Ur­ban Wildlife

On ge­netic dis­rup­tion, frag­mented habi­tat and why there is al­ways a white squir­rel in Toronto’s Trin­ity Bell­woods Park

Canadian Wildlife - - CONTENTS - By Matthew Church

On ge­netic dis­rup­tion, frag­mented habi­tat and why there is al­ways a white squir­rel in Toronto’s Trin­ity Bell­woods Park

There is a squir­rel in my lo­cal park that is fa­mous. Its like­ness ap­pears on nov­elty items in lo­cal kitsch shops, on T-shirts and in nu­mer­ous web­sites. This squir­rel has in­spired the name of a hip coffee shop, and it has its own Twit­ter ac­count. It is the “white squir­rel of Trin­ity Bell­woods.” It is beloved and much pho­tographed. A lo­cal pa­per re­cently de­scribed it as “mythic.”

Hardly. Hav­ing lived in this neigh­bour­hood for nearly 30 years, I have seen it count­less times. There’s noth­ing mythic about it. And more im­por­tantly, given that squir­rels gen­er­ally don’t live longer than six years (and of­ten a lot less) in the ur­ban wild, it isn’t an it; it’s a they. Over three decades, I have en­coun­tered maybe 10 gen­er­a­tions, per­haps more. And as cap­ti­vat­ing a vi­sion as they tend to be for passersby, they are more in­ter­est­ing still for il­lus­trat­ing sev­eral fas­ci­nat­ing re­al­i­ties of ur­ban wildlife.

The first thing to know is these likely are just plain old eastern gray squir­rels, Sci­u­rus car­o­li­nen­sis, the dom­i­nant species in eastern North Amer­ica. And they are not al­bi­nos — if it were al­binism, their eyes would be pink, the colour of blood vis­i­ble through the ves­sels and skin due to an ab­sence of a pig­ment called melanin. They would be nearly blind in bright light and doomed to a short ex­is­tence. No, this fa­mous squir­rel, with its dark eyes, is leu­cic. Re­lated to al­binism in that it is also caused by ge­netic dis­rup­tion of pig­men­ta­tion but some­what more com­mon, leu­cism is the loss of sev­eral dif­fer­ent types of pig­ment caus­ing white or pale feath­ers, fur, skin and scales (but not pink eyes). It is com­mon in nu­mer­ous species, among rep­tiles, birds and a few mam­mals. In its par­tial, var­ie­gated form, it gives us piebald horses, dogs, pi­geons and more. An ex­pres­sion of a ge­netic dis­po­si­tion that usu­ally goes un­ex­pressed, leu­cism oc­ca­sion­ally ap­pears in a given pop­u­la­tion and then dis­ap­pears again within a gen­er­a­tion or two, the gene lost and over­whelmed in the ever-widen­ing ge­netic field.

A re­cent study dis­cov­ered eight un­known in­stances of leu­cism among small ro­dents in south­west­ern Ecuador. Pre­vi­ously un­heard of in the re­gion, re­searchers found per­sis­tent, even ris­ing rates of leu­cism in the Cordillera de Chilla, de­scribed as “an im­por­tant bridge for the pas­sage of wild an­i­mals be­tween the Pa­cific slope, the An­des and the Ama­zon.” Or at least it used to be. Now, be­cause of an in­flux of hu­mans, and road-build­ing, ranch­ing and farm­ing through­out the area, the bridge has been breached and nat­u­ral habi­tats have been par­celled, lim­ited and iso­lated. In the frag­ments of habi­tat that re­main, species are re­duced to fi­nite ar­eas, and there’s lit­tle op­por­tu­nity for mi­gra­tion and dis­per­sal. Ge­netic va­ri­ety rapidly dwin­dles. In­ter­breed­ing in­creases and anom­alies be­come more fre­quent. Leu­cism man­i­fests.

Is it the same phe­nom­e­non that has been oc­cur­ring across the street in our park? For ur­ban wildlife, habi­tat frag­men­ta­tion is a fact of life. En­tire pop­u­la­tions of cer­tain species are un­able to evolve nat­u­rally: hemmed in by busy streets, rail lines, bridges, ex­press­ways and other im­pass­able bar­ri­ers, there is not the usual in­flux and out­flow of ge­netic vari­abil­ity. Dis­tinct ge­netic colonies can emerge.

In ad­di­tion to shrink­ing pop­u­la­tions, this leads to in­ter­est­ing lo­ca­tion-spe­cific traits, like leu­cism. Oc­ca­sion­ally, in ar­eas that are con­ducive, ge­netic drift sta­bi­lizes to form a per­ma­nent gene pool with cer­tain on­go­ing vari­ants. Trin­ity Bell­woods Park, bounded by busy streets on each side, is suf­fi­ciently self-con­tained: with nearly 15 hectares of trees, wa­ter, co­pi­ous nat­u­ral and an­thro­pogenic food sources, it is squir­rel par­adise.

Their ex­is­tence in the city raises an in­ter­est­ing is­sue. Does leu­cism af­fect their lives pos­i­tively or neg­a­tively? In the wild, their bright colour­ing would present an el­e­vated ex­is­ten­tial risk due to height­ened vis­i­bil­ity and there­fore pre­da­tion. In the ur­ban con­text, where the num­ber of nat­u­ral preda­tors is re­duced, vis­i­bil­ity might not be such a big is­sue and may even be ad­van­ta­geous. First, bright white squir­rels are less likely to be run over when they do ven­ture into streets. Fur­ther, their strik­ing ap­pear­ance means that hu­mans are more likely to feed them, of­fer­ing a dis­tinct leg up in the lo­cal strug­gle for sur­vival. Go­ing even fur­ther, given that ur­ban an­i­mals gen­er­ally tend to be more ac­tive in win­ter­time than their ru­ral cousins (due to higher am­bi­ent tem­per­a­tures, year-round abun­dance of food, even dis­rup­tive ef­fects of noise), be­ing with­out colour­ing of­fers ideal win­ter cam­ou­flage from their nat­u­ral neme­ses and preda­tors.

There is no way of know­ing quite how white squir­rels’ lives are af­fected by the ab­sence of pig­men­ta­tion — though we can safely sup­pose that fame doesn’t mat­ter much to them. Per­haps get­ting a few ex­tra hand­outs and avoid­ing be­ing flat­tened in traf­fic has en­abled them to per­sist. Not mythic per­haps but the stuff of leg­ends still.

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