Squirrel Mapper

Measuring evolution in action
in urban and rural landscapes.

Exploring the data: the geography of squirrel morphs

The question that we ultimately want to answer is why do black squirrels and gray squirrels occur at different frequencies in different environments? First we have to find out if they actually do occur at different frequencies than what we would normally expect.

We can do this is by taking sample observations of squirrels to make inferences or conclusions about the nature of the entire population. This helps us answer questions without having to look at every single squirrel in the population. Our samples are the records that you have reported of your squirrel observations. We will pool all these samples together to see if different factors affect the frequencies of the two morphs. (see technical FAQ)

Are black squirrels subject to natural selection?

If the squirrel population did not experience any selective pressures, we would expect to find the two morphs in proportion to their genetic frequency (see evolution FAQ). Since the black allele is dominant, we would expect to find black squirrels in a 3:1 ratio to gray squirrels. Let's look at the samples reported to SquirrelMapper across the native range of Sciurus carolinensis in recent times. Let's tally the observed morph frequency and test if it is different from expected morph frequency.

Are black squirrels more common in urban areas?

Urban areas have low hunting pressures because you generally can't hunt in cities. And urban areas may have a visual environment more similar to old growth forest (both have lots of dark shade and more conifers than second growth forest). So we might expect that frequencies of black squirrels to be higher in urban areas than rural areas. Based on the latitude and longitude of the squirrels you have reported we have linked each observation to a global land use dataset to determine if each squirrel occurs in an urban area or not. This lets us contrast squirrel morph frequency between urban and rural areas. Let's tally the observed morph frequency and test if it varies between urban and rural areas.

Do black squirrels occur more in colder regions?

First, let's look at how the frequencies change by broad geographical areas, such as the southern part of the range versus the northern part of the range. Due to black squirrels' greater heat retaining capacity in colder conditions (they have thicker fur and a higher basal metabolic rate), we might predict that black squirrels will be more common farther north in the native range of the gray squirrel (i.e., northern U.S. and Canada). Is this the case? Based on the latitude of the squirrels you have reported we can breakdown squirrel morph frequency by latitude. Recall that higher latitudes are more northern and hence colder. Let's tally the observed morph frequency and test if it varies among latitudes.

What happens when gray squirrels move to new regions?

Next, let's look at areas in the gray squirrel's range where it has been introduced versus areas where it occurs naturally. Sciurus carolinensis has been introduced mainly to parts of Europe and western North America but also to Italy and South Africa. We might expect that frequencies in the locations where the squirrels have been introduced will have a higher black to gray ratio because black squirrels have been released from the natural predators present in their original ranges. Based on the latitude and longitude of the squirrels you have reported we have linked each observation to whether it falls within the native (eastern North America) or introduced range (for example, western North America, United Kingdom, South Africa, and Italy) of the species. Let's test if observed morph frequency differs among native and introduced parts of the gray squirrel's range.

Future predictions

So what is the future of the black morph of the gray squirrel? Will it become more or less common? Consider three important trends:

  • Urbanization is spreading around the World.
  • Climates are generally warming.
  • The gray squirrel is not only being introduced to new areas, it is spreading on its own from areas where it has already been introduced.

So will the black morph regain its original numbers? Or will it forever stay rare and restricted to certain areas?

Do you want to learn more?

Then check out our detailed dataset for Onondaga County, New York where you can test more refined hypotheses.

Now let's look at squirrel morphs on the scale of Onondaga County, New York. Of all places, why Onondaga County? Onondaga County is located in central New York State. A large city — Syracuse — occurs in the middle of it. A mix of farmlands, forests, and villages surround it. You can't hunt in the city or the villages. Squirrels abound throughout the county. Because SquirrelMapper operations are based at SUNY-ESF in Syracuse we have recorded many squirrel observations in the area. Also, we have access to more refined environmental data for the county (where you can and can not hunt, for example) than we do range-wide. This lets us test more detailed predictions about the occurrence of black versus gray squirrels.

Are black squirrels in Onondaga County subject to natural selection?

If the squirrel population did not experience any selective pressures in Onondaga County, we would expect to find the two morphs in proportion to their genetic frequency (see evolution FAQ). Since the black morph is genetically dominant, we would expect to find black squirrels in a 3:1 ratio to gray squirrels. Let's look at the samples that we have collected across the whole range of Sciurus carolinensis in relation to those in Onondaga County. Let's tally observed morph frequency and test if it differs from expected morph frequency.

The "urban effect"

One possible selective pressure in Onondaga County is the environmental difference between urban and rural environments. We might predict that black squirrels are more frequent in urban areas because of the darker visual environment and generally lower human predation pressures (no hunting). Based on the latitude and longitude of the squirrel observations reported in Onondaga we have assigned each to whether it occurs in an urban or rural area. Let's evaluate if squirrel morph frequency differs between urban and rural areas.

The "hunting effect"

Another selective pressure in Onondaga County is hunting. We might predict that gray squirrels will be more frequent where hunting occurs because hunting generally happens in areas where black squirrels are more conspicuous and therefore more likely to be shot. Based on the latitude and longitude of the squirrel observations that you reported we have assigned each to whether it occurs in hunted or unhunted area. Let's evaluate if squirrel morph frequency differs between hunted and unhunted areas.

Interactions between land use and hunting

Land use and hunting activity could interact with each other in complicated ways. For example, there are extensive areas of secondary forest in urban areas, in parks and cemeteries, where you can't hunt, thus controlling for hunting effect and focusing on urban effect. This is where it gets really interesting! Let's contrast morph frequencies in urban areas that are developed or not developed where no hunting is allowed in either... is there still a stand-alone urban effect?

Are black squirrels more or less common in Onondaga County than elsewhere in their native range?

Just as background, let's look at the overall frequencies of the two morphs to see if they differ between Onondaga County and elsewhere in the species native range (eastern North America).

Future predictions

So what do you predict for the future of the black morph of the gray squirrel? Will it become more or less common in Onondaga County? Urbanization is spreading outward from the city boundaries. And people are hunting less in the countryside. Based on these trends and the analysis you have just completed, what do you predict for the future evolution of morph coloration in the gray squirrel in Onondaga County? Will it regain its former abundance?