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Mating Walnut Flies: Mating strategies and optimal foraging theory

Optimal foraging theory predicts that an animal will forage in a patch of food until it reaches the highest instantaneous rate of ingestion of food, at which point it will switch to a different patch. The assumption is that the gain curve is not linear, but saturates, so that increased foraging time results in smaller and smaller benefits over time. The time of greatest benefit depends in part on how long it takes to reach the next patch. If the travel time is large, it benefits the forager to stay in a patch longer than if the travel time is short.

Searching for mates is analogous to foraging for food. If the OSR is male-biased, and it takes longer to find the next available female, then the "travel time" to the next female is greater than if the OSR is female-biased. Therefore, optimal foraging theory predicts that (if copulation duration is controlled by the male, of course) copulation durations will be short when the OSR is female-biased, and long if there is an overabundance of males.

(Optimal foraging links)

Actual Research

This is exactly what was found by Henar Alonso-Pimentel and Dan Papaj, researchers at the University of Arizona. They manipulated the sex ratios of walnut flies in the lab, and observed behavior and sex ratios in the wild. As predicted, male-biased sex ratios were associated with short copulations, and female-biased sex ratios were associated with long copulations. They found that it was the actual ratio that was important, not the absolute densities of males and females.

References:

Alonso-Pimentel, H., and D. R. Papaj. 1996. Operational sex ratio versus gender density as determinants of copulation duration in the walnut fly, Rhagoletis juglandis (Diptera: Tephritidae). Behavioural Ecology and Sociobiology 39:171-180.

Alonso-Pimentel, H., and D. R. Papaj. 1999. Resource presence and operational sex ratio as determinants of copulation duration in the fly Rhagoletis juglandis. Animal Behaviour 57:1063-1069.

Read the abstracts of these research papers.

Still up for grabs (an open research question) is the role of female choice in the mating behavior of these flies!