Effects of nutrition on age determination using eye lens weights

Populations of hispid cotton rats (Sigmodon hispidus) were studied in 6 young loblolly pine (Pinus taeda) plantations in South Carolina, USA. Each plantation had 2, 1-ha, live-trapping grids; 1 to sample a Sigmodon population which continually received supplemental food, the other to sample a control population. After 15 months of live-trapping, the Sigmodon were killed and their eye lenses removed, dried and weighed. Birth dates were calculated for 425 Sigmodon which were first captured when young. Patterns of eye lens growth for food-supplemented and control animals were compared using the Richards' growth functions and were significantly different. For animals less than 165 days of age, the functions predicted greater eye lens weights for control animals than food-supplemented animals. The reverse was true for animals older than 165 days of age. Because of the influence of nutrition on eye lens growth patterns, the assumption that laboratory animals or known-age animals from one population can be used to estimate the ages of animals from another population is not always valid.