Death and disappearance: estimating mortality risks associated with philopatry and dispersal

Abstract

Models dealing with the relationship of inbreeding to dispersal, the evolution of cooperative breeding, and the adaptive significance of reproductive suppression all involve a parameter describing die probability that emigration ends in death. Surprisingly, estimates of this parameter (or equivalently, of the probability of dispersing successfully) are virtually absent from the literature. There are formidable difficulties in estimating dispersal risk by direct observation of emigrants, but we contend that many investigators have already collected demographic information that can be used to estimate it indirectly. The root problem is that the proportion of the population diat emigrates from its natal group or home range, e, is not generally observable. Nevertheless, for each value of e diere is only one possible survival rate of emigrants (st) and of philopatric animals (sf) diat can explain the overall survival rate. For study sites Uiat are neither sources nor sinks, we describe a graphical framework within which census data and behavioral observations can be used to place bounds on e, s,, and sp. We use data from a Serengeti population of the dwarf mongoose (HelogaU parvula) to illustrate how different types of data and assumptions allow us to construct better estimates of these parameters. For dwarf mongooses, die approach reveals unexpected differences in dispersal risk among sex and age classes.