Ejaculate expenditure strategies in a polygynandrous bird, the red junglefowl, Gallus gallus

Plastic strategies of ejaculate expenditure are a fundamental aspect of male reproductive fitness, especially in polygynandrous species where multiple mating opportunities expose males to the risk of sperm depletion, and sperm competition favours large, high-quality ejaculates. Little however remains known about factors that modulate ejaculate expenditure behaviour. In this thesis, I explored how four different factors affect male ejaculate expenditure plasticity in the polygynandrous red junglefowl, Gallus gallus: temporal effects, male social status, female reproductive quality and female sexual novelty.
Firstly, I show that male ejaculate expenditure has a marked seasonal pattern, gradually peaking in June and decreasing later in the season. However, contrary to previous studies in domestic fowl, ejaculate expenditure patterns were largely insensitive to the time of the day (morning, midday, afternoon). Secondly, social status mediates access to females and the level of sperm competition faced by the ejaculates of a male. Dominant males have a higher copulation success and face a lower risk of sperm competition than subordinate males. Inconsistent with the findings of previous work in domestic fowl, I found no evidence that male social status predicted male’s ejaculate expenditure nor did I find a significant change in ejaculate expenditure resulting from a change in a male’s social status. Thirdly, I found that males exposed to large-combed females have higher ejaculate investment than males exposed to small-combed females, which suggests that males have the ability to assess female reproductive quality, consistent with previous work in fowl. Importantly, a male’s ejaculate expenditure in the intermediate-combed female in the final trial was similar to his expenditure in the previous four trials such that the intermediate-combed female received more sperm from males that had previously mated with large- versus small-combed females. These results suggest that males may have a carry-over effect where previous experience influences ejaculate expenditure in the current mating opportunity. Fourthly, we experimentally confirmed previous findings that males have a preferential sperm investment in sexually novel females (the Coolidge effect). Males were also more likely to mate with a novel female and ejaculated sooner than males exposed to an experimental control. In addition, we found that in the first rial, when males were naïve with respect to experimental treatments, expenditure patterns were similar across treatments and showed no evidence for a preferential investment in novel females which suggests that males may require previous experience to generate an expectation of the novelty of the female in the next mating opportunity and plan ejaculate expenditure in advance (i.e. reserve some sperm for the second female). There was also some evidence that males exposed to different treatments had different patterns of ejaculate expenditure in the final trial where all males were exposed to two females sequentially. However, the extent to which these differences were explained by previous experience is less clear. Collectively, the findings of these studies demonstrate that the temporal change, social status, female quality and female novelty may have profound effects on plastic male ejaculate expenditure.

Keywords: female quality
female novelty
cognitive mechanism
sexual selection
social status
experience
sperm allocation strategy
red junglefowl
Advisor: Pizzari, T