Sex-specific differences in dispersal propensities and their consequences for grey mouse lemurs (Microcebus murinus)

Abstract

Dispersal, the permanent relocation or change of social group of an organism, is an almost ubiquitous phenomenon among taxa. Sex-biased dispersal (SBD) propensities and/or distances are of one of the most common general dispersal patterns. With the invention of genetic tools the number of studies identifying dispersal trends has increased exponentially. Still, aspects of the dispersal process remain unknown for most species, impeding not only our understanding of the evolution of SBD, but also knowledge about the connectivity of populations, which finally determines the dynamics of populations. Consequently, many of the assumptions about the costs induced by dispersal like increased mortality remain of theoretical nature. In this thesis, I investigated proximate aspects of the dispersal process and consequences of sex-biased natal dispersal of a small, solitary primate, the grey mouse lemur (Microcebus murinus) by means of behavioural observations and radio-tracking of 90 subadult grey mouse lemurs as well as capture-mark-recapture and genetic analyses. I found that male grey mouse lemurs have a uniform movement strategy during dispersal with highly directed movements and spatially very concentrated explorative activity. The length of the whole dispersal process varied, because individuals varied in the period during which they commuted between natal home range and prospected sites. This observation indicated that unfamiliarity with the habitat during immigration presented the biggest challenge during dispersal for grey mouse lemurs in terms of dispersal-related costs. This assumption was further supported by another finding, derived from capture-mark-recapture and genetic data collected for a long-term study. Prior to emigration, grey mouse lemurs needed to accomplish a minimum degree of development and growth. This minimum level of maturity required for dispersal served probably as a preparation for the negative effects of dispersal on the energy balance. Such condition-dependent dispersal strategies seem to be very common, since they allow for flexibility in dispersal behaviour such as the exact timing of emigration, which probably helps to increase the success probability of dispersal. Possibly, such preparations freed dispersal distances from constraints of physical condition in grey mouse lemurs, which was neither determined by body mass nor by body condition. Once the critical threshold was overcome other factors determined the exact timing of emigration. Which proximate factors, remains to be determined, but promising directions for future investigations represent the study of personality differences or physiological changes. In this thesis, I also introduced an approach that could be used to detect behavioural changes during dispersal and how these changes are related to factors such as personality of hormonal changes. The approach was exemplified by modelling feeding sequences of subadult males and females. Finally, I used a ten-generation capture-mark-recapture and genetic data set collected for a long-term study to evaluate, whether male-biased natal dispersal effectively eliminated the risk of inbreeding, which is one of the factors which is generally accepted to play an important role in the evolution of SBD. No signs for inbreeding depression in terms of survival could be detected and natal dispersal decreased inbreeding risk substantially. However, only in combination with demographic (such as mortality) and behavioural factors (roaming of males during mating season and promiscuity) was inbreeding risk reduced to such an extent, that additional dispersal of either males or females becomes unnecessary. This situation probably allowed SBD propensities to become and stay an almost fixed event in the life history of male grey mouse lemurs, which is rather insensible to external cues and deterministic concerning the emigration decision, but allows for some flexibility in the timing of dispersal, which enables males to improve their prospects of successful dispersal. However, which factors caused the evolution of SBD in grey mouse lemurs remains to be determined.