Sexual Conflict and Division of Labour during Incubation

Individuals have limited time and energy and so they face a trade-off between the resources they allocate to parental, mating and self-maintenance activities and between care of current and future offspring. Within a biparental system, models predict that an individual will do better if it can reduce its own level of investment and rely on its partner to compensate, but that the partner should not compensate completely or it will be exploited. The costs and benefits of care can differ between males and females, leading to sex-specific differences in optimum resource allocation to a given breeding attempt. Sexual conflict can occur over evolutionary or ecological time, as predicted by ‘sealed bid’ and ‘negotiation’ models of parental investment, respectively. While researchers have traditionally used handicapping or mate removal to induce variation in paternal effort and then measure the female response, I achieved this by carrying out two experiments in which I separately manipulated (a) paternal certainty and (b) male extra-pair copulation (EPC) opportunity during incubation in a biparentally incubating passerine, the Zebra Finch, Taeniopygia guttata. Both experiments were based on a ‘crossover design’, which took place over two breeding attempts: in the first attempt half the pairs experienced a treatment (i.e. reduced paternal certainty or increased male EPC opportunities, depending on the experiment) while the remaining pairs were control; in the second breeding attempt the type of treatment was switched between groups. This allowed me to explore the effects of previous events on current effort. I also statistically controlled for male attractiveness, based on association times obtained from mate choice trials, as this varies between males, and females may respond differently to it. In the first round of the experiments, when all subjects lacked breeding experience, treatment males reduced incubation effort relative to controls. However, when the treatments were switched, males continued to invest the same level of effort that they had shown previously, which suggests that individuals may use early life experiences to shape their later responses. Males and females responded to each other’s contributions over ecological time, and control and treatment pairs negotiated different patterns of effort. Contrary to predictions of most game theoretic models, treatment females compensated fully for the reduction in partner effort. Since there can be a trade-off in investment between reproductive stages and attempts, I tested whether compensating females showed reduced reproductive output in a subsequent breeding attempt. In the second attempt, clutch size was negatively related to the amount of time spent incubating the first clutch in females that had previously fledged a large brood. I also found that the number of offspring surviving to maturity increased with total incubation attentiveness. Together, these findings suggest that incubation attentiveness is an appropriate proxy for parental investment since it enhances offspring survival at a cost to the mother’s future fitness. Incubation effort can be measured in terms of temperature as well as time, and the sexes may differ in the quality of the incubation they provide for a given unit of time. In the Zebra Finch, the female alone possesses a brood patch, a bare area of thickened, loose ventral skin which facilitates heat transfer from parent to egg. I examined sex differences in incubation ability by comparing the ventral surface temperature of incubating males and females using infra-red thermography (IRT) under standard incubation conditions and also under more challenging conditions where the energetic costs of incubation had been experimentally increased by a clutch size manipulation. The ventral surface was warmer in females than males under standard conditions, and ventral temperature was higher than baseline body temperature in females but not males. Moreover, females increased ventral temperature after the clutch enlargement, whereas males showed no response. These results suggest that the sexes differ in their ability or willingness to incubate, presumably due to sex differences in parental certainty or mate availability. Since the female has a warmer ventral surface and spends more time incubating than the male, I predicted that she would be able to re-warm cool eggs more quickly and incubate eggs to a higher temperature. I used IRT to measure egg surface temperature and a temperature probe inside a dummy egg to measure rates of re-warming. Following the clutch size enlargement, the variance in egg temperature increased with clutch size more rapidly in males than in females, but no sex differences were otherwise detected in clutch surface temperature or rate of re-warming. This suggests that there are differences in the quality of incubation provided by males and females for a given contribution of time, but that these are subtle. In conclusion, variation in male and female incubation effort can be influenced by paternal certainty, male EPC opportunity, clutch size and the partner’s behaviour and these effects can be observed over ecological time. The flexibility of an individual’s response depended on the type of cue and the stage in an individual’s reproductive life that the cue was presented. Variation in parental effort can influence offspring fitness, and females that invest more effort in one breeding attempt may be unable or unwilling to do so in a subsequent attempt. Male and female parents can differ in their abilities to care and this can lead to differences in the amount and quality of care provided.