Evolutionary Studies of the New Zealand Coastal Mosquito Opifex Fuscus (Hutton) Ii
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EVOLUTIONARY STUDIES OF THE NEW ZEALAND COASTAL MOSQUITO OPIFEX FUSCUS (HUTTON) II. COMPETITION FOR MATES by ELISABETH SLOOTEN and DAVID M. LAMBERT1)2)3) (Marine Research Laboratory, R. D. Leigh, and Evolutionary Genetics Laboratory, Department of Zoology, University of Auckland, Private Bag, Auckland, New Zealand) (Acc.15-IV-1983) Introduction As pointed out by ALEXANDER (1975), LLOYD (1979) and THORNHILL (1980), few data exist on the question of non-random mating with.in mating aggregations of insects. This is especially true of species in which males do not control any resources important for female reproduction (e.g. territory, oviposition site, food etc. ). The mating swarms of the mos- quito Opjfiex fuscus and many other species of Diptera fall in this category. Aspects of the general mating behaviour of a few swarming Diptera have been described (e.g. ALCOCK, 1973; DOWNES, 1969; THORNHILL, 1976a, 1976b, 1980). However, little is known of the possible operation of selec- tion in the mating swarms of such dipteran species. This is surprising in view of the fact that the sex ratio in dipteran mating swarms is typically very unequal (DowNES, 1969), making them particularly important for detailed study. In an earlier paper (SLOOTEN & LAMBERT, 1983) we described the unusual mating behaviour of the New Zealand coastal mosquito Opjfiex fuscus. The males of this species form mating aggregations on the water surface of the pools from which the adults emerge. They approach pupae which are close to emergence (and briefly at the surface to breathe), and grasp these using their well-developed front legs. They then hold on to the 1) Publication No. 3 from the Evolutionary Genetics Laboratory, University of Auckland. 2) We thank Brian McARDLE, Steve DAWSONand Randy THORNHILLwho read previous drafts of the manuscript. The research was partly funded by the Auckland University Research Committee Grant number 141Z129 to D.M.L. and by the Depart- ment of Zoology, University of Auckland. 3) For reprint requests. 2 pupa with the genital claspers for some 5-15 minutes, until the younger individual emerges. If the latter is a female, copulation begins just before she has completed emergence. If the emerging individual is a male, they will break off the interaction as soon as the genitalia come into contact, just before emergence is completed. Males cannot discriminate between male and female pupae untill genital contact is made. The females thus mate as early as possible in the adult stage of the life cycle. Opifex males also become sexually active relatively early in adult life compared to other mosquito species (when 6-24 hours old, PROVOST & HAEGER, 1967) although not as early as the females. As is apparently the case for most mosquito species, the females mate only once while the males will mate with several females if given the opportunity. Hence, males compete for access to females. The Operational Sex Ratio (OSR, the average ratio of sexually active and fertilizable females to sexually active males at any given time, see EMLEN & ORING, 1977) was found to be approximately 1 female to 260 males (SLOOTEN & LAMBERT, 1983). Such a marked bias in the OSR creates a strong potential for polygamy and a greater variance in reproductive success among males (see EMLEN & ORING, 1977). In our earlier paper (SLOOTEN & LAMBERT, 1983) we speculated on the possible evolutionary history of the mating behaviour of Opifex fuscus. We put forward the hypothesis that early mating and pupal grasping by males have been favoured by selection caused by competition among males for mates. We argued that the strongly skewed OSR causing such intense competition among males would be likely to have resulted in selection for males which are better able to gain access to mates (for ex- ample, males that are able to mate younger or with younger females, or males that are more successful in competitive interactions over access to females). We argued that support for our hypothesis could come from an in- vestigation of competition between Opifex males, assessing the possible ac- tion of selection on the mating system today, and from information about other insect species with similar mating systems. Here we present evidence for non-random mating among Opifex males and discuss our results with respect to other insect species with similar mating systems. In order to investigate possible non-random mating in Opifex fuscus, the size of males found in in the field was to the size of ran- copulation compared ' domly caught males, not in copulation. Materials and methods The body size of males observed in the field varies considerably. For example, winglengths of males caught in the same area on the same day were typically found to .