Evolution & Ecology, LSTD 3443

By Brian Brus

Professor Bryan Coppedge
University of Oklahoma

May 9, 2013

I certify that I have read A Student’s Guide to Academic Integrity at the University of Oklahoma, and this paper is an original paper composed by me for this course. Except where properly cited and attributed, it has not been copied or closely reworded from any other source and has not been submitted as a whole, or in part, for credit in any other course at OU or any other educational institution. It has not been created or submitted for any other purpose such as a job assignment at my workplace or any other agency.

SQUIRREL TOURISM 2

Abstract

In the face of such strong, persistent, evolutionary forces of natural selection and sexual selection, what are the limits to which human communities can establish and maintain novel genetic variants such as albino or white squirrel populations? Several communities in North America have been trying to do just that for tourism reasons, with mixed outcomes.

Keywords: squirrel, tourism, evolution.

Evolutionary Limits of Squirrel Tourism

Novelty drives interest, and some of the most obvious novelties in nature are organisms that diverge from their species’ structural norm. Ecology and evolution has taught us that even the tiniest genetic variance from parent to offspring can lead to a wide range of expression, but most would be too inconsequential for the casual observer to notice; alternately, toxic mutations can lead to a foreshortened lifespan when the organism is badly mismatched for its environment, and those individuals pass by without our notice entirely. But suppose the animal in question had a blatantly obvious difference that still allowed it to function normally, such as a different color? An black-as-midnight tiger, for example, or piebald python? Or white squirrel?

If a community were able to gather a large number of such unusual individuals, the collection could easily serve as a tourist attraction and economic enhancement — travelers have proven they are attracted by such esoterica as historic writers’ homes and large balls of twine; a white squirrel colony is no less viable. Several North American towns have tried to do just that, such as Marionville, Mo.; Kenton, Tenn.; Northern Keys, Fla.; Charlotte, N.C.; and Exeter in Ontario, Canada, to name just a few (Vinzant). Marionville, for example, with a small population of just under 2,500, boasts on its municipal website, “Visitors come to Marionville in hopes of catching a glimpse of our white squirrels. The totally white creatures roam the yards and are protected by city law. The squirrels have been here, reportedly since the late 1800 & 1939;s” (City of Marionville). And the 1,300 residents of Kenton hold an annual white squirrel festival and 5- kilometer race to draw attention to the community (Groves). Squirrel expert D. MacClintock says most adult tree squirrels of the rodent family Sciuridae have fur in grey or red hues, allowing them to blend in their shadowy tree environs (1970). However, white anomalies can occur for several different reasons, such as leucism, a condition caused by a recessive allele that reduces the pigmentation in all types of an animal’s tissues (Cieslak, Reissmann, Hofreiter & ; Ludwig, 2011). A leucistic animal might still display ghostlike markings, suggesting the species’ typical patterns — a leucistic zebra could seem totally white, but careful inspection will reveal pale stripes. Patches of leucism produce a piebald surface.

Albinism, on the other hand, also due to recessive gene alleles, leads to the absence of melanin, a derivative of the tyrosine amino acid (Thompson & Thompson). Other unaffected pigments might still be present, which can yield white animals with highlight colors, such as a yellowish but otherwise white snake. The condition of melanism is just the opposite, involving the production of even more dark-colored pigments than normal. The community of Brevard boasts of white squirrels with dark eyes that reveal the presence of pigmentation; a dark patch on the head suggests they might not be truly leucistic. Olney likewise is proud of its pink-eyed albino squirrels. Those pink eyes are worth noting as more than merely a cosmetic difference, as the lack of melanin typically is associated with vision problems in a wide range of animals. This is because tyrosinase is a key enzyme in the production of retinal melanin, so an albino individual is likely to suffer from reduced visual acuity (Esteve & Jeffery, 1998). Good eyesight is obviously important to survival in the wild — foraging, reactive defense and mating behaviors come to mind immediately — so albinism would seem to be a less successful adaptation for most animals, unless the condition was also tied to a benefit of some sort. It could be argued that subjective “cuteness” might be such a benefit if pink-eyed, white squirrels receive more free food or hunting protections from human admirers than their darker neighbors.

