Creature Feature: California ground squirrel

If you enjoy hiking in North America, you’ve probably been advised to steer clear of rattlesnakes. When you find yourself face-to-rattle with this venomous snake: stop, listen, and slowly back away. That’s the best way to avoid an attack from a snake that might be threatened by your large and looming presence. Most animals would agree: rattlesnake venom is primarily composed of two neurotoxins [1] and smaller mammals and birds would do well to avoid a deadly strike from this predator. Not, however, the California ground squirrel (Otospermophilus beecheyi). Instead of quietly avoiding rattlesnakes, they employ bravado and spunk to protect themselves from ambush. California ground squirrels exhibit a remarkable suite of behavioral and physiological adaptations that have given them a leg-up against rattlesnakes.

California ground squirrels look rather unassuming, but you’d be surprised how spunky these little fellas can be when confronted by would-be predators. [Photo taken by: Wade Tregaskis, Source]

Part 1: Ground Control to Major Otospermophilus

First, what exactly is a ground squirrel? The allergy-inducing alternative to a tree squirrel? Ground squirrels are a taxonomic grouping of squirrels that live, unsurprisingly, on the ground rather than in trees. This group also includes marmots, groundhogs, chipmunks, and prairie dogs. California ground squirrels are common throughout western North America, and are sometimes referred to as Beechey ground squirrels [2]. You’ve probably seen them scurrying amongst their burrows, which can often be found dotting large grassy patches around highways and parking lots. Most people confuse ground squirrels for regular tree squirrels, but one easy way to tell them apart is a dark triangular patch of fur running across their upper backs. Of course, if you notice one entering a ground burrow and there’s a general scarcity of trees in the area, you can be pretty confident that this is in fact O. beecheyi. You might be thinking, does it really matter whether they live on the ground or up in trees? Well, one of the biggest threats to small, ground-dwelling animals that tree-dwelling animals can avoid is terrestrial predators. But this added threat doesn’t seem to deter O. beecheyi from living a full and happy subterranean life. And although they don’t use their tail to balance the same way their arboreal cousins might, they still make good use of this fluffy appendage.

Note the dark patch of fur that can help to distinguish the California ground squirrel from tree-dwelling species [Photo taken by: David A. Hoffman, Source]

Part 2: Shake ya tail feather

Researchers at UC Davis spearheaded much of the ground-breaking [pun-very-much-intended] work on this unassuming squirrel species. Dr. Donald Owings and former students of the Animal Behavior Graduate Group studied inter-species interactions between ground squirrels and rattlesnakes. Ground squirrels will often approach and harass both venomous and non-venomous snakes [3]. This kind of mobbing behavior is not uncommon among mammal species, but ground squirrels take things a step further [4]. First, they can discriminate between venomous rattlesnakes and non-venomous gopher snakes [3]. Gopher snakes and rattlesnakes have remarkably similar and cryptic patterns, and gopher snakes are known to imitate the rattling sound of rattlesnakes [5]. Since the squirrels encounter snakes both above ground and in their burrows, the theory is that they have evolved both a visual and non-visual detection system for these predators [3]. Why does this discrimination matter to a squirrel? After all, a hungry snake is a hungry snake. One reason is that the squirrels have a different defensive approach towards the more deadly rattlesnake. In addition to mobbing and kicking dirt at rattlesnakes, squirrels engage in “tail flagging”. This entails lifting the tail up in the air and waving it from side to side (check out the video below to watch this behavior in action).

In addition to mobbing and kicking dirt at rattlesnakes, California ground squirrels will often engage in tail flagging behaviors towards these venomous predators. [BBC, Source].

Shaking one’s tail feather is about more than just fancy moves. California ground squirrels actually direct warm blood into their tail when tail flagging [6]. Some snakes rely on infrared radiation (i.e., heat signals) to locate their prey. A raised, waving, and heated squirrel tail looks rather more threatening than a low-lying scurrying critter, even when it’s not kicking dirt at you. What is even more fascinating about this behavior is that it is only directed towards rattlesnakes: gopher snakes cannot detect infrared radiation and rely on other cues to find their prey [6], so the tail flagging display would not be an effective tactic against them.

