Black squirrels are mutants, but not thugs

The first time I saw black squirrels was while sitting in Manhattan’s Union Square Park. Naturally, I assumed I was just looking at a particularly sooty specimen of a more commonly seen New York City rodent – the eastern gray squirrel. But identically dark-coated squirrels manifested during subsequent lunch hours until I finally acknowledged that, unlike cats at night, not all squirrels are gray.

Black is the new gray

Black squirrels are melanistic versions of the species Sciurus carolinensis – the eastern gray squirrel. Aside from their dramatic pigmentation, they’re just like the fairer-haired members of their species. Their lifestyles are probably similar to the squirrels in your own neighborhood.* They live in trees, eat nuts, and make those squirrel sounds at you when you’re leaving for work in the morning. They are “scatter-hoarders,” which means they bury food they wish to store for later in numerous locations rather than in one or two well-guarded caches. Sometime they bury their leftovers in your outdoor flower pots, uprooting a perfectly good basil plant in the process. For this and similar offenses, squirrels are often viewed as pests by humans. As with the eastern grays, black squirrels occupy portions of the eastern and midwestern United States, as well as southeastern areas of Canada. And at some time during the past century, the black squirrel managed to find its way to the British Isles.

Controversy from across the pond

Great Britain has its own native squirrel, the red squirrel (Sciurus vulgaris). They were not pleased when our gauche American gray squirrels escaped into their British countryside (after being introduced in captivity by some thrill-seeking noble) and largely out-competed the dainty red squirrel. In recent years, black squirrels, whose presence in the wilds of the UK also resulted from captive-living mishaps, have grown in population and now threaten to outnumber even the country’s detested gray squirrels.

Much indignant speculation as to the cause of the demographic shift has ensued in the British press. A 2008 article in the Daily Mail claimed that the “mutant pack of black squirrels” were nudging out grays as a result of a higher levels of testosterone conferred by the pigment mutation. (I’ve yet to find any scientific documentation of hormonal variations between the differently colored squirrels.) Black squirrels are often described as being more aggressive, though this is based on anecdotal observation, and perhaps a dash of xenophobia. The less sensationalistic BBC wrote that black squirrels were increasing in number simply because the mutant gene responsible for their pigmentation was dominant over the wild-type allele. This is partially accurate, or perhaps “incompletely” accurate.

Incomplete dominance

In 2009, British scientists finally rounded up some squirrels and attempted to sort out what was going on in terms of both genotype (the actual genes) and phenotype (outward appearance of the animal). They concluded that there were three, not two, coat variations in Sciurus carolinensis. In addition to the black and gray squirrels there exists an in-between phenotype – a brown-black squirrel with an orange underbelly (as opposed to the white underbelly of the original gray squirrel). Genetic testing showed these phenotypes to correspond exactly to differences in a single pair of alleles.† Using E+ to indicate the wild-type allele and Eb for the melanic allele, the authors demonstrated that the mutant version was incompletely dominant over the wild-type. That is, squirrels with two E+ alleles had the familiar eastern gray coloring, squirrels with two Eb alleles were entirely black, and those with one of each allele exhibited the mixed, brown-black phenotype. Complete dominance of the mutant gene would not yield this in-between squirrel variant. If the Eb allele were dominant, even the heterozygous animal (E+ Eb) would be all black.

Why all the black squirrels then?

If black squirrels don’t posses a quickly-spreading dominant gene or any demonstrated hormonal or behavioral advantage, then what might explain their rising numbers in the UK? Again Britain’s favorite tabloid has a thought: sexual selection. Surely stirring discomfort amongst pale English readers, the Daily Mail suggested that female gray squirrels prefer to mate with black squirrels. However, as with claims of the blacks’ bullying barbarism, rumors of their greater success with the ladies is thus far only hearsay. If one counts the brown-black mixed squirrels as black squirrels, then the combined animals could eventually outnumber grey squirrels by gene prevalence alone. But, of course, there are always other factors to consider.‡ Even the folks who sequenced all that squirrel DNA don’t rule out the possibility that additional genes may be involved in making grey squirrels black. It’s frontier science. If you really want to know the answer, you’ll need to get yourself some squirrels, a microscope, and a bunch of expensive DNA materials. Let me know if you find anything interesting.

* If you live in Austin like me, you’re probably in the company of fox squirrels (Sciurus niger). New Yorkers are more likely to see Eastern gray squirrels. And the rest of you will have to do your own research if you want to learn which rodents inhabit your vicinity.

† A quick introductory genetics recap: Alleles are different versions of a gene, each occupying the same spot (locus) on one of two chromosomes. One chromosome (and thus one allele for the gene of interest) is inherited from each parent.

‡ For instance, it has been suggested that the black squirrels’ dark coat (which absorbs more heat from sunlight) might be better suited for cold climates than the lighter fur of the traditional Eastern gray squirrel.

More Lifeforms from Alex Reshanov:
Banded Sea Kraits are swimming, tree-climbing, venomous beauties
Tasmanian Devils are not quiet, cuddly vegetarians

March 3, 2011

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