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In 1847, German biologist Carl Bergmann illustrated a pattern in which larger-sized species of animals are typically found in colder environments, while species of a smaller size reside in warmer areas. Bergmann’s rule states that animal populations and species of larger size are found in colder environments, and the reverse is true in warmer regions.
Although 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 recognized subspecies include the Northern woodland (O. v. borealis), Dakota (O. v. dacotensis) and Northwest (O. v. 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 (O. v. margaritae) that 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 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 the Northern portions of the whitetail’s geographic range.
Built to Cope with Extreme Environmental Conditions
More importantly, 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. Since 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 in order to do so.
Optimum Birthing Dates Maximize Survival
Regardless of their birth date, fawns that are malnourished during summer and autumn are not likely to achieve their maximum growth potential prior to facing their first stressful winter season, which decreases their prospects for survival. Even if stunted fawns do 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.
The large borealis subspecies, whose members live on the rich farmlands of the Midwest, probably demonstrate Bergmann’s rule most clearly. While occurring at low to moderate densities throughout Indiana, Illinois, Ohio, southern Michigan and southern Wisconsin, these whitetails enjoy rich forage year round and face winter weather of only moderate severity. Given such ideal nutrition, minimal social stress and favorable climate, even the late-born farmland fawns stand a good chance of achieving their maximum growth potential at adult age.
In the Upper Great Lakes region and in the Northeast, however, not all whitetails grow to their large size potential. Although unusually small deer sometimes are the result of late births, poor nutrition is more commonly the cause of poor growth among Northern deer. In the North, strong selective pressures prevail that favor physical fitness, because the harsh winter climate there normally dictates deer welfare and tends to weed out the small, the weak and the genetically inferior.
Usually, the frequent occurrence of smaller-than-normal deer on Northern range is symptomatic of dietary problems due either to soil infertility or, more commonly, depletion of preferred (more nutritious) plants due to excessive foraging by overly abundant deer. In either case, the stunted-deer phenomenon sends a clear message that all is not well with the local Northern deer herd and that populations are too high for the food resources available. Given such dietary shortage, Bergmann’s rule cannot operate to its fullest extent, and many small, unhealthy deer are likely to succumb to malnutrition or fall victim to predators, even during those winters of only moderate severity.
The importance of Bergmann’s rule transcends far beyond mere academic interest. For those whitetails normally subjected to harsh winter climates, achieving maximal body growth prior to winter is crucial to their future reproductive success and, ultimately, to their very existence.
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