Quantifying partial migration with sex-ratio balancing

Abstract

Partial migration, the phenomenon in which animal populations are composed of both migratory and nonmigratory individuals, is widespread among migrating animals. The proportion of migrants in these populations has direct influences on population genetics and dynamics, ecosystem dynamics, mating systems, evolution, and responses to environmental change, yet there are very few studies that measure the proportion of migrants. This is because existing methods to estimate the proportion of migrants are time-consuming and expensive. In this paper, we demonstrate a new method for estimating the proportion of migrants in a population based on sex ratio measurements. Many partially migratory taxa exhibit sex-biased migration or residency, and in these cases, the sex ratios of migrants and nonmigrants are fundamentally related to the proportion of migrants in the population. We define this relationship quantitatively and show how it can be used to infer the proportion of migrants in a population through a process we term “sex-ratio balancing”. We obtain Bayesian estimates of proportion of migrants and quantify the uncertainty in these estimates with highest posterior density intervals. Lastly, we validate the sex-ratio balancing approach with a Chinook salmon (Oncorhynchus tshawytscha Walbaum in Artedi, 1792) data set. Sex-ratio balancing holds promise as a tool for quantifying partial migration and filling a key data gap about partially migratory taxa.

Male-Biased Partial Migration in a Giraffe Population

“…In addition to competition for spatiotemporally varying forage resources, competition for mating opportunities may be a potential factor contributing to male giraffe movement strategies. Interestingly, available literature on ungulate migration describes largely female biased migration (Ohms et al, 2019), however the unique reproductive strategies of giraffe among ungulates may help explain this discrepancy. Since female giraffes are as seasonal, asynchronous breeders, and since they are only sexually receptive for a few days during a biweekly estrous cycling, male giraffe allocate much of their time in all seasons moving among scattered herds of females to assess their sexual receptivity (Pratt and Anderson, 1985;Bercovitch et al, 2006).…”

“…In partial migration, some members of the population are resident while others exhibit migratory behaviors (Dingle and Drake, 2007;Chapman et al, 2011). Identified in a diverse suite of taxa (Chapman et al, 2011;Ohms et al, 2019) and observed on a wide range of spatial scales, from several kilometers (Mysterud, 1999;Gaidet and Lecomte, 2013) to hemispheres (Shaffer et al, 2006), partial migration, because of the inherent variation of movement strategies with a population, provides a useful process to evaluate the causes of intraspecific variation in movement behaviors and the fitness consequences of different space-use strategies (Chapman et al, 2011). Although these variations in movement may have a genetic basis in some systems (Berthold and Helbig, 1992;Bensch et al, 2011;Hess et al, 2016) researchers are increasingly identifying scenarios in which these alternative movement strategies are conditional on the state of an individual and may be plastic over the life of an individual (Sutherland, 1998;Found and St. Clair, 2017).…”

“…Despite a growing body of research on the ecological mechanisms for the emergence and maintenance of individual variation in movement strategies, there is a lack of studies evaluating this variation at the population level (Ohms et al, 2019). Many studies examining varying movement strategies use tracking devices (GPS, VHF, PIT tags) to monitor the movements of focal individuals and extrapolate these processes to the population level (Struve et al, 2010;Mysterud et al, 2011;Cagnacci et al, 2015).…”