A year ago, I decided to follow in the footsteps of biologist Richard M. Lehtinen, who, over the course of about eight and a half years, took walks through the neighborhoods around The College of Wooster, in Wooster, Ohio, and tallied each black and gray squirrel he spotted. The data he gathered were then used as part of a study he co-authored about the distribution of the black (melanistic) color morph of the eastern gray squirrel (Sciurus carolinensis) in the Great Lakes region. (Dispatches From the Neighborhood Watch: Using Citizen Science and Field Survey Data to Document Color Morph Frequency in Space and Time, Ecology and Evolution, Volume 10, 2020.) I discussed this and other research on melanistic eastern gray squirrels in a 2023 post.
Lehtinen walked seven set routes daily in his study area - one route was followed for the entire length of the survey period (July 22, 2010 - December 31, 2018), five others were followed for six years, and one was followed for five years. He recorded a total of 39,966 observations, 66 percent of which were of melanistic individuals. These data do not provide an unduplicated count of squirrels sighted, given that many of the same individual squirrels were sighted and tallied on more than a single day. In the aggregate, though, they do gauge the overall melanistic representation among eastern gray squirrels living in the area and along the routes followed.
It's a fairly simple research activity that generated data important for explaining possibly why distribution patterns for the melanistic squirrels are what they are. Lehtinen et al. found significant differences in the presence of these squirrels among the routes followed, and that these differences changed over time, some consistently in one direction or the other (increase or decrease in melanistic percentage), and others evincing little change. These walking survey data, coupled with additional population data assembled by the authors, showed the melanistic distribution in Wooster, across much of Ohio, and in the Great Lakes region to be decidedly mosaic in nature. In essence, different locations showed sometimes substantially different concentrations of black squirrels, a finding true for areas that were geographically close to each other with similar habitats which, the authors posited, exhibited few, if any, obvious distinctions in selective forces that might be in effect (e.g., presence of predators). Hence, it would appear that something other than natural selection might be driving the ebb and flow of these black squirrel populations: the authors suggested that chance in the form of genetic drift was at work.
Beginning in November, 2023, and continuing through December 29, 2024, I conducted my own citizen-science project in which I tallied each sighting I made of a black or gray squirrel during my daily walks through my own extended neighborhood. My primary goal was quite simple: estimate, as best I could, the overall representation of melanism in the squirrel population in this area. A secondary objective was to see how the black squirrels were distributed among several localities within the study area.
The area in question is part of Silver Spring, Maryland, a suburb of Washington, D.C.. The study area is shown roughly outlined below in a screenshot from a portion of the map of Silver Spring from Apple Maps. My walks fell within this area.
My effort differed from Lehtinen's in several significant ways. The primary and most important one is that mine covered a much shorter time period than his. I began tallying my squirrel observations on November 5, 2023, and, for this post, used data up through December 29, 2024, a period encompassing 421 days. I recorded one or more squirrel observations on 269 days or about 64 percent of the days in this period. Travel, nasty weather, health issues, and an absence of sightings account for the missing days. In contrast, Lehtinen's survey was conducted for about eight and a half years - from July 22, 2010 to December 31, 2018 - a period of 3,085 days. During that time period, he made 5,166 surveys, about 1.7 each available day.
Among other differences in the two neighborhood surveys is the larger area covered by mine. His routes traversed an area of 0.33 square kilometers or 0.13 square miles. My routes fell within an area of very roughly 0.73 square kilometers or 0.28 square miles.
In contrast to his effort, I did not have precisely prescribed routes. As described below, I traversed some streets, avenues, and parks in the neighborhood much more frequently than others. To minimize double counting of individual squirrels in any one day, the daily circuits I followed through my study area were nearly all loops. In the event my path did cover any of the ground twice on a particular day, only tallies that exceeded those from the initial passage were included.
Finally, I would stress that the topographical nature of the area being studied in each of these projects may differ. My study area included several relatively quiet, tree-lined residential roads, and three busy and noisy thoroughfares, among them, one undergoing substantial reconstruction in this period. Included in my study area were two large parks, one abutting a small woods and one bordered by woods and a creek, and one small, partly treed, park bounded by neighborhood roads. Lehtinen did not describe his study area beyond noting that "each of [his] routes followed streets in residential neighborhoods adjacent to the College of Wooster campus and Cornerstone Elementary School and are situated along parallel or perpendicular streets." (p. 1529) After looking at a satellite view of the presumed area in which he gathered his data, I believe his to be more consistently residential than mine if his routes lay only along an axis running directly from the College of Wooster to Cornerstone Elementary School. In that case, his study area, unlike mine, featured few, if any, natural or manmade barriers to squirrel movement among the routes.
The additional important issue of identifying color morphs should be addressed. As I noted in the previous post on these squirrels, analyses by biologist Helen R. McRobie of the genetic basis of the color morphs in the eastern gray squirrel found three distinct such morphs: gray, black, and brown-black. The genetic source of the last is a heterozygous version of the allele controlling color. (Helen Rebecca McRobie, The Genetic and Molecular Basis of Melanism in the Grey Squirrel (Sciurus carolinensis), Anglia Ruskin University, Doctoral thesis, September 2014; and Helen R. McRobie, et al, The Genetic Basis of Melanism in the Gray Squirrel (Sciurus carolinensis), Journal of Heredity,Volume 100, Number 6, 2009.) To my untrained eye, there is some fluidity in the color variations sported by individual eastern gray squirrels, fluidity that goes beyond the three distinctions McRobie identified. (See, for example, the discussion of several other color variations "in the vicinity of Washington, D.C.," by zoologists Richard W. Thorington, Jr., and Katie Ferrell in Squirrels: The Animal Answer Guide, 2006.) Nevertheless, despite detecting some slight variations in the coloring of a very few of the squirrels I observed, I had no issue distinguishing black from gray individuals in my study area. All my observations fell into of those two categories. Lehtinen et al. did not indicate that such color distinctions were an issue for them.
