Effects of Old Age on Female White-Tailed Deer in Illinois
Photos courtesy of the Illinois Department of Natural Resources.
The presence of numerous small refuges from hunting scattered throughout the landscape, and the reluctance of some hunters to kill female white-tailed deer, has allowed many females in Illinois to survive well past the prime reproductive ages, considered to be 3 to 7 years old.
To examine the productivity and survival rates of these older deer, 26 females were captured and marked with radio transmitters or plastic collars when 3 years old or less. Their life histories were monitored until they died. All 26 deer lived to 8 years of age, and one to age 18. These females were marked on two areas in Illinois, each with some area protected from hunting.
The number of fawns produced by these females showed only a gradual decline as deer aged instead of an abrupt decline after age 7 as might be expected. In fact, between ages 13 and 17 years, these females averaged nearly two fawns per year, only slightly less than for the prime ages.
Although their reproduction was not greatly diminished, annual survival of these older females was lower compared to younger deer. Survival began to decline at about age 10, and although many hunters preferred to take bucks, hunting was the primary cause of death for these females.
Social position is an important factor in survival for white-tailed deer. Some females become dominant, while most of the other females remain in a subordinate social position. Dominant females provide leadership of groups comprised of both sexes.
Social position also affected life spans. Of seven marked subordinate females, six died between 8 and 10 years of age. In contrast, 16 dominant females survived much longer, with four dying during their eighth year, two at age 9, two at 10 years, three at 11, and one each for ages 13 through 17 years. While four subordinate females died from road accidents, only one dominant female died from a highway accident.
In Illinois, where females usually are in good physical condition and not subject to harsh winters or excessive predation by wolves and other predators, females may be expected to survive well past their prime years and continue to have young. This applies to both dominant and subordinate females. Dominant deer tend to live in home ranges that avoid dangerous situations. These include avoiding highways, taking advantage of any refuge areas present to avoid hunters, and locating safe foraging sites for themselves and their offspring. This behavior extends their life span and contributes to social bonding among female relatives.
Charlie Nixon is a wildlife ecologist retired from the Illinois Natural History Survey.