Rather than wait for white squirrels to show up of their own accord, towns have imported animals, effectively injecting a specific genetic factor into the local squirrel families. Brevard’s white squirrels, for example, are believed to have been founded by the family of Barbara Mull Lang in the 1950s when a pair were passed from person to person after escaping from the circus. Lang said she became their caretaker before they were inadvertently set free near town (2005). The city of Brevard now has an ordinance against harming or trapping white squirrels, as does Olney. In the latter town, it’s widely held that a local hunter circa 1902 rescued two infant orphans and/or a breeder brought a pair in from nearby Sumner; documentation supports both accounts (City of Olney). Olney residents have tried to support their white squirrels’ survivability even further, actually giving the animals right of way on streets and requiring that
local cats be kept under control so that they don’t act on natural predator instincts against squirrels as prey.

These efforts have produced mixed results, leading one to wonder to what degree white squirrel variants could thrive without human intervention; even less likely seems to be the possibility of a fully distinct albino species arising. Brevard for several years has held a survey of its tree-dwelling residents via the White Squirrel Research Institute to track the health of the town’s largest tourist attraction. Accordingto the summary of the latest published account,  “Brevard’s white squirrels would seem to be doing well” (City of Brevard, 2011). In 2012, only six of a planned 35 sectors were counted because of administrative problems in the program, but those numbers also seemed to indicate strength of whites among the more commonly-tinted Eastern Grey Squirrel (Sciurus carolinensis) at approximately 44.5 percent of the total. Since 1997, the white squirrel ratio has dropped as low as 21.6 percent, but as the institute reports, “For whatever reason, the percent white is stable, i.e., it rose dramatically from almost zero in 1951 to approximately one out of four by the 1990s but has reached equilibrium” (City of Brevard). In addition to engendering
human favor, it might be inferred that white coloration conveys some benefit to Brevard squirrels.

The Olney colony, on the other hand, doesn’t seem to be faring as well. In the period from 1977 to 1986, the town’s squirrel count revealed that white squirrels counted for as little as 19.6 percent of the total local carolinensis population and no higher than 25.2 percent (Stencel & Ghent, 1987). As the local newspaper reported more recently, at the end of 2012 the white-to-gray ratio had declined to as few as one-to-eleven, or about 8 percent, compared with a ratio of one-to-nine, or about 10 percent, the previous year (Wells, 2012).

The city clerk and count coordinator at the time attributed the decline to drought and predatory cats, common sense assumptions without further data to weigh. Olney and Brevard are distinctly different environments more than 500 miles apart, or an eight-hour drive across Kentucky, so it would be unreasonable to directly compare them without considerations of weather patterns, tree nut yield and other such factors; likewise, variables of each municipality’s
squirrel management policies are numerous.

White variants of dark-colored animals would seem to be at a disadvantage in any case, as they tend to stand out in stark contrast to their habitat instead of benefitting from camouflage, snowy seasons excepted. At the risk of setting aside hard evidence in favor of hypotheticals, one can imagine a hawk choosing between two equally tasty squirrels wrestling with nuts at the tops of identical trees — which target would be easier to visually track? MacClintock numbers among a squirrel’s enemies foxes, bobcats, weasels, snakes, raccoons, cats, owls and hawks. (1970). M.
Steele and J. Koprowski explain that when a typical squirrel forages, a dark pelage hides it from hunting eyes on high (2001). Further, some fox squirrel colonies in the Southeast are characterized by even more dark fur, the researchers note, with higher rates of occurrence correlating with environments that have experienced more forest fires, suggesting a camouflage benefit against charred tree trunks. Nature would seem to be selecting for some level of
concealment. But as for the lighter-hued squirrels, other evidence suggests just the opposite outcome.

As S. Brewer, M. Lucas, J. Mugnano, J. Peles and G. Barrett conclude their breeding experiments with voles, “We hypothesize that albinism is disadvantageous … where conspicuousness of coat color likely results in increased rates of predation” (Brewer et al, 1993).

D. Kaufman and C. Wagner studied the same thing much earlier, noting the lower survivability of white house mouse under the watchful eyes of predatory owls (1973). A squirrel’s bushy tail serves several functions, such as aiding in balance during climbing along thin branches, providing air resistance like a parachute in falls, and twitching as a
distraction to predators. MacClintock also notes that squirrels snuggle into their tails for additional warmth on cold days and hide under them for shade on sunny days (1970). On that point, we should consider whether un-tinted (and un-tented) squirrels are at a disadvantage for lack of melanin, which blocks damaging ultraviolet radiation. As the National Organization for Albinism and Hypopigmentation also notes, the global latitude of an environment can also make a difference in exposure rates, with more UV radiation closer to the equator. So it might be argued that albino squirrels face more risk of developing skin damage, but that’s merely a logical assumption here, lacking supporting data. Appropriately enough, the squirrel’s Latin nomenclature is derived from the use of its tail: sciurus is composed of skia, meaning shadow, and oura, meaning tail, literally a creature that sits in the shadow of its tail. (MacClintock, 1970).

