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The evolution and development of cooperation in mammalian societies

Final Report Summary - THCB2011 (The evolution and development of cooperation in mammalian societies)

In a minority of animals, including some mammals, individuals spend part or all of their lives assisting others to rear offspring and explaining the evolution of reproductive cooperation has become one of the principal focuses of research on the evolution of animal breeding systems. The first aim of this project was to understand the development, distribution and evolution of reproductive cooperation among mammals, using a combination of quantitative interspecific comparisons and phylogenetic reconstruction to investigate contrasts between species. This involved the construction of a comparative dataset covering the life history parameters and breeding systems of more than 2,000 mammals that allowed us to reconstruct evolutionary sequences and examine the correlations between different social and ecological traits. Our analyses showed that altruistic forms of reproductive cooperation (activities that increase the fitness of others at a net cost to the individual’s own reproductive success or survival) are associated with high average levels of kinship between group members; have usually developed in lineages where monogamy was the ancestral breeding system; are restricted to litter-bearing (polytocous) species; and are most commonly found in species living in arid, unpredictable environments, where a single dominant female suppresses breeding in other adult female group members. Unlike some previous studies, we have found no evidence of any consistent association between cooperative breeding and longevity. In contrast, in species where kinship between group members is low, asymmetrical forms of cooperation are seldom well developed, though group members commonly have individually differentiated relationships with each other and often form temporary alliances used in competition for breeding opportunities and resources.

The second aim of the project was to investigate the extent, causes and development of individual differences in cooperative behaviour in two of the most specialised cooperative mammals (Kalahari meerkats Suricata suricatta and Damaraland mole-rats Fukomys damarensis). Our fieldwork on meerkats was based on a combination of observation and experiments on more than 700 wild animals from thirty different groups while our work on mole-rats involved over 500 marked individuals in wild groups and a similar number of individuals from sixty different captive colonies maintained in transparent tube systems in a purpose-built facility at our field-site. In both species, a single female in each group virtually monopolises reproduction. Breeders as well as non-breeders contribute to all cooperative activities, their contributions to different activities are inter-correlated and there is no evidence of functional specialisation of individuals in different activities. Individual contributions to cooperative activities vary widely and increase with age but are unrelated to variation in kinship. Individual differences in cooperative behaviour appear to be unrelated to variation in sex hormones but are commonly affected by variation in cortisol levels, though the extent and direction of these effects appears to differ between the sexes. In meerkats, experimentally induced increases in cortisol levels in pregnant mothers are associated with increases in the growth and competitiveness of their offspring. In contrast to some eusocial insects, there is little evidence of differences in development between breeders and non-breeders before breeders acquire the breeding position. However, once individuals of both sexes acquire breeding status, they re-start skeletal and muscular growth leading to increases in size and weight and to a more elongate body shape which, in females, appears to facilitate the gestation of large litters Though Damaraland mole-rats are often portrayed as eusocial and their life-history and behaviour has been compared with that of termites, our results suggest that, in most respects, their behaviour and life histories more closely resemble those of other cooperative vertebrates.