Tuesday, March 26

On Prairie Dogs

Not exactly my model species, but I talked about birds a short while ago, so why not prairie dogs? They're at least mammals with strong social organization. Anyway, I paper came to my attention, about dispersal in prairie dogs, and it was published in Science, so of course I had to read it.

Prairie dogs disperse when all close kin have disappeared

Prairie dogs pull up stakes and look for a new place to live when all their close kin have disappeared from their home territory--a striking pattern of dispersal that has not been observed for any other species.

Original article is here (behind Science's paywall). As previously discussed, baboons disperse when they are on cusp of full adulthood. They find a new troop, and usually stay with that troop until they die.

Prairie dogs differ from baboons in several striking ways. They live in large groups, called colonies, and the size of these groups can vary quite a bit, which is nothing unusual to a primatologist. But unlike baboons, prairie dop groups can range from five to thousands. Colonies can be further subdivided, into wards, and then coteries. Needless to say, prairie dogs almost certainly do not possess baboons' rich understand of who's who in the group. Nevertheless, these subdivisions are oddly reminiscent of the four-level hierarchy found in Hamadryas baboons.

Coteries are the closest thing there is a basic unit of prairie dogs. Coteries are sort of like harems: they consist of a male, several females, plus juveniles and infants. The juvenile males disperse soon after they are a year old. They leave their natal territory, settling about 1.5 miles away, on average. Females tend to stay put.

The author of this paper, Hoogland, references the esteemed Hamilton and May. Their theory was that dispersal occurs because reducing the amount of competition (for mates, food, etc) between related individuals is good for inclusive fitness. On the other hand, the potential for cooperation between related individuals might outweigh the costs of competition. Over the past few decades, Hamilton and May have been supported by findings in the field.

Hoogland has found contradictory evidence in his prairie dogs. When zero relatives are around, females are much more likely to disperse, 2.5 to 12.5 times more likely.

These prairie dogs have no opportunities for competition between relatives, but also no opportunities for familial cooperation. You might think that the dangers of dispersal would still be a major impediment, but apparently these populations live pretty close together, and the females usually just move one colony over, so the risks are low.

Hoogland's own words, to sum it all up:
"The absence of close kin in the natal territory is thus a proximate cause of natal dispersal by prairie dogs, but the ultimate cause is presumably the opportunity to find either a new territory that offers the benefits of cooperation with close kin that dispersed there previously (rare), or a new territory in which survivorship and reproductive success might be less dependent on cooperation with close kin (common)."
This may not be a study about primates, but the laws of dispersal and inclusive fitness govern all animals. When we discover a behavior violates our conceptions about how life must act in order to maximize fitness, there are two main possibilities:
  1. The environment that this species lives in has given rise to a different approach to the challenge; unusual local factors are altering behaviors on the fringe.
  2. The foundations of the behavior are not what we think they are. There are factors not being considered, which are crucial to understanding why these behaviors happen. Just because our model is correct most of the time, doesn't mean the model accurately represents why animals behave the way they do, in this case.
And anyone seriously studying this stuff ought to pause and consider new evidence in this light, if only briefly. In this case, the first option is probably at work. Prairie dogs are mammals, not so different from primates, but as Hoogland states, this is the first evidence of its kind, irrespective of species. Who knows, maybe a re-examination of dispersal behavior in primates will find similar evidence that's been overlooked. If you find it, chances are you'll get it published in Nature or Science.

Hoogland, J. (2013). Prairie Dogs Disperse When All Close Kin Have Disappeared Science, 339 (6124), 1205-1207 DOI: 10.1126/science.1231689

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