Water Striders
Total Page：16
File Type：pdf, Size：1020Kb
Se x u a l se l e c t io n a n d intersexual c o n f l ic t s in WATER STRIDERS Göran Arnqvist Umeå 1992 Department of Animal Ecology University of Umeå S-901 87 Umeå Sweden AKADEMISK AVHANDLING som med vederbörligt tillstånd av rektorsämbetet vid Umeå Universitet för erhållande av filosofie doktorsexamen i ekologisk zoologi kommer att offentligen försvaras fredagen den 23 :e oktober 1992, kl. 10.00 i hörsal F, Humanisthuset. Examinator: Prof. Christian Otto, Umeå. Opponent: Dr. Andrew Sih, Lexington, USA. ISBN 91-7174-704-4 Cover by Görel Marklund Organization Document name UMEÅ UNIVERSITY DOCTORAL DISSERTATION Department of Animal Ecology S-901 87 Umeå, Sweden Date of issue October 1992 Author Göran Arnqvist Title Sexual selection and intersexual conflicts in water striders Ab st ract This thesis deals with evolutionary aspects of the reproductive biology of water striders (Heteroptera: Gerridae), a group of semi-aquatic insects. The adaptive significance as well as the evolutionary consequences of a number of reproductive behaviours were studied. Eggs of water striders are parasitized by the parasitic wasp Tiphodytes gerriphagus. Female water striders of some species lay their eggs in groups, whereas others spread out single eggs. A theoretical model was developed to predict the effect of egg laying strategy on the infestation risk. It was predicted that solitary eggs and eggs in large groups should suffer lower infestation risks than eggs in groups of intermediate size. The results of an empirical test of the model were in accordance with the predictions. Male water striders guard the females after copulation. In a series of experiments, where partly sterilized and normal males were used, it was determined that the last male to mate a multiply mated female fertilized approximately 80% of the female's eggs. The postcopulatory mate guarding behaviour exhibited by males was thus interpreted as a paternity assurance strategy, to avoid sperm competition. Female water striders are reluctant to mate, and the sexes engage in a precopulatory struggle where females attempt to dislodge males attempting copulation. Males of the water strider Gerris odontogaster are provided with two abdominal processes, which were shown to function as grasping devices during mating. The length of the processes was subjected to intersexual selection in field populations, and the length of the processes was related to a male's ability to endure the precopulatory struggle. Contrary to predictions, the heritability of the processes was high (h2 =1.01 ± 0.28). The results of a series of experiments showed that there is a basic conflict of interest between the sexes regarding the mating decision and the mating frequency. While males generally benefit from matings in terms of increased paternity, females experience several costs of mating. Mating females suffered three times as high predation risk as did solitary females, and a lower mobility resulting in time/energy costs. However, mated females did not experience any energetic or genetic benefits from rematings, nor did the females benefit from replenishment of sperm supplies. A theoretical optimization model was developed to predict female reluctance behaviour in a sexual conflict situation, and the predictions were tested empirically. It was concluded that female reluctance during the precopulatory struggle represents a general reluctance to mate (intersexual conflict) rather than an adaptive female mate assessment strategy. Further, female reluctance varied with population density and sex ratio, and this variation was predicted to affect the pattern and intensity of sexual selection. Estimates of sexual selection were made in field populations with multivariate statistical techniques, and several traits, including parasite load, body size and secondary sexual traits, were found to experience sexual selection. As predicted, the selective regimes varied considerably between different populations, and the importance of spatial and temporal variations in selective regimes is discussed. Key words Sexual selection, mating behaviour, parasitism, sperm competition, mate guarding, sexual dimorphism, natural selection, sexual conflicts, water striders, Heteroptera, Gerridae, Gerris, Scelionidae, Trypanosomatidae. Language English ISBN 9 1 . 7 1 7 4 . 7 0 4 .4 Number of pages 95 Signature -------------- Date 1992 - 08 - 28 3 C o n t e n t s List of papers ........................................................................................................... 4 Introduction ..............................................................................................................5 Natural history of water striders............................................................................ 5 Patterns of egg deposition and parasitism .............................................................6 Sperm competition and paternity assurance ......................................................... 7 Intersexual conflicts................................................................................................ 8 Conflicts over the mating frequency and the mating decision ................. 8 Precopulatory fighting ..................................................................................9 Sexual selection.....................................................................................................11 The peacock's tail: male abdominal processes ................. 11 The mechanism of selection: intrasexual selection or female choice?. 12 Good genes or nonadaptive female choice? ............................................. 12 The role of parasites .................................................................................... 13 A mechanistic approach: predicting and measuring selection ............... 14 Local adaptation in metric traits ................................................................ 15 Concluding remarks and future prospects ...........................................................16 Acknowledgements ............................................................................................... 18 Literature cited.......................................................................................................20 A p p e n d ic e s : Papers I - VIII 4 - L ist o f p a p e r s - This thesis is a summary and discussion of the following papers, which will be referred to in the text by their Roman numerals: I. Arnqvist, G. and Byström, P.1991. Disruptive selection on prey group size: a case for parasitoids? American Naturalist 137:268-273. II. Arnqvist, G. and Mäki, M. 1990. Infection rates and pathogenicity of trypanosomatid gut parasites in the water strider Gerris odontogaster (Zett.)(Heteroptera: Gerridae). Oecologia 84:194-198. III. Arnqvist, G. 1988. Mate guarding and sperm displacement in the water strider Gerris lateralis Schumm. (Heteroptera: Gerridae). Freshwater Biology 19:269-274. IV. Arnqvist, G. 1989. Sexual selection in a water strider: the function, nature of selection and heritability of a male grasping apparatus. Oikos 56:344-350. V. Arnqvist, G. 1989. Multiple mating in a water strider: mutual benefits or intersexual conflict? Animal Behaviour 38:749-756. VI. Arnqvist, G. 1992. Precopulatory fighting in a water strider: intersexual conflict or mate assessment? Animal Behaviour 43:559-567. VII. Arnqvist, G. 1992. The effects of operational sex ratio on the relative mating success of extreme male phenotypes in the water strider Gerris odontogaster (Zett.) (Heteroptera: Gerridae). Animal Behaviour 43:681-683. VIII. Arnqvist, G. 1992. Spatial variation in selective regimes: sexual selection in the water strider, Gerris odontogaster. Evolution 46:914-929. The papers are reproduced by permission from the publishers. 5 Introduction Ever since Darwin, evolutionary biologists and ecologists have been puzzled by the tremendous diversity and magnitude of sexual dimorphism. Why do males and females typically differ considerably not only in morphology, but also in behaviour during reproduction? Darwin proposed that such dimorphic traits evolved by sexual selection, which "depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction" (Darwin, 1871, p. 256). Darwin also made a distinction between sexual selection due to male-male competition (intrasexual selection) and sexual selection due to female choice (intersexual selection). But even though some major insights were made early in this century (e.g. Fisher, 1930), it is not until more recently that the topic has received increased attention. The study of sexual selection and the evolution of animal mating systems have experienced a very rapid development during the lasts decades, and has grown to be one of the most vivid and productive areas of evolutionary biology (Bradbury and Andersson, 1987; Harvey and Bradbury, 1991). During this period, several seminal papers have generated bursts of empirical and theoretical attention to various conceptual areas of the topic, some of which I will review briefly below. This thesis deals with evolutionary aspects of the reproductive biology of water striders, a group of semiaquatic insects. I have focused on two main questions: (/) to assess the adaptive significance of a set of reproductive behaviours, and (ii) to examine the processes and patterns of sexual selection in natural water strider populations. I will start, however, by giving some basic background information about the biology