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Mating Walnut Flies: Abstracts:

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.

Abstract: In laboratory and field studies of the walnut fly, Rhagoletis juglandis Cresson (Diptera: Tephritidae), we assessed the effect of operational sex ratio on copulation duration and partitioned the sex ratio effect into component effects due to male density and female density. In our first laboratory experiment, results were clearly consistent with theoretical expectation: increases in male density were associated with significant increases in copulation duration while increases in female density were associated with significant decreases in copulation duration. These component effects yielded a striking composite effect of operational sex ratio (OSR) on copulation duration in which male-biased ratios were associated with low frequencies of short copulations and female-biased ratios were associated with high frequencies of short copulations. Consistent with a priori expectations concerning costs of territorial behavior, the effect of male density on copulation duration was stronger than that of female density. There was no significant interaction between the effects of gender density on copulation duration: each gender density contributed additively to the composite OSR effect on copulation duration. In contrast to the effect of OSR, overall density had little effect. Field data corroborated these findings fully and showed additionally that OSR in the vicinity of fruit tended in nature to be male-biased. III a second laboratory experiment, we measured copulation duration for individuals exposed alternately to male-biased and female-biased ratios. Individual flies consistently copulated for longer in male-biased environments than in female-biased ones, We propose that this plasticity permits individuals to track changes in local sex ratio over space and time and respond appropriately.

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.

Abstract: The effects of the interaction between the operational sex ratio (OSR) and a resource (i.e. oviposition site) on mating dynamics have rarely been considered. We examined the effect of the resource presence and its interaction, with the effect of OSR on copulation duration in Rhagoletis juglandis, a tephritid fly species characterized by a resource-defence mating system in which males defend territories on walnut fruit. In this species, copulation duration varies from 30 s to over Ih and was shown previously to respond strongly to changes in OSR. In the field, short copulations tended to begin and end on fruit, whereas most long copulations generally began on fruit but ended in the foliage, suggesting a possible effect of resource presence on the copulation duration. In laboratory assays of isolated pairs, copulations were significantly shorter in the presence of a surrogate fruit, confirming the effect of resource presence. In another laboratory assay, in which we manipulated OSR independently of resource presence, resource presence and OSR were additive in their effects. Results are discussed in the context of sperm competition theory.
© 1999 The Association for the Study of Animal Behaviour.