That brings us back to a particularly striking difference between the Brevard whites and Olney whites that should be given more consideration: albinism, not just color. As noted earlier, the condition of albinism tends to be related to more ocular development problems. Researchers Esteve and Jeffery noted that in the ganglion cell layer of a squirrel’s eye, the pigment-deficient variants expressed cell densities less than 5 percent difference from those with normal pigment. The researcher wrote, “Squirrels are the only known albino mammal to survive successfully in the wild, reinforcing the notion that their visual deficits are minor.” (1998). That may be the case, but evolution studies in other animals have shown that even the slightest change can be a matter of mortal importance, even in dining habits — a centimeter’s difference in the beaks of C. Darwin’s finches in the Galapagos Islands is a good example, as ever-so-slightly stronger beaks provide access to seeds that weaker beaks cannot crack (Weiner, 1994). In competition and survival in the wild, a few more seeds or nuts can make a world of difference in whether a creature survives to the next breeding season to pass on its genes. Steele and Koprowski provide an excellent overview of squirrels’ diets and a cost-to-benefit analysis of each food source (2001). They found that although squirrels prefer to eat food dense with
nutritional value, those materials also carried a heavier risk of predation — i.e. larger pine cones or heavier nuts requiring more effort and expenditure of energy. This is, at its core, very similar to the trade-offs that Darwin discovered with finches, as well as other researchers who followed in his Galapagos footsteps, that better seeds required greater inputs. Steele and Kaprowski also found that squirrels that sensed higher risk tended to move toward less nutritious, easier to obtain food. What this means in our discussion of albino squirrels is a two-fold impact (at least): First, every foraging effort presents a slightly higher risk of predation for squirrels that are easier to
spot by predators, which will probably affect their feeding options under the cost-to-benefit equation; and second, even if all other factors were equal, squirrels with vision deficiencies may very well have less time to react to predators during dinner and throughout the day.

It would be disingenuous to ignore this paper’s earlier reference to camouflage in winter, as it might seem obvious that a white coat provides greater protection from predators against a snowy background. However, as MacClintock notes, a typical gray squirrel’s survival during the coldest months depends more on the animal’s body fat and nut caches, both of which are accumulated during foraging in the non-snowy seasons (1970). In other words, that particular winter benefit would not seem to outweigh a white squirrel’s summer penalty. Further, as Steele and Koprowski explain, dark melanics are common in the northern edges of grey squirrels’ ranges, especially in Ontario, Canada. There, black morphologies withhold more heat energy in the cold, they said (2001).

So it would seem that white squirrels have the deck stacked against them; or rather, the deck is stacked in exactly the way that evolution deems successful for squirrels with more melanin. In addressing the apparent success of whites among greys in Brevard, White Squirrel Research Institute scientists reported, “The fact that two variants have come to reach some kind of non-trivial equilibrium suggest that there is a trade off.  That is, there must be both a cost and
a benefit of being white that off set each other at the 25 percent level” (City of Brevard). That trade off appears to be missing in Olney.

Still, though, as a recessive gene trait, the potential for Olney’s albino count to rise in successive generations is still a possibility if the right pairings occur. This depends greatly on whether other squirrels are willing to mate with the white variants — i.e. sexual selection, mentioned earlier — and whether the alleles involved in albinism cause other problems as well. Brewer et al. found that in breedings of squirrels’ distant cousins, albino voles, the inheritance or recessive albinism followed expected Mendelian ratios of expression in their offspring, suggesting that the trait could be passed for generations before resurfacing. In the meantime, researchers Stencel and Ghent suggested that humans should not react punitively to grays’ ascendence over whites in Olney. They concluded that killing normally colored squirrels to give whites a competitive advantage would be counter-productive to the vitality of the genetic pool and albino propagation because the fitness of the genotypes rely on each other (2011). As the researcher noted, a better management route involves supporting all the squirrels in Olney, regardless of their coloration.

In short conclusion, although it is laudable to celebrate nature’s diversity and turn to genetics as a low-impact economic input to help a community’s tourism revenues, there appears to be only so much we can do to counter natural selective forces. Grey squirrels have evolved the color they are because that’s the color which has proven to be most successful. Unless creatures can adapt or find their own niche in a new environment, offsetting drawbacks with genetic benefits, the humans who transplanted them must continue to be active participants in the survival of the less-than-fittest.

