Snakes Are Good Neighbors

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?

A pair of male Western Diamond-backed Rattlesnakes in combat over an unseen female
A pair of male Western Diamond-backed Rattlesnakes (Crotalus atrox) in combat over an unseen female. For more on rattlesnake combat (including videos!), click on the image.

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.

A female Western Diamond-backed Rattlesnake (tan rattlesnake with brown blotches), swallowing a brown rodent.
Allison, female Western Diamond-backed Rattlesnake (Crotalus atrox), swallowing a rodent.

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.

Western Diamond-backed Rattlesnakes (Crotalus atrox) often share their dens with other animals, including Gila Monsters (Heloderma suspectum), photograhed by Jeff Smith. Click on the image for more examples of sharing.

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.

Further Reading

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?

Still don’t want vipers for neighbors? Make friends with kingsnakes.

In contrast to the story told here, some snakes are closely tied to their prey (frogs).

The Travelling Taxonomist discusses the ecological and cultural roles of snakes in Madagascar.

What is the role of snake sheds in the ecosystem?

The most famous example of snake’s impact on an ecosystem.

While humans’ fear of snakes is somewhat irrational, for other animals it’s very real.

Finally, changing attitudes toward snakes in the next generation.