Health of the mother is known to affect the sex-ratio of the offspring in sexually dimorphic mammals. Both density-dependent and environmental factors (conditions at present, or seasonal conditions the year before) could impact the health of the mother. There are however two opposing hypotheses for whether a mother in good health should invest more in a strong son (that can father many offspring) or in a daughter (that can stay in the same area and use good resources). Fetuses from 1,485 female white-tailed deer (Odocoileus virginianus) were collected over a 10 year period (2002-2012), across 75 counties in Arkansas. For each adult female deer we estimated age, recorded live and dressed weight, calculated the kidney fat index (KFI), back-calculated the date of conception, and recorded the number and sex of fetuses. Our objective was to investigate whether female Arkansas deer that are in good health tend to produce male or female offspring. We developed a set of generalized linear models, with a binomial response (male versus female offspring), to examine the effects of year, month of conception (Oct, Nov & Dec), age of the mother, number of fetuses, and KFI on fetal sex ratios.
Results/Conclusions
We found that age of the mother, and number of fetuses, had slight negative effects on the probability of producing female offspring. Conception during November had the highest probability of producing female offspring (β = 0.30 ± 0.16), whereas the probability of producing a female fetus decreased with an increase in number of fetuses (-0.71 ± 0.11). Females in better health (higher KFI) tended to produce more females (β =0.0001 ± 0.001), but older females had a higher likelihood of producing male offspring. Our results seem to partially support both of the offspring sex-ratio hypotheses, with no distinct patterns. It is possible that there is a geographic relationship and that closer investigations might reveal clearer patterns, perhaps correlations with distance from the Mississippi River. We hope that the results of our analysis will form a baseline against which herd health in Arkansas can be measured in the future. Especially, since chronic wasting disease was detected in white-tailed deer in Arkansas in 2016, and there is a consequent need for close monitoring of the deer herd.