Biology of Reproduction, Vol 21, 1099-1104, Copyright © 1979 by Society for the Study of Reproduction

Age at First Conception in Black-tailed Deer

¹ Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6

Under captive conditions, some 50% of female black-tailed deer (Odocoileus hemionus columbianus) fawns conceive in their first year. Wild fawns first conceive in their second year of life. The growth of wild and captive fawns was similar until mid-October. Wild fawns then ceased to grow. Captive fawns that subsequently gave birth in June grew faster than captive fawns that did not reproduce. During the breeding season of November-December the range of body weights of samples of wild fawns, of captive fawns which bred and of captive fawns that did not breed, all overlapped. It is suggested that the timing of puberty is regulated by photoperiodic influences and that fawns can first conceive when they reach their individual critical body weight if nutrition is adequate.

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ACKNOWLEDGMENTS We thank Mrs. E. Gadsby for her high standards of conscientious animal husbandry and J. Littleboy for assistance in raising fawns. Mr. J. Walters (Director, UBC Research Forest) and his staff have given ongoing support to this research. The project was funded by operating grants from the N.R.C. of Canada and from the B.C. Provincial Fish and Wildlife Branch. We especially thank the staff of the latter organization (Nanaimo office) for permission to use data collected during the North-West Bay study. We thank R. Frisch, D. Seip and R. Wilen who kindly commented on an early draft of this paper and the anonymous referees whose comments helped us clarify our concepts.