The Reproductive Behaviour and the Nature of Sexual Selection in Scatophaga Stercorarial. (Diptera: Scatophagidae)

Abstract

AbstractFemales in all stages of their reproductive cycle around dung are equally attractive in eliciting encounters from searching males. On contact, all females which still have eggs to lay are mated (or remated), generally (i.e. over 75%) in their first encounter. All such females copulate for a normal duration, irrespective of previous matings. However, females which have completed oviposition are much less attractive to males on contact and though about 35% of post-oviposition females copulate as a result of their first encounter, only about half of them will have begun genital contact even if allowed six encounters with males. The female behaviour does not appear to be involved in this effect. Matings with post-oviposition females last only 43% of the normal duration. This effect may be due at least in part to the side to side ('swaying') movement performed by the female. It is extremely rare in nature for females to oviposit without a male mounted in attendance (i.e. in the 'passive phase'), though females will do so quite readily if separated artificially. The female initiates separation only after all the egg batch is laid, and after separation she flies immediately upwards and away from the dropping. Female quiescence appears to be necessary for the male to adopt the passive phase; swaying during genital contact and immediately after precedes separation without a passive phase. Females remain quiescent if they still have eggs to lay (even when already mated) and fresh dung is present, though the continuous reception of fresh dung is not necessary. Females sway fairly persistently if they have completed oviposition or there is no dung stimulus. Most females examined on arrival at the dropping (and before being found by a male) already had sperm in their spermathecae, presumably from matings at previous ovipositions since all females mate on arrival at the dung and there are several successive batches of eggs. It is predicted that receptivity before all the eggs are laid may now be of selective advantage to the female. The performance of non-receptivity as exhibited by post-oviposition females involves the female in an average rejection delay of 2.7 min. per encounter. At the average density of searching males present on the dung this rejection delay time would involve the female in much more time waste than full receptivity, since after mating the passive male undertakes the rejection of other males during oviposition. Even allowing for time wasted in recopulation following take-over (i.e. when a second male takes possession of the female), a female with full receptivity would save about 50 min. per oviposition cycle compared to one showing rejection. The passive phase is presumably of advantage to the male in preventing further inseminations, sperm from which could compete with his own for the fertilisation of the female's eggs. After oviposition, the female initiates termination of the passive phase by swaying reactions which cause the male to dismount. She then flies from the dropping.