ROBERT SIEGEL, host:
From the Rudyard Kipling story, "Rikki-Tikki-Tavi," many of us learned early on that even the most venomous cobra is no match for a valiant mongoose, a carnivorous kin to a weasel. When Kipling's story was turned into a 1975 animated feature, Rikki, the mongoose, came out looking suspiciously like a squirrel. And from what we learned today in the Proceedings of the National Academy of Sciences, the California ground squirrel is also praiseworthy for its ability, not so much to fight as to scare off venomous snakes.
Aaron Rundus, who is now doing post-doctoral research at the University of Nebraska at Lincoln, studied the squirrel's remarkable defenses when he was a graduate student at the University of California-Davis. And Dr. Rundus joins us now from Lincoln. Welcome to the program.
Dr. AARON RUNDUS (Behavioral Biologist, University of Nebraska-Lincoln): Hi. Nice to be here.
SIEGEL: And tell us first about the snakes that a California ground squirrel might encounter.
Dr. RUNDUS: In northern California where we did our work, most likely they would run into either Northern Pacific Rattlesnakes or Pacific Gopher Snakes.
SIEGEL: The rattlesnake is venomous, and the Gopher Snake, I gather, is not?
Dr. RUNDUS: That's correct.
SIEGEL: And the squirrels actually react differently to the venomous and non-venomous snakes.
Dr. RUNDUS: Yes. They do a signal or a display that we've called tail flagging when they encounter snakes, and that's where they bush up their tail fur and they wave their tail from side to side. And if you just look at the visual characteristics of that, it looks very similar, irregardless of what species they're tail flagging to, but when you actually look at these under an infrared imaging camera to look that the heat or the thermal properties, you find that the squirrels are increasing the amount of heat they emit from their tails during the rattlesnake encounters, and not when they encounter Gopher Snakes.
SIEGEL: But you're saying that the squirrels are emitting heat and also a visual signal, the tail waving back and forth.
Dr. RUNDUS: Right. So the - it's what we would classify sort of as a multimodal signal, so we can see that as visual information or a visual signal. We also are - that at least the rattlesnakes are capable of picking out the thermal or the heat component of that signal as well.
SIEGEL: And I assume, since there's still are California ground squirrels around, this is relatively successful and it has protected them from rattlesnakes?
Dr. RUNDUS: It appears to be so. And so, in our study we actually looked to see how the signal is functioning and we found that when the infrared component is included in the tail flagging display, it goes a lot farther in terms of shifting rattlesnakes from a predatory type of mode into a more defensive type of mode.
SIEGEL: You saying that the squirrel waves its tail, emits heat, and the snake backs off at that.
Dr. RUNDUS: That's correct. And we have several theories for why that's might be - have this particular type of effect on rattlesnakes. Probably our best guess is that tail flagging itself signals the rattlesnake that it's been discovered. And rattlesnakes are ambush hunters, so they'll usually take up residence and wait to its natural vulnerable pup when they can. So if you're a rattlesnake and you see a tail-flagging squirrel, you can be pretty sure that you've been discovered.
SIEGEL: The California ground squirrels sounds like it has adapted amazingly well to the predators and its environment.
Dr. RUNDUS: That's correct and these squirrels and these snakes have been, sort of, co-evolving together for well over a million years. So there's been quite a bit of time for both of these species to try to, if you will, sort of, circumvent each other's defenses.
SIEGEL: In the arms race between the rattlesnakes and the ground squirrels, have the rattlesnakes come up with anything remotely as interesting or sophisticated as this tail flagging and heat emission?
Dr. RUNDUS: So we do see, in terms of the venom profiles of rattlesnakes, there's been a fair bit of research done now looking at how local populations of rattlesnakes have been adjusting to types of venom that they have and use in order to try to circumvent the resistance that the adult squirrels possess to those venoms.
SIEGEL: How many years did you study squirrels for this project?
Dr. RUNDUS: The actual project lasted for about five and a half years total.
SIEGEL: And did you develop a healthy respect for the squirrels during that time?
Dr. RUNDUS: Absolutely. When you first, look at this squirrel-snake system, you can't help but feel a little bit sorry for the squirrels being undersized by these venomous rattlesnakes. But when you can see the kinds of defenses that they have against these predators, it becomes apparent that they're pretty well matched and equal, sort of, adversaries to one another. And so you really get a healthy respect and appreciation for what both participants in the system can accomplish.
SIEGEL: Well, Aaron Rundus, thank you very much for talking with us.
Dr. RUNDUS: Thank you. It's been my pleasure.
SIEGEL: That's Aaron Rundus of the University of Nebraska at Lincoln, who studied the California ground squirrel's defenses against venomous rattlesnakes.
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