In addition to malaria parasites, we also use an unusual species of parasitoid wasp (Melittobia acasta) to understand life-history decisions in the context of co-operation and conflict. Parasitoid wasps in the genus Melittobia are unusual in that males often fight to death during competition for mates. This research is in collaboration with Stu West.
Sex ratios and fatal fighting
The life history of these wasps suggests that they should fit the same basic sex ratio pattern as malaria parasites (Local Mate Competition), but they don't. Sex ratios are always extremely female biased, regardless of the number of females laying eggs in the same host. Lethal combat has been suggested as a likely explanation for this apparent failure to fit theory, because as earlier emerging (older) males are expected to have a considerable competitive advantage. This scenario would suggest that the first female to arrive and lay eggs on a patch should produce just enough sons to fertilise their sisters, but there is no advantage of later females adjusting the sex ratio (proportion male) of their offspring in line with Local Mate Competition.
However, our data show that although earlier emerging males do have a competitive advantage, this can be reduced by an interaction with body size, because larger males are much more likely to win fights and is only true for a narrow critical period around the time at which the younger males emerge from their pupae. This suggests that lethal male combat cannot fully explain the lack of sex ratio shift observed in Melittobia. An alternative explanation could be that if co-founding females are usually very closely related, theory would predict only small sex ratio shifts in response to Local Mate Competition. The natural history of Melittobia suggests that this could be the case because females often produce non-dispersing daughters who lay their own eggs on their natal host.
The existence of dispersing and non-dispersing daughter morphs offers a novel context to investigate sex allcoation and other life-history trade-offs in the context of social evolution.
In the past
Sarah has dabbled in sex allocation strategies in Nasonia (parasitoid wasps) and Callosobruchus (bean beetles), as well as wondering why temperature sex determination has evolved in Chelonia (sea turtles).