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