Náttúrufræðingurinn

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Náttúrufræðingurinn - 1997, Síða 25

Náttúrufræðingurinn - 1997, Síða 25
this century is given. Then the main findings of G. A. Parker’s great contribution lo the research on this species is summerized. The rest of the paper describes the results of the author who studied size variation in the yellow dungfly, how it arises and the effect it has on male mating tactics. Size variation is mainly caused by larvae competition for food. Environmental factors are therefore very inlluential in shaping adult size, but the role of genetic factors cannot be ruled out completely as Simmons and Ward (1991) have shown. Population fluctuations are marked with spring and autumn peaks in most countries. In Iceland there is no summer de- cline and density is very high. Size of males is greatest in August and September. Males seem to be able to assess their size and act accordingly. They are not distributed randomly around oviposition sites; instead, the larger males are more likely to search and copulate on cow pats than in the surrounding grass, and they are more likely to guard their females until oviposition is completed. This distribution is not influenced by pat age, at least up to 80 min. old droppings. Analysis of behaviour of individual males re- vealed that large males spend more time than small nrales on the pat than in the grass and increased densily makes larger males more ac- tive while smaller males are not affected. More large males caught females than small males in spite of being less numerous. Density affects males’ behaviour in many ways; they touch each other more, make more rnoves, spend relative more time on the pat than in the grass and spend more time searching at each oviposition site. Staytime (searching time) of individual males was not dependent on size. Their fighting tactics are shaped by their RHP and the value of the fernale. Recent research by Geoffrey Parker and associates has further re- vealed that copula duration is dependenl on phenotype and larger males are at an advan- tage. It is concluded that larger males are more successful in gaining females and defending ihem in fights. The result is that large males are more likely than the small males to be paired to large females. Pairing is size assortative both in the grass and among ovipositing pairs. The large rnales are clearly at an advantage in competition for females while the smaller males play the best of a bad job. The extensive research that has been carried out on the reproductive behaviour of this spe- cies shows how impressive the forces of evolu- tion can be when shaping the behaviour of dif- ferent phenotypes. PÓSTFANG HÖFUNDAR/AuTHORSADDRESS Hrefna Sigurjónsdóttir Kennaraháskóla íslands v. Stakkahlíð, IS-105 REYKJAVÍK Netfang/E-mail hrefnas@khi.is 19

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