References

Cieslak, M., Reissmann, M., Hofreiter, M., & Ludwig, A. (2011, November). Colours of
domestication. Biological Reviews, 86(4), 885-899. Retrieved April 3, 2013, from
http://libraries.ou.edu/access.aspx?url=http://search.ebscohost.com/login.aspx?direct=tru
e&db=edswsc&AN=000296475300008&site=eds-live .

Brewer, S., Lucas, M., Mugnano, J., Peles, J., & Barrett, G. (1993, October). Inheritance of
albinism in the meadow vole (Microtus pennsylvanicus). American Midland Naturalist,
130(2), 393-396. Accessed at

http://libraries.ou.edu/access.aspx?url=http://search.ebscohost.com/login.aspx?direct=true&db=e
dswsc&AN=A1993ME26400016&site=eds-live

City of Brevard (2011). Compare population fluctuations and percent of the white variant.
Welcome to Heart of Brevard. Retrieved April 1, 2013, from
http://www.brevardnc.org/whitesquirrelinstitute/fluctuations.htm. Article now resides on https://whitesquirrelinstitute.org/squirrel-count/15-year-summary-of-squirrel-count

City of Brevard (2011). Summary of the 2011 squirrel count. Welcome to Heart of Brevard.
Retrieved March 28, 2013, from original posted on http://brevardnc.org/summary-of-the-2011-squirrel-
count . Article now resides on https://whitesquirrelinstitute.org/squirrel-count/summary-of-the-2011-squirrel-count

City of Marionville (n.d.) About us. City of Marionville MO. Retrieved March 22, 2013, from
http://www.marionvillemo.com/html/about_us.html .

City of Olney (n.d.) White squirrel history. Welcome to City of Olney. Retrieved March 22, 2013,
from http://www.ci.olney.il.us/visitors/white_squirrels/white_squirrel_history.php .

Esteve, J., & Jeffery, G. (1998). Reduced retinal deficits in an albino mammal with a cone rich
retina: a study of the ganglion cell layer at the area centralis of pigmented and albino grey
squirrels. Vision Research, 38(6), 937-940. Retrieved March 29, 2013, from
http://dx.doi.org/10.1016/S0042-6989(97)00229-0 .

Groves, D. (n.d.). Events. West Kentucky Star. Retrieved March 23, 2013, from
http://www.westkentuckystar.com/Events/Northwestern-Tennessee/White-Squirrel-
Festival-in-Kenton-TN.aspx .

Kaufman, D. W., & Wagner, C. (1973). Differential Survival of White and Agouti Mus musculus
under Natural Conditions. Journal Of Mammalogy, (1), 281. doi:10.2307/1378896 ….
from http://libraries.ou.edu/access.aspx?url=http://search.ebscohost.com/login.aspx?direct=tru
e&db=edsjls&AN=edsjls.10.2307.1378896&site=eds-live

Lang, B. M. (2005, June 6). The true story about the white squirrels. Welcome to Heart of
Brevard. Retrieved March 20, 2013, from
http://brevardnc.org/whitesquirrelinstitute/BarbaraMullLang.htm .

MacClintock, D. (1970). Squirrels of North America. New York: Van Nostrand Reinhold.
Steele, M. A., & Koprowski, J. L. (2001). North American tree squirrels. Washington:
Smithsonian Institution Press.
Stencel, J. E., & Ghent, A. W. (1987). Analyses of annual surveys of white and gray squirrels
(Sciurus carolinensis) in Olney, Illinois, 1977-1986. American Midland Naturalist, (2),
251. doi:10.2307/2425782. Retrieved April 2, 2013, from
http://www.jstor.org/stable/2425782 .

Thompson, R., & Thompson, K. (n.d.). What is albinism? National Organization for Albinism
and Hypopigmentation. Retrieved March 25, 2013, from
http://www.albinism.org/publications/what_is_albinism.html .

Vinzant, C. (n.d.). Animaltourism.com. Animal Tourism. Retrieved March 21, 2013, from
http://animaltourism.com/animals/squirrel.php .

Weiner, J. (1994). The beak of the finch: a story of evolution in our time. New York: Knopf.

Wells, L. (2012, November 29). Drought, cats blamed for decline in Olney's white squirrel
population. Evansville Courier & Press. Retrieved April 2, 2013, from
http://www.courierpress.com/news/2012/nov/29/no-headline—white_squirrels .

We would like to thank Brian Brus for letting us post his article Evolution of Squirrel Tourism on our website.

Great read.