The lens as an index of age in the pronghorn antelope (Antilocapra americana Ord.)

Abstract

The rate of growth and subsequent density variations in a natural population depend upon natality rate, mortality rate and age structure, but in order to determine each of these rates one must first know the age of the individuals from which data are taken. The population characteristics of big game species are poorly known, partly due to the fact that adequate age criteria are still not available. The criteria used to establish the age of an animal must satisfy several requirements, especially if these criteria are to be uniformly applicable to members of different populations living in different habitats. If an age character is a function of growth and development, it should not be affected by such variables as temperature and nutrition. If it is subject to wear, the rate of wear should be uniform between different individuals and populations, regardless of habitat. Finally, if the animal is relatively long-lived, as is the case with most, if not all, big game species, the character must change at a predictable rate throughout the life of the individual. Age criteria which satisfy all of the above requirements are rare among animals. Many attempts have been made to discover annular growth rings, like those in trees, in animal structures, but the only known examples are the growth rings in fish scales, those in the canine teeth of seals and their relatives (Scheffer, 1950) and the growth rings on the horns of bighorn sheep (Ovis Canadensis) and their relatives (Murie, 1944). Zones of annual growth have been reported in incisor teeth of moose (Alces alces) by Sergeant and PimIott (1959) but this character is only discernible for the first few years of age and remains to be fully evaluated in any event. A structure in mammals which grows continually throughout life but which has only recently been studied with respect to its applicability as an indicator of age is the lens of the eye. Lord (1959, 1961) found that the lens served as a reliable age indicator in cottontail rabbits (Sylvilagus floridanus) and gray fox (Urocyon cinereo­argenteus floridanus). The purpose of the present study is to describe the rate of growth of the lens of the eye in pronghorn antelope (Antilocapra Americana) and to establish, within the limitations of the data available, an age index for this species based on lens growth and size.