Another Year’s Squirrel Data

by Carl Strang

Fox and gray squirrels are familiar animals in northeast Illinois. They are easy to tell apart. Gray squirrels have pale gray belly fur, and their tails are mainly gray toned.

Gray squirrels also are smaller than fox squirrels.

Fox squirrels have reddish brown fur beneath, and their tails are reddish and black.

The orange pigment also occurs in a fox squirrel’s bones, and the lenses of its eyes. Fox squirrels have built-in sunglasses.

As I mentioned a year ago, fox and gray squirrels are adapted to different habitats. Gray squirrels are forest animals, while fox squirrels live in the more open savannas. Both species occur at Mayslake Forest Preserve, where the savannas are high in quality but the forests are poor. By “poor” I mean they have lots of buckthorn, very little diversity of herbaceous plants, and the trees are mainly young members of weedy (fast-growing, relatively short lived) species.

I have been keeping records of where I see fox and gray squirrels at Mayslake. As there is more savanna than forest habitat there, I expect to see more fox squirrels. To what extent do the two squirrel species stick to their expected habitats? Here are the results of two years’ study. Observations total 853 for fox squirrels, 209 for gray squirrels. As expected, the former species appears to be much more abundant.

If the two species had no preference, i.e. occurred in forest and savanna areas in proportion to those areas’ sizes, I would expect 528 of the 853 fox squirrel observations to be in savanna, 325 in forest. Instead the numbers are 726 and 127, respectively. This shows the expected strong preference for savanna (for what it’s worth, the χ 2 test statistic value is a whopping 194.8).

If gray squirrels were showing no habitat preference, I would expect 129 observations in savanna, 80 in forest. The observed numbers were 160 and 49, respectively. This likewise reveals a preference for the superior savanna habitat (chi-squared = 19.5, also plenty large).

The preference for savanna does not appear to be quite so strong in the gray squirrel, however. Is there a difference between the species? If I base my expectations for gray squirrels on the ratios seen in fox squirrels, I should have seen 178 gray squirrels in savanna, 31 in forest. Again, the observed numbers were 160 and 49. Here the χ 2 = 12.3, large enough to support a difference between the species’ degree of preference. I will continue to monitor these two species, to see if there are any changes over the years.

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1 Comment

  1. April 6, 2011 at 5:58 am

    […] of places animals need for food, shelter, all the requirements for survival. Last week I presented summaries of habitat data for fox and gray squirrels at Mayslake Forest Preserve. Today I want to share results for two species of birds that nest at […]


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