Modelling caprine age-at-death profiles using the Gamma distribution

Abstract

Age-at-death profiles constructed from archaeozoological data have been used for decades to infer the goals of prehistoric herd management strategies. Several ‘ideal’ profiles have been proposed as models for the optimal kill-off profiles that represent specific husbandry strategies, such as maximising milk or meat yields, which can then be compared to archaeological profiles. We evaluate the goodness of fit of ten caprine archaeological age-at-death profiles to five published idealised profiles, whilst properly accounting for sampling error and data where the age classes of observations are uncertain. We statistically reject all tested idealised profiles as plausible models to explain the data, and instead propose that a Gamma distribution provides a simpler and better general model to represent possible herd management strategies. Furthermore, we show that archaeological profiles can be summarised well using Gamma parameters, which allow multiple datasets (and models) to be easily compared and graphically represented together with minimal information loss, thus allowing clearer inferences to be drawn. Finally, we calculate likelihood distributions of the Gamma parameters, which provide confidence intervals that fully account for the uncertainties from small sample sizes and uncertain age classes. We have developed an R package ‘GammaModel’ to enable users to apply these tools to any age-at-death count data.