Effect of population density on juvenile growth rate in white-tailed deer (Odocoileus virginianus)

Background/Question/Methods

Adult body size in ungulates displays variability according to nutritional availability.  While this phenomenon is established, the precise mechanisms behind realizing this variability are not well understood.  Animals may increase body size by one or any combination of three strategies: by increasing birth weight, by increasing growth rate during juvenile development, or by increasing the length of the maturation period, depending on possible fitness tradeoffs between these strategies.  The purpose of this study is to test the hypothesis that differences in adult body size are realized by increasing juvenile growth rate for white-tailed deer (Odocoileus virginianus), a cervid that demonstrates high levels of phenotypic plasticity in adult body size.  In particular, this study is an assessment of the effect of increased nutritional availability resulting from a decrease in population density on growth rate over time.  Harvest records for the Fort Hood military base in central Texas are used to construct population-level growth rate estimates by logarithmic non-linear least squares curve fitting.  Results of this method are compared to those of a previous method of mammalian growth-curve modeling that includes additional parameters for birth weight and asymptotic (adult) weight.  

Results/Conclusions

The annual logarithmic rate of growth increases during the study period.  Adult body mass increases in response to reduced population density and a concomitant rise in nutritional availability that results in higher growth rate.  Most of this effect occurs within the first two years of life indicating that growth rate plasticity is realized primarily in the juvenile stage.  More complex models that estimate multiple parameters, such as birth weight, adult weight, and maturation period may not work with harvest data in which estimates of these parameters are prone to error. This is likely due to inaccuracies stemming from age estimation using tooth wear, the most common method for age determination in white-tailed deer.  Such error renders growth rate estimates from complex models too variable to detect inter-annual changes in growth rate that are captured in this simpler model, which estimates growth rate only.