Comparative patterns of philopatry and dispersal in two common mole-rat populations: implications for the evolution of mole-rat sociality

1. The aridity food-distribution hypothesis (AFDH) maintains that ecologica l constraints and foraging risks in arid environments curtail dispersal and promote the evolution of cooperatively foraging social groups within Afric an mole-rat populations. To assess the influence of habitat constraints upo n mole-rat social behaviour and hence the validity of the AFDH, we investig ated the differential patterns of dispersal and philopatry in two populatio ns of the common mole-rat, Cryptomys hottentotus hottentotus Lesson, one fr om a more 'optimal' habitat (mesic) and another from a 'suboptimal' habitat (arid). 2. Rates of immigration and emigration were markedly lower in the arid rela tive to the mesic site, reflecting divergence in the ecological constraints upon dispersal. 3. The rates of dispersal at both sites increased as a function of group si ze. However, for any given group size, colony attrition was significantly g reater at the mesic site than at the arid site, probably reflecting the rel axation of constraints upon dispersal in mesic areas. 4. There was no sex bias in dispersal at either study site. 5. At the mesic sites dispersing and nondispersing individuals were of comp arable mass, whilst at the arid site there was a trend for dispersing anima ls to be heavier than their nondispersing counterparts. 6. This investigation revealed marked differences between the study populat ions in the nature of dispersal. This reflects adaptive variation in social behaviour between the regions, and the results suggest that delayed disper sal and cooperation may be more crucial to individual survival in arid than in mesic areas. As such, these findings provide support for the underlying contention of the AFDH that ecological constraints on successful dispersal and colony formation in arid areas have promoted a greater degree of socia l cohesion in mole-rats occurring in these regions, ultimately advancing th e evolution of group-living and cooperative foraging in the Bathyergidae.