Christer Wiklund

The Effect of Flexible Growth Rates on Optimal Sizes and Development Times in a Seasonal Environment

The interrelationships among development time, growth rate, and adult size are investigated using simple optimization models of a seasonal life history in which larger adults have greater reproductive output. Unlike most previous studies, our models assume that growth rate is an adaptively flexible character that can be increased at the expense of a greater juvenile mortality rate. Three components of fitness are considered: the cost of developing at a suboptimal time of the year, the reproductive advantage of larger adult size, and the increased mortality from rapid juvenile growth. The study focuses on the optimal responses of size, development time, and growth rate to changes in the amount of time available for completion of the life cycle. The models show that the optimal growth rate and size at maturity may respond in several different ways. Perhaps the most likely effect is that growth becomes faster and size smaller with less time available. It is also possible, however, for either growth rate or size (but not both) to stay constant; in other cases, less time available leads to slower growth or larger size. The effects of increased mortality on the juvenile stage are also explored; here, the optimal size is likely to decrease, but growth rate and development time may increase or decrease.

The evolutionary relationship between adult oviposition preferences and larval host plant range in Papilio machaon L.

SummaryIn Papilio machaon host plant utilization is argued to be proximally guided by adult and larval preferences, both of which are genetically determined. To decide whether these preferences are controlled by one or two gene complexes, oviposition preferences and larval survival of a Scandinavian population were investigated with regard to all potential host plants in the region. The experiments revealed a substantial difference between the potential host plant ranges of the adults and the larvae, indicating that adult and larval host plant preferences are determined by separate gene complexes.Based on this inference, a theoretical model is developed for the co-evolutionary relationship between adult oviposition preferences and larval feeding preferences for species in which the host plant choice is exercised by the adults.

Adaptive variation in growth rate: life history costs and consequences in the speckled wood butterfly,Pararge aegeria

An important assumption made in most lifehistory theory is that there is a trade-off between age and size at reproduction. This trade-off may, however, disappear if growth rate varies adaptively. The fact that individuals do not always grow at the maximum rate can only be understood if high growth rates carry a cost. This study investigates the presence and nature of such costs inPararge aegeria. Five females from two populations with known differences in life history (south Sweden and Maderia) were allowed to oviposit in the laboratory and their offspring were reared in environmental chambers under conditions leading to direct development. We measured several aspects of life history, including development times, pupal and adult weights, growth rate, female fecundity, longevity and larval starvation endurance. In both populations there seemed to be genetic variation in growth rate. There was no evidence for a trade-off between age and size at pupation. As predicted, larvae with high growth rates also lost weight at a relatively higher rate during starvation. High weight-loss rates were furthermore associated with a lower probability of surviving when food became available again. This is apparently the first physiological trade-off with growth rate that has been experimentally demonstrated. In both populations there were significant differences in growth rate between the sexes, but the populations differed in which sex was growing at the highest rate. In Sweden males had higher growth rates than females, whereas the reverse was true for Madeira. These patterns most likely reflect differences in selection for protandry, in turn caused by differences in seasonality between Sweden and Madeira. Together with the finding that males had shorter average longevity than females in the Swedish, but not in the Maderiran, population, this indicates that a lower adult quality also may be a cost of high growth rate. We argue that for the understanding of life history variation it is necessary to consider not only the two dimensions of age and size, but also to take into full account the triangular nature of the relationship between size, time and growth rate.

Why do males emerge before females

SummaryIn butterflies and many other insects there is a general tendency for males to emerge before females. This is known as protandry. In this paper we advance the hypothesis that protandry is a reproductive strategy of males, resulting from competition for mates, and should primarily occur in species maintaining female monogamy. Our hypothesis is corroborated by applying a mathematical treatment to a theoretical population with seven defined properties, all of which are argued to be reasonable assumptions for natural populations.

