TABLES FOR ESTIMATING AGES AND BIRTH DATES 



OF COTTONTAIL RABBITS, With Suggestions For Handling Lenses 



William R. Edwards 



Lord (1959) first presented the concept of estimating 

 the ages of cottontail rabbits {Sylvilagus ftoridanus spp.) 

 on the basis of the dry weight of the eye lens. He gave 

 a graphic representation of the relationship between age 

 (x) and the dry weight of the lens (y) for approximat- 

 ing the ages of cottontails in days (Lord 1959:360). 

 Dudzinski and Mykytowycz (1961:159), using Lord's 

 data (1959:359), reduced the relationship of dry lens 

 weight to age for cottontails to the algebraic form 



log,„ y = 2.4890 — 



68.7927 



(x + 41) 



Manipulation of this equation to the form 

 68.7927 



] 



X — 



41 



(2.4890 — log,„y) 



allows the biologist to estimate the age of a cottontail 

 in days. However, the equation is tedious to use with 

 samples of appreciable size. Consequently, the tables 

 presented in this paper were prepared to facilitate the 

 compilation and processing of cottontail lens data. 



Estimated ages in days were computed for lens 

 weights of 11-210 mg (Table 1). Because of the rela- 

 tively low precision of the estimating equation, as sug- 

 gested by the work of Dudzinski and Mykytow\cz (1961 : 

 158), there is little justification for using the equation 

 for growth of cottontail lenses in estimating ages beyond 

 the first-year class. \Vhen one knows the dr\- weight of 

 a lens in milligrams, it is a simple matter to find that 

 weight in one of the columns in Table 1 and to read 

 the estimated age in days in the adjacent column. 



Statistics gathered at the Illinois Natural History 

 .Survey suggest a high degree of bilateral SNinmetry in 

 weight between a rabbit's lenses. \Ve now believe that the 

 difference in weight between lenses of a pair is primarily 

 the result of sloughing off of tissue of one or both during 

 handling. Thus, we suggest that only the weight of the 

 heavier lens of each pair be used in estimating age even 

 when both lenses are in apparently good condition. 



Table 2 was prepared to simplify determination of 

 an estimated date of birth after an estimate of age has 

 been obtained. In this table davs of the vear are num- 



Table 1. — Estimated ages of cottontails from dry weights of eye lenses. 



