Maternal investment tactics in cooperative breeding systems

Whenever multiple individuals contribute to the care of offspring, the optimum level of investment for each carer depends on the behaviour of the others. Previous theoretical and empirical work has largely focused on carer contributions within a single stage of a breeding attempt, neglecting the potential for investment during earlier stages to influence later care decisions. Typically, mothers have much greater control than other carers over the number and quality of offspring, and hence by altering her investment during offspring production a mother can adaptively adjust offspring phenotype to match or exploit the predicted care paradigm. In this dissertation, I use theoretical, empirical and comparative methods to investigate the influence of maternal tactics on investment rules in cooperative breeding systems, where ‘helpers’ care in addition to parents. In three chapters I model maternal control of offspring quality and offspring number across a cooperative breeding attempt, and investigate how the costliness of different reproductive stages, the kin-structure of the care group, and the consequences of offspring early-life condition influence the investment rules of carers. During offspring rearing, fair division of labour within a cooperative group can theoretically be resolved using simple turn-taking rules, leading to efficient outcomes for all carers. To test whether such a rule is employed in nature, a later chapter analyses empirical provisioning data from the chestnut-crowned babbler (Pomatostomus ruficeps), a cooperatively breeding bird endemic to the Australian outback. I use a Markov chain Monte Carlo approach to determine whether individuals alter their provisioning rate when other carers visit the nest, and identify both ‘passive’ and ‘active’ turn-taking behaviour. Finally, I present a comparative analysis of studies on provisioning rules in cooperatively breeding birds, and investigate whether the level of investment mothers must contribute to offspring influences the later care paradigm observed. My results indicate that maternal costs contribute to variation in both breeding group size and female provisioning behaviour. I conclude that maternal investment tactics are an underappreciated influence on carer investment rules in both the theoretical and empirical literature, and that incorporating them is crucial to understanding variation in cooperative care behaviour in nature.

This PhD was funded by a studentship from the Natural Environment Research Council, and supplementary support from Peterhouse, Cambridge.