The data provided below are based on tallies of observations on days on which I actually walked the neighborhood and spotted at least one squirrel, whether gray or black. This means that the few days on which I walked but failed to spot a single squirrel are missing from these data. The aggregate data provided below regarding the overall representation of gray and black squirrels, as well as the data used to determine differences in melanistic presence in the several specific subsets of neighborhoods I covered (see below), are based on the entire data set of days when squirrels were tallied.
Aggregate Observations
Over the course of the entire study period, I recorded 1,565 observations of squirrels in my neighborhood. Of this total, 1,152 or 73.6% were of gray squirrels and 413 or 26.4% were sightings of melanistic individuals.
I have no prior baseline data for this or nearby areas with which to compare this melanistic representation in my neighborhood population. I take with the proverbial grain of salt the unsubstantiated and vague estimate offered up in a recent article in The Washington Post by freelance writer Ashley Stimpson. She stated that black squirrels make up "as much as half of the District's squirrel population." (Why We Celebrate Black Squirrels, But Think of Gray Ones as Pests, The Washington Post, October 8, 2024.) As a source for this estimate, she linked to a CBC News article which posited that "about half of Washington's squirrel population" is melanistic. This article offered no source for its estimate. (Matt Kwong, White House, Black Squirrels: How 8 'Desirous' Canadian Rodents Ended Up Overrunning Washington, CBC News, July 12, 2018.)
Melanism Representation in Several Distinct Localities of the Study Area
Although I did not follow fixed routes during my survey, there are at least five distinct portions of the study area in which I frequently walked. Some of these localities, though often adjacent to one another, have features potentially posing some physical boundaries (e.g., busy roads) that separate them. I describe each locality below.
Locality 1: Two tree-lined, relatively quiet residential roads whose houses share backyards. The roads are connected by a grassy walkway. This locality is separated from the others by a busy road, though squirrels do cross it, sometimes unsuccessfully (see data below on squirrels killed by traffic).
Locality 2: The open and partially wooded area of a park with several ball fields, separated from Locality 1 by a busy street. Part of the park abuts Localities 4 and 5.
Locality 3: The broad lawn with mature oak trees along the side of a church, bounded by a very busy road undergoing construction, the parking lot of a small cluster of stores, and a small apartment building. It seems quite isolated from the rest of the study area.
Locality 4: A triangular area bounded by tree-lined, relatively quiet residential roads, encompassing a small park with a soccer field and tennis court, partly contiguous to Localities 2 and 5.
Locality 5: Two, tree-lined, relatively quiet residential roads, that cross each other, partly contiguous to Localities 2 and 4.
My walks took me to each of these localities with varying frequencies. Of the 269 days on which I recorded any observations, 49.4 percent included observations made in Locality 1, 33.5 percent in Locality 2, 11.5 percent in Locality 3, 26.4% in Locality 4, and 19.7% in Locality 5. Nearly all of my daily walks ventured into more than one of these localities. I made observations in each locality in all seasons of the year.
The table below shows the aggregate counts of gray and black squirrel observations in each locality, and the percentage of the total observations in each locality that were of melanistic individuals.
I would note that the data Lehtinen et al. provided in a supplementary data table to their study ("wooster_summary_route_year") show that Lehtinen's Route 1 contributed significantly more data to his survey data than did his other routes. Approximately 44.0 percent of the total observations from his entire walking survey were accounted for by Route 1 and 46.9 percent of all melanistic observations came from that same route. No other route he followed came close. The nearest was Route 2 with 17.0 percent of total observations and 15.7 percent of melanistic observations. Nevertheless, given the high representation of melanistic squirrels across all of his routes (the range was from about 49 percent to over 73 percent with a median of approximately 69 percent and a mean of about 66 percent), the aggregate data he reported were not disproportionately influenced by his Route 1.
It would appear that my study showed melanistic squirrels to be distributed across these nearby localities in a mosaic pattern with some substantial differences, often much larger than those from Lehtinen's walking survey data. Even those localities abutting each other without physical barriers (Localities 2, 4, and 5) showed sizable differences.
It may be that, due to the physical barriers demarcating some of my localities, my findings are somewhat more reflective of the mosaic pattern that Lehtinen et al. found in the separate surveys they conducted of 55 urban areas in Ohio. The frequencies of melanistic squirrels across these urban areas was "extremely broad." (p.1534)
I don't know whether the habitats of my localities were similar in the selective forces that might be at work in each. A few observations are in order. Several of these localities are bounded by high-traffic roads presenting a threat to squirrels seeking passage across them (particularly Localities 1 and 3). Over the course of my survey, I counted five dead gray individuals on the road separating Localities 1 and 2. (I observed one other dead squirrel in this period, a melanistic squirrel inside Locality 1.) Locality 3 is effectively isolated by the road barriers. Human traffic and activities, including dog walking, were evident in all of these localities. Locality 2 also featured well-attended organized sports in the spring and fall. With regard to natural predators, there are hawks in the area and several were observed or heard in Localities 1, 2, 4, and 5 at various times in the year.
Although more limited in its duration and less structured in its execution than the walking survey by Lehtinen, I believe that the results of my citizen-science effort are not without their utility, offering some support for the findings Lehtinen et al. described in their 2020 study of melanistic gray squirrels in the Great Lakes region.
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