How Bergmann’s Rule Helps Northern Whitetails Survive

BergmannAlthough some subspecies of white-tailed deer (Odocoileus virginianus) might differ only in subtle facial, body and tail markings and colorations, others might differ greatly in body size. The largest of 38 subspecies include the northern woodland (borealis), Dakota (dacotensis) and northwest (ochrorous) whitetails, which inhabit the northern United States and Canada. These deer stand about 40 inches at the shoulder, and males can weigh up to 400 pounds.

In contrast, the smallest whitetails tend to come from warmer, southerly climates. For example, the Margarita Island subspecies (margaritae), which lives off the coast of Venezuela, might weigh less than 40 pounds at maturity.

This geographic trend in body size — largest in the North, smallest near the equator, with intermediate sizes occurring at mid-latitudes — is also seen in other mammals and is referred to as Bergmann’s Rule, after the scientist who first observed and described the phenomenon.

This relationship of body size to latitude is of interest to deer hunters and deer managers because body size and antler size generally go together. That is, the largest-bodied bucks generally grow the largest antlers and tend to come from northern portions of the whitetail’s geographic range.

More important, the biological manifestation of Bergmann’s Rule has special survival value for deer in that it permits them to better cope with extreme, unfavorable temperatures. Because a large body has less surface area in relation to total body mass, a large-bodied deer retains body heat more efficiently than does a small-bodied deer. Such an energy-conserving advantage could mean the difference between life and death for Northern whitetails faced with prolonged exposure to periods of sub-zero temperatures, periods often accompanied by limited supplies of food of low nutritive value. Conversely, small-bodied deer living in areas of hot weather are better able to dissipate surplus heat and maintain body temperature equilibrium.

Although genetic factors largely account for the body size differences articulated in Bergmann’s Rule, prevailing environmental conditions also play a vital role. The large Northern subspecies of whitetails have the innate ability to grow to large size, but they must be born on schedule and be well nourished to do so.

Regardless of their birth date, fawns that are malnourished during summer and autumn are not likely to achieve their maximum growth potential before facing their first stressful winter, which decreases their prospects for survival. Even if stunted fawns survive their first winter, they seldom demonstrate compensatory growth later in life, especially if subjected to continued dietary deficiencies; most grow up to be undersized adults.

— John J. Ozoga

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