A Cottontail Rabbit Lens-Growth Curve from Southern Wisconsin

A lens-growth curve, with confidence limits, was established from 188 known-age, pen-reared cottontail rabbits (Sylvilagus floridanus) between 2 and 297 days old. Excessive variation in lens weights for any given age-class made it unlikely that year classes, other than young-of-the-year from adults, could be separated. Pen-reared cottontails from southern Wisconsin had heavier lenses and heavier body weights than those from central Illinois, at least up to 150 days old. After death, lens-weight loss due to decomposition increased with temperature. Greatest weight loss occurred during alternate freezing and thawing. At least 12 days were needed to dry lenses in a gravity convection oven. This paper reports on lens weights of 188 known-age cottontails between 2 and 297 days old that were born and raised in pens, and lens weights of 69 animals between 9 months and 4% years old that were born in the wild and held in pens after being captured at an age of between 4 and 8 months. A few animals were raised in 1961, but the majority were raised in 1962 and 1963. Since animals were reared primarily for use in experiments involving the transfer of calcium 45 from females to offspring, the lens weights were incidental to the main study. For this reason, animals of exactly known age were not available over 297 days old. Since Lord (1959) first reported using the lens for age determination, lens-growth curves have been developed for many species. Curves from which an age prediction with confidence limits could be obtained, however, have been few. Dudzinski and Mykytowycz (1961) presented a growth curve with confidence limits for rabbits (Oryctolagus cuniculus) in Australia, as did Bauer et al. (1964) for the fur seal (Callorhinus ursinus). Confidence limits were put on the Australian rabbit and fur seal lens curves after making models linear and eliminating heterogeneity of array variance. For advice throughout the course of this study, I am indebted to R. A. McCabe, J. J. Hickey, and L. B. Keith. Statistical advice and aid in calculating regression equations were provided by D. B. Siniff.