Mass and production rate of ejaculates in relation to monandry/polyandry in butterflies

SummaryThe mating system maintained in a species has a strong effect on the degree of sperm competition, and certainty of paternity should accordingly influence the optimal sperm content, nutrient content, and mass of the ejaculate. We investigated how ejaculate mass relates to the degree of polyandry in 20 species of butterflies belonging to the families Pieridae and Satyridae. We found that the degree of polyandry has a substantial effect on the reproductive performance of males. The allometric line between ejaculate mass and male body mass has a higher elevation in the pierids compared to the satyrids. The mean number of matings performed by the pierid species is also higher compared to the mean of the satyrids. Thus, the relative ejaculate mass is larger in the family in which polyandry is more pronounced. A within family effect of degree of polyandry on relative ejaculate mass was also detected in the pierids. Since males of polyandrous species on average mate more often than males of monandrous species, they should be expected to have a higher capacity for producing many ejaculates. We investigated how this capacity was influenced by the degree of polyandry, by allowing males of seven different species (Danaus plexippus, Lasiommata megera, Papilio machaon, Pararge aegeria, Pieris napi, Pieris rapae, and Polygonia c-album) to mate twice, with different time intervals between matings. The results showed that not only is the mass of the ejaculate greater in more polyandrous species, but also the rate at which males are able to produce sperm and accessory substances is greater. Hence our data indicate that sperm competition is important for explaining variation in ejaculate mass in butterflies.

Why do males emerge before females? protandry as a mating strategy in male and female butterflies

SummaryThe reproductive strategy of butterfly males can be defined as being to maximize the number of females mated. We have earlier shown that, if the eclosion period of females is regarded as given, males should emerge before females to achieve maximal reproductive success. However, females may also be considered to have a reproductive strategy with respect to the issue “when to emerge”. In this paper we assume that females are selected to minimize the time spent unmated (to minimize prereproductive death), and analyze when females should optimally emerge in relation to males to achieve this end. We show that there is no conflict between the sexes with respect to the timing of eclosion when the length of the eclosion period is approximately equal for males and females. Thus, protandry should be considered a reproductive strategy of both males and females.

Polyandry and its effect on female reproduction in the green-veined white butterfly (Pieris napi L.)

SummaryIn many insects nutrients transferred by the male to the female at mating are later incorporated into both the eggs and soma of the mated females. Accordingly, it has been suggested that female insects can use these male-derived nutrients both for somatic maintenance and to increase both the number and quality of their offspring. Moreover, much discussion is presently devoted to whether the male nuptial gift represents paternal investment, defined as “any increase in given males total surviving progeny by increasing the reproductive output by a given female”, or mating effort which obtains “if a male gains by increasing the proportion of eggs he fertilizes from a given female” (Parker and Simmons 1989). If the male nuptial gift represents parental investment it should be expected to benefit predominantly the offspring sired by the donor, whereas the “physiological fate” of the male nuptial gift is somewhat irrelevant under the mating effort explanation. In this paper we test these issues by studying the lifetime fecundity, egg weights and longevity of two groups of females of the polyandrous green-veined white butterfly, Pieris napi, one group of which was allowed to mate only once and the other of which was allowed to mate at liberty, the latter group of females mating on average 2.28 times. Moreover, to test the incorporation rate of male-derived nutrients, we performed a second set of experiments where females were allowed to mate with radioactively labelled males. The results showed that polyandrous females had higher lifetime fecundity compared to monandrous females, laying on average 1.61 as many eggs, and that the difference in cumulative fecundity between the two groups was statistically significant from the 5th day of egg-laying onwards. Polyandrous females also lived longer and maintained egg weight at a high level for longer than monandrous females. Largely concomitant with egg-laying rate, incorporation rate of male-derived nutrients peaked 3–4 days after mating, subsequently tapering off to stabilize at about 40% of the maximum. Given the opportunity, female P. napi remated after 3–5 days, the duration of the refractory period being positively correlated with ejaculate mass. Hence, although the nutrient investment of the first male to mate with a female “subsidizes” the progeny of later-mating males, the male nuptial gift in P. napi clearly qualifies as both paternal investment and mating effort.

Proximate Causes of Rensch’s Rule: Does Sexual Size Dimorphism in Arthropods Result from Sex Differences in