Pine and Montane Vole Age Estimates from Eye Lens Weights

Abstract

Lens weights were a satisfactory basis for estimating ages of 124 pine voles (Microtus pinetorum) and 517 montane voles (M. montanus) of known age. Other body and cranial measurements were of little added value for estimating ages when considered over a full two-year age range. Analyses of covariance showed no differences attributable to sex, season of birth, or laboratory generation in the lens weight-age regressions. Montane voles from small (1-4), medium (5-6), and large (7-10) litters had significantly different regressions, although pine voles from small (1-2) vs. large (3-4) litters did not differ. J. WILDL. MANAGE. 39(3):550-556 Various species of microtine rodents are central problems in both ecological theory (Krebs et al. 1973) and pest management (Forbes 1972, Federal Cooperative Extension Service 1959), and age specific data are needed to understand their basic population processes (e.g., Schaffer and Tamarin 1973). In pest management situations age specific birth and death rates are frequently the essential basis for accurately estimating effects of proposed or actual management decisions. If, as seems possible, reproductive inhibitors should replace toxic mortality agents in vole management programs (Richmond and Stehn, unpublished data), then data on age specific rates and age structure will be even more important, because in general birth rate perturbations will have a greater impact on population age structure (and thereby on potential rate of increase) than death rate changes. For these and other applications we sought to develop age estimation formulae for pine and montane voles. Our data are established mammalian aging criteria: eye lens weights and several morphologic measurements from known-age series of laboratory animals. Morris (1972) thoroughly reviewed this field, and Friend (1968) reviewed the lens weight technique sp cifically. We thank M. E. Richmond for assistance throughout the project and S. G. Fisher and D. Q. Thompson for reviewing the manus ript. R. Stehn, J. Jannett, J. Stone, and K. Tonnessen provided essential technical assistance. R. H. Foote kindly lent the electrobalance. The Cornell University Office of Computer Services and the University of Massachusetts Computing Center supplied machine time. METHODS AND MATERIALS The pine voles used here were of the first through fourth laboratory generations from animals originally collected near New Paltz, Ulster County, New York. The montane voles were first through third or fourth generation laboratory descendants of animals collected near Moran, Teton County, Wyoming. The pine voles were born between 1 February 1969 and 12 January 1 Supported by U.S. Department of Interio Award Contract 14-16-0008-1037 and a Ford Foundation Grant to the Cornell Program in Pest Management Ecology. A contribution from the New York Cooperative Wildlife Research Unit (U.S. Fish and Wildlife Service, Cornell University, New York State Department of Environmental Conservation, and Wildlife Management Institute, cooperating). 2 Present address: Division of Biological Sciences, University of Missouri, Columbia 65201. 550 J. Wildl. Manage. 39(3):1975 This content downloaded from 157.55.39.96 on Sun, 02 Oct 2016 06:40:27 UTC All use subject to http://about.jstor.org/terms VOLE AGE ESTIMATES ? Gourley and Jannett 551