Ecological determinants of social systems: Comparative and experimental feeding ecology of two mouse lemur species ( Microcebus berthae, M. murinus )

The socio-ecological model (SEM) integrates ecological factors with characteristics of social systems and allows predictions about the relationships between resource distribution, type of competition and consequences for social organization and structure. Both theoretical and empirical research within this framework has mainly focused on explaining the evolution and maintenance of variation among group-living primates and other mammals. The main aim of this thesis was to illuminate ecological determinants of variation in the social organization and structure of solitary species, which exhibit yet unexplained diversity in these variables. I studied two nocturnal solitary primates (Microcebus berthae, M. murinus), which differ in two characteristics of female associations: (1) ranging patterns and (2) sleeping associations. Beginning in August 2002, I monitored individuals of sympatric populations in a 25ha study area in Kirindy Forest/CFPF (Madagascar) and (re-) captured and marked a total of 177 M. berthae and 291 M. murinus. Between March 2004 and November 2007, I recorded data on spatial patterns and behaviour of 18 M. berthae and 17 M. murinus females by means of direct observation and radio-tracking in more than 1700 hours. I quantified intra-specific differences in diet based on feeding behaviour, analysis of faecal samples, and stable nitrogen and carbon isotope analysis of hair and potential food sources. Relative resource availability was assessed by year-round phenological monitoring of trees and standardized capture of arthropods from different guilds. The main results supported basic predictions of the SEM and revealed that resource characteristics and the resulting competitive regimes did indeed relate to differences in female spatial-temporal patterns and social structure of solitary foragers. The major food sources of M. berthae occurred in small dispersed patches, leading to strong within-group scramble competition and over-dispersed females with a low potential for female associations. In contras t, M. murinus also used patchily distributed, high quality (large) resources. Within and between-group contest competition over these monopolizable resources allowed females of this species to cluster in space. When food distribution was experimentally manipulated, females of both species flexibly adjusted their spatial patterns. Moreover, food availability was identified as a major determinant of female spatial-temporal patterns, which affected these mouse lemur species unequally, mainly due to pronounced differences in feeding niche width and energy strategies to survive the lean season. In contrast to opportunistically feeding and seasonally inactive M. murinus, the more specialized M. berthae females experienced seasonally low food density, which enhanced female spatial avoidance. In addition, inter-specific competition between these ecologically similar mouse lemurs added substantial variation to the relative availability of resources for M. berthae and M. murinus females, respectively. Nevertheless, spatial competitive coexistence is possible due to life history trade-offs that result in species aggregation and a relative increase in the strength of intra-specific over inter-specific competition. In conclusion, the present study successfully applied existing socio-ecological theory to solitary foragers, tested a recent model on competitive coexistence and contributed a comparative data set to our understanding of inter-specific behavioural and ecological variation in solitary foragers.