Evaluation of the Eye Lens Technique for Aging Columbian Black-Tailed Deer

Abstract

In order to evaluate the eye lens technique for aging Columbian black-tailed deer (Odocoileus hemionus columbianus), lenses from 83 known-age animals were fixed, desiccated, and weighed in a standardized manner. Several potential sources of error in the preparation and weighing of lenses were explored, including length of desiccating time, length of cooling period before weighing, removal of lenses from eyeballs before fixing, and freezing prior to fixing. The effects of freezing and removal of the lens before fixing were investigated on a series of domestic sheep (Ovis aries) eyes. Freezing had no significant effect, but it was determined that fresh lenses were easily damaged with consequent loss of weight. For statistical comparisons of the influence of sex, nutrition, body weight, and geographical variation, a mathematical expression of the lens weight-age relationship was necessary, and the equation Y = a + b log X was found to give a satisfactory representation of the data. A comparison was made with weights of lenses from 20 known-age Rocky Mountain mule deer (Odocoileus hemionus hemionus) from Utah, and significant differences were found. Lenses from male and female deer differed significantly in weight. The effects of nutrition on lens weight are not clear cut, and even though there is a good general correlation with body weight, trends vary with increasing age. Using the regression, a test was made of the usefulness of the lens technique by estimating the ages of the knownage deer. The margin of error was found to be directly proportional to age, but did not bear a constant relationship and increased rapidly in deer older than 5 years. Accuracy was improved by comparing males and females with their appropriate regressions. In general, this technique is considered to be satisfactory for aging deer at least through 5 years, especially if only year classes are desired. Since Lord (1959) described the use of the dry weight of the eye lens for aging cottontail rabbits (Sylvilagus floridanus), this technique has been applied to a number of other mammals and birds with varying degrees of success. Lord (1961) has also investigated aging of the gray fox (Urocyon cinereoargenteus) by this means, and Sanderson (1961) reported on a study of the raccoon (Procyon lotor). Other applications of this technique have been published by Beale (1962) on fox squirrels (Sciurus niger); Martinson et al. (1961) on swamp rabbits (Sylvilagus aquaticus); Wight and Conaway (1962) on cottontails; and Kolenosky and Miller (1962) for pronghorn antelope (Antilocapra americana). Lord (1962) also applied this method to white-tailed deer (Odocoileus virginianus) and described the relationship between lens weight and nutritional status of the deer. In this paper he also mentioned unpublished studies on the deer mouse (Peromyscus maniculatus) and the Norway rat (Rattus norvegicus). Montgomery (1963) has studied the effects of freezing and decomposition on the weights of raccoon lenses and attributed some of the variability in Sanderson's data (1961) to these causes. In Australia, Dudzinski and Mykytowycz (1961) have also used this method for aging wild rabbits (Oryctolagus cuniculus). Various investigators have attempted to use the lenses of birds for aging purposes. Payne (1961) reported this method to be unsatisfactory for house sparrows (Passer domesticus) and cited unsuccessful studies with other birds. Campbell and Tomlinson (1962) attempted to age chukar partridges (Alectoris graeca) with similarly discouraging results. Generally birds appear to have too much individual variation and overlap in weights between age-classes for successful use of this technique. The principal objective of the present