a family of venomous snakes that includes Copperheads, Cottonmouths (Water Moccasins), and Rattlesnakes in North America.
The sound of domestic cats fighting is probably familiar to many readers. Like most predators, cats defend their turf and the resources within. Prey is often scarce and starvation a real possibility, so it makes sense to fight for access to these resources.
But even amongst those who spend their lives observing vipers, it is rare to see a snake fight. Why? Unlike most predators, vipers are not territorial, only males fight, and only for access to females.
Vipers don’t need to eat as often as endothermic (warm-blooded) predators that feed on similar-sized prey; they can go weeks or even months without food, while many birds and mammals eat daily. If prey is not limiting, why waste energy fighting over it?
Not only may vipers not view their neighbors as competitors, their presence may actually increase their hunting efficiency! Vipers use chemosensory cues to select hunting sites and preferentially choose sites where conspecifics (other members of their species) have been successful. We have also observed different viper species using the same hunting sites, so the presence of other viper species may increase hunting effectiveness as well.
Another important resource for vipers and other ectotherms (cold-blooded) is an appropriate over-wintering shelter (den). In this situation, vipers are also tolerant of others. Vipers often share their dens with conspecifics and many other species too. In some climates dens are a limiting resource, which can explain this behavior. But that is not always the case; some vipers are social and choose to aggregate.
Arizona Black Rattlesnakes at a communal den site in southern Arizona.
Vipers’ home ranges overlap with many conspecifics, as well as other snake species. Even male vipers tolerate others in their territory outside the breeding season and encounters between them don’t usually end in a fight. Being good neighbors enables them to live in higher densities than mammal and bird predators.
In some cases, vipers may be 100-1000 times more abundant than their mammalian competitors!
But vipers need less food, so do these facts cancel each other out? Turns out they have a few other tricks up their sleeves.
Vipers can fast for long periods, so if prey populations crash, viper populations do not necessarily follow. Moreover, since most vipers give birth every two to three years, prey population crashes may not have an immediate effect on their reproduction. When other predator populations have crashed (for example, classic predator-prey models like lynx and hares), and prey populations start to increase again, vipers are waiting for them.
Vipers can have a greater impact on prey populations than bird or mammal predators. Perhaps this could change the tune of those who think the only good snake is a dead snake?
This post was largely inspired by the following article from Dr. Erika Nowak and colleagues:
Nowak, E. M., Theimer, T. C., and Schuett, G. W. 2008. Functional and numerical responses of predators: Where do vipers fit in the traditional paradigms? Biological Reviews 83:601-620.
Details about vipers (Timber Rattlesnakes) selecting hunting sites where others have been successful can be found here:
Clark, R. W. 2007. Public information for solitary foragers: timber rattlesnakes use conspecific chemical cues to select ambush sites. Behavioral Ecology 18:487-490.
This post was part of #SnakesatyourService blog carnival in 2013 (minor updates in 2022). Want to find out more about ecosystem services offered by snakes?
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