California ground squirrels will tail flag towards infrared-sensitive rattlesnakes (A) and not towards infrared-insensitive gopher snakes (B). Notice how the squirrel’s elongated, warm tail looks somewhat like a snake ready to strike when viewed with an infrared camera [Source].

Part 3: Your toxins I’m slipping under (my skin)

But wait, there’s more! Recent research has revealed that California ground squirrels will chew up shed rattlesnake skins and apply the scent across their bodies [7]. And no, this isn’t just because rattlesnakes have an alluring scent. Researchers tested three main hypotheses for why snake scent application (SSA) may have evolved: (1) to avoid ectoparasites; (2) to use as a threat signal to conspecifics, and (3) to serve as anti-predator defense [7]. The first two hypotheses were found to have less support than the third, anti-predator defense hypothesis. Consistent with this hypothesis, snake scent application was found to be more common in adult females and juveniles compared to adult males [7]. This makes sense intuitively: females and juveniles are more at risk for predation than adult males. On top of this behavioral response, California ground squirrels can even resist a certain level of envenomation by rattlesnakes; a compound in squirrel blood effectively binds the venom and neutralizes its deadly effect. Research suggests that this physiological adaptation is population-specific and depends on the density of rattlesnakes in the habitat [8]. Squirrels found in areas where snakes were more common could resist higher concentrations of injected rattlesnake venom [8].

Adult female and juvenile California ground squirrels have been observed to chew up rattlesnake skin and subsequently licks parts of their body, including their tails. This snake scent application (SSA) behavior is thought to serve as an anti-predator defense [Source].

So, in case all the frantic tail flagging, dirt kicking, and scent wafting proves fruitless, some California ground squirrels can avoid death even after they’ve been bitten by a rattlesnake. Now that’s a superpower! The next time you find yourself face to face with a cute little California ground squirrel, look out for these dastardly critters. They might be gearing up for their next encounter with a deadly rattlesnake. While we’re busy doing a stop, listen, and back away, the California ground squirrel takes a different approach: chomp, kick some dirt up, and shake, shake, shake.


References:

  1. Patel, Virat, and Richard J. Hamilton. “Rattle Snake Toxicity.” StatPearls [Internet]. StatPearls Publishing, 2019.
  2. Jameson, E. W., & Peeters, H. J. (2004). Mammals of California (Vol. 66). Univ of California Press.
  3. David F, H., & Donald H, O. (1978). Snake species discrimination and the role of olfactory cues in the snake-directed behavior of the California ground squirrel. Behaviour65(1-2), 115-123.
  4. Owings, D. H., & Coss, R. G. (1977). Snake mobbing by California ground squirrels: adaptive variation and ontogeny. Behaviour, 50-69.
  5. Sweet, S. S. (1985). Geographic variation, convergent crypsis and mimicry in gopher snakes (Pituophis melanoleucus) and western rattlesnakes (Crotalus viridis). Journal of Herpetology, 55-67.
  6. Rundus, A. S., Owings, D. H., Joshi, S. S., Chinn, E., & Giannini, N. (2007). Ground squirrels use an infrared signal to deter rattlesnake predation. Proceedings of the National Academy of Sciences104(36), 14372-14376.
  7. Clucas, B., Rowe, M. P., Owings, D. H., & Arrowood, P. C. (2008). Snake scent application in ground squirrels, Spermophilus spp.: a novel form of antipredator behaviour? Animal Behaviour75(1), 299-307.
  8. Poran, N. S., Coss, R. G., & Benjamini, E. L. I. (1987). Resistance of California ground squirrels (Spermophilus beecheyi) to the venom of the northern Pacific rattlesnake (Crotalus viridis oreganus): a study of adaptive variation. Toxicon25(7), 767-777.

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