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- Bergström, Jonas, 1970-
(author)
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The evolution of mating rates in Pieris napi
- 2004
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Doctoral thesis (other academic/artistic)abstract
- In the green-veined white butterfly (Pieris napi), females obtain direct fitness benefits from mating multiply and studies have shown that fitness increases seemingly monotonically with number of matings. The reason is that at mating males transfer a large nutritious gift (a so called nuptial gift) to the females that the females use to increase both their fecundity and lifespan. In addition, if exposed to poor food conditions as larvae, females mature at a smaller size compared to males. Accordingly, it was suggested that smaller females could compensate for their size through nuptial feeding by, for instance, mating more frequently. We did not find any support for that hypothesis. On the contrary, larger females remated sooner and had a higher lifetime number of matings. Neither were smaller females able to compensate in any other way, because singly mated females and multiply mated females suffered to the same extent from their smaller size. This thesis also shows that despite the positive relationship between fitness and number of matings, there is a large variation in female mating frequency in wild populations and about every second female mates only once or twice. This variation is not dependent on how often females get courted by males, because female mating frequency was shown not to be affected by male courtship intensity. Hence, the reason for the low mating frequency could either be that males have evolved the ability to manipulate females to mate at a suboptimal rate as a measure of protection against sperm competition, or alternatively, that female mating rate is suppressed by some costs. Using two selection lines, artificially selected for either a high or a low mating rate, we showed that the variation in mating rate was mainly a female trait because which line the females were from affected their mating rate whereas which line the male was from did not. This implies that females mate at a low rate due to hidden costs or due to constraints. The same study also showed that females with a high "intrinsic" mating rate lived shorter, but only when denied remating. This led us to test the hypothesis that the cost females face is to have the ability to mate at a high rate but the cost is only paid when remating opportunities are scarce. However, we found no support for such an idea, because females with a high intrinsic mating rate held in a cold environment where the butterflies were prevented from flying and feeding did not live shorter. Neither was there an effect of a female’s mating rate on her ability to quickly break down and convert male nutrient gifts into egg material. Female mating rate did, on the other hand, affect dispersal tendency, with low mating rate females being more inclined to fly between different habitats. The underlying reason for this is still to be explored.
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2. |
- Sjöberg Lindfors, Virpi, 1963-
(author)
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Butterfly life history and mating systems
- 1998
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Doctoral thesis (other academic/artistic)abstract
- The general hypothesis tested in this thesis is whether butterfly mating strategies are affected by their life history. Several components of mating behaviour, morphological and physiological traits were studied and related to various life histories. Male and female interests are often in conflict. Males maximise their reproductive output by mating repeatedly, whereas females maximise their reproductive output by soliciting matings to obtain viable sperm and male donated nutrients, but avoiding harassment by males while searching for host plants.Male butterflies increase their reproductive output either by delaying, or preventing, their mates from remating, because in most species the latest male to mate with a female has sperm precedence. One way in which males can delay or prevent female remating is by delivering a large spermatophore at mating. There are two ways in which males can increase ejaculate mass: 1) by growing larger - since large males produce larger ejaculates, or 2) by feeding as adults and mate at an older adult age. These two strategies are in conflict, since a larger size results from growing for a longer time as a larva. We show that large males rather than older males deliver the heaviest ejaculates. Therefore the strategy males should adopt is dependent on the mating system. In monandrous species early male emergence is especially important, since receptive females will be available during a short period. In polyandrous species it may be more beneficial to deliver large ejaculates that delay female remating; hence polyandrous males should grow for a longer time as larvae.Males in polyandrous species deliver heavier ejaculates, and polyandrous females incorporate male donated nutrients in their eggs, and increase their reproductive output by remating. However, a comparative study of seven butterfly species failed to provide evidence that polyandrous butterfly females in general are more efficient in sequestering nitrogen from spermatophores than are monandrous females.
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