A recent NPR radio show covered the research of the biosemiotician Con Slobodchikoff of the Univeristy of Arizone on prairie dog calls. The piece is very public-orientated, but still might be worth listening to.
We’ve all (I hope) heard of the vervet monkeys, which have different alarm calls for different predators, such as for leopard (Panthera pardus), martial eagle (Polemaetus bellicosus), and python (Python sebae). (Seyfarth et al. 1980) For each of these predators, an inherent and unlearned call is uttered by the first spectator, after which the vervet monkeys respond in a suitable manner – climb a tree, seek shelter, etc. It appears, however, that prairie dogs have a similar system, and that it is a bit more complicated.
Slobodchikoff conducted a study where three girls (probably underpaid, underprivaleged, and underappreciated (under)graduate students) walked through a prairie dog colony wearing shirts of the colors green, yellow, and blue. The call of the first prairie dog to notice them was recorded, after which the prairie dogs all fled into their burrows. The intern then walked through the entire colony, took a break for ten minutes, changed shirts, and did it again.
What is interesting is that the prairie dogs have significantly different calls (important, as they are pretty much exactly the same to human ears) for blue and yellow, but not for yellow and green. This is due to the dichromatic nature of praire dog eyesight (for a full study of the eyesight of retinal photoreceptors of subterranean rodents, consult Schleich et al. 2010). The distinction between blue and yellow is important, however, as there isn’t necessarily any reason that blue people are any more dangerous to praire dogs than yellow ones. “This in turn suggests that the prairie dogs are labeling the predators according to some cognitive category, rather than merely providing instructions on how to escape from a particular predator or responding to the urgency of a predator attack.” (Slobodchikoff 2009, pp. 438)
Another study was then done where two towers were built and a line was strung between them. When cut out shapes were slung down the line, the prairie dogs were able to distinguish a triangle from a circle, but not a circle from a square. So, the prairie dogs are not entirely perfect at encoding information. The conclusion still stands however that more information is encoded in the calls than is entirely relevant to a suitable reaction (unless one were to argue that evolutionary pressure existed on prairie dogs to distinguish blue predators from yellow ones.)
NPR labels this ‘prairiedogese’, which makes me shiver and reminds me of Punxatawney Pennsylvania, where Bill Murray was stuck on a vicious cycle in the movie Groundhog Day, forced every day to watch the mayor recite the translated proclamation of the Groundhog, which of course spoke in ‘groundhogese’. Luckily, however, there won’t be courses in this ‘language’.
References:
Schleich, C., Vielma, A., Glösmann, M., Palacios, A., & Peichl, L. (2010). Retinal photoreceptors of two subterranean tuco-tuco species (Rodentia, Ctenomys): Morphology, topography, and spectral sensitivity The Journal of Comparative Neurology, 518 (19), 4001-4015 DOI: 10.1002/cne.22440
Seyfarth, R., Cheney, D., & Marler, P. (1980). Monkey responses to three different alarm calls: evidence of predator classification and semantic communication Science, 210 (4471), 801-803 DOI: 10.1126/science.7433999
Slobodchikoff CN, Paseka A, & Verdolin JL (2009). Prairie dog alarm calls encode labels about predator colors. Animal cognition, 12 (3), 435-9 PMID: 19116730
Thanks for the interesting post! I’ve been meaning to write something about Slobodchikoff’s research myself for ages, especially with these critical language log posts in mind (here and here, but somehow just didn’t get round to it.
One thing that bugs me about this research is the word “encode.” So okay, prairie dogs produce different calls depending on what colour/object they see. But how does that have anything to do with information supposed to be encoded in the call? Is there any evidence that other prairie dogs actually “get” what is “encoded” in the call other than showing an automated flight response?
As far as I know (but alas I can’t remember where exactly I read it) it is actually a quite common feature in the alarm calls of birds (or at least some or one bird species ;-)) that their calls differ according to the degree of excitation produced by the predator/object/etc. Importantly, other birds are in fact sensitive to these differences and show differential behaviour according to the degree of excitation (e.g. they show up in greater number according to the size of the predator, If I remember correctly).
But nobody would say these birds “encode” any information in their call, as their calls differ simply because of low-level perceptual features leading to different excitation with different predators/objects.
Is there anything that actually suggests prairie dog calls should be interpreted any differently?
Their argument that, as you write “there isn’t necessarily any reason that blue people are any more dangerous to praire dogs than yellow ones” doesn’t seem very convincing to me, as the only thing needed is a differential level of excitation according to the colour they see which then influences their calling behaviour. This in turn doesn’t seem very cognitive to me, so I really do not get how they jumped to that conclusion in their paper. I have to say though that I haven’t read the paper so maybe they answer my question but at the moment I’m not convinced.
P.S. Please take this rant as a sign of a succesfull post as you really made me think about the natureof prairie dog calls! 🙂
Ah! I missed that. Or, at least, I hadn’t come around to the information in the right way – while I was writing this, I was conserding what sort of environmental pressure would have caused the prairie dog to encode this information in the first place. I couldn’t come up with a compelling reason. (Why I didn’t include this in the post, I don’t know).
Your comment that the other prairie dogs don’t necessarily hear this information is a good one. The paper does deal with this, a bit – it mentions that it is important to note the difference between a dog and a coyote, which might be different colours, as their predatory activities differ. The paper brings up an example of a coyote sitting by the hole for up to an hour, waiting for the prairie dog to emerge. Considering that prairie dogs tend to only stay under for ten minutes, it is very useful to encode the information in the first alarm call to stay down. That’s about the only situation where having, say, color indicated is useful. Whether the prairie dogs can really distinguish between a wolf and a coyote, which would traditionally probably be more likely predators with different hunting techniques due to pack size, is unknown. How good are their color perceptors, I wonder? It’s also worth noting that I don’t think every call managed to do this correctly.
I would be very surprised if prairie dogs did not also change the nature of their calls due to excitation and stress. That’s fairly common, and it’s very useful – it’s more advantageous for others to know that a coyote has you in its mouth as opposed to being twenty metres away.
FYI, the Bad Linguistics blog did a nice review of this research here: http://badlinguistics.posterous.com/hold-the-front-page-prairie-dogs-still-dont-h
Thanks Chris! That’s an awesome post.