Monday, 27 October 2008

Not a reference to a Meme.

So, wait, why do we care where badgers go? Well, you might not, but scientists do. We're talking specifically about eurasian badgers, Meles meles, not its North American doppleganger Taxidea taxus (taxus being the latin for `divided`, like how the badger is pinstriped). Badgers are Mustelids, which means they think with their stomach, if they think at all; I'm fond of Mustelids, and not just the classically fur-bearing ones. Even the exceptionally clever ones are wonderfully stupid in their own way.

Now, badgers tend to disperse, or relocate to a place away their birthplace. This can be permanent, or non-permanent. Actually, dispersal is a common feature of a lot of species, but MacDonald et al. 2008 happened to focus on badgers. Part of the reason animals tend to disperse is to get away from competing with close kin. Interestingly, "even in a stable and saturated environment in which the mortality of dispersing individuals is 100%, dispersal rates , 0.5 are not evolutionarily stable (Hamilton and May 1977)." This I did not know. I'm not sure I believe it; I'll have to go read Hamilton and May.

Eurasian badgers are interesting, because in part, they're arranged in large social groups that stem from natal group-formation. In other parts, they're largely solitary (think Mr. Badger from Wind in the Willows). Further, they're polygynandrus, or promiscuous, with a minority of males from outside a given social group siring the majority of cubs. Dispersal might play a key role in achieving large degrees of reproductive success in badgers, one might surmise.

MacDonald et al. used about 17 years of live-trapping data from a location north of Oxford, England. Sparing you the gruesome details of the methods, they estimated that 19% of badgers dispersed in their study period, and the bulk of them were to adjacent social groups. They found no significant deviation between males and females in dispersing, or dispersal distances. Males apparently dispersed more in autumn or spring, and females moreso in summer - the former in a period where females tend to be in oestrous.

The authors figure that about 10% of individuals who became yearlings dispersed, but 40% of those who survived to 8 years dispersed. This is a fairly high level of dispersal, and interestingly, density didn't appear to impact dispersal. That is to say, frequently, when animal density is high, it's thought that violent interactions will prevent many dispersers from being successful. However, it appeared that males who dispersed had much higher levels of injury, which seems to give the `social fence` hypothesis a bit of a respite.

With the evidence of extra-territorial matings, the authors seem to suggest that the inbreeding avoidance aspect of dispersal might not be as important as other mechanisms. They found that dispersals didn't tend to be permanent, and they could easily satisfy the needs of extra-territorial mating for inbreeding avoidance. The badgers, in short, get to have their cake and eat it too, not surrendering territory to get outside their natal group to avoid inbreeding.

Why do I care?

First, badgers are neat critters. But more importantly than that, inbreeding avoidance is an important part of life-history. Some of my last lab's work had a strong focus in inbreeding avoidance in animals, so it's a topic near to my heart. Further, several ungulate have the same genetic patterns of paternity, though with different social structures. I'm thinking, in particular, about Pronghorn, where multiple paternities are fairly common in twins. Now, the folks who did the pronghorn work suggested moose guys check for the same patterns, and I'd be curious to see if they bear out. But with what I know about moose biology (i.e., less than a moose knows) I'd be curious now to check to see if the gigas sub sp. (Alaskan moose) tend to show the same male-life time short-dispersals. It's more of a stretch from badger social ecology to moose, because moose largely lack social structure through most of the year, but that's not to say they don't distribute themselves across the land in a similar manner.
It bears looking at, anyhow.

DAVID W. MACDONALD, CHRISTOPHER NEWMAN, CHRISTINA D. BUESCHING, AND PAUL J. JOHNSON.
MALE-BIASED MOVEMENT IN A HIGH-DENSITY POPULATION OF THE EURASIAN BADGER (MELES MELES) J. Mammalogy. 89(5):1077–1086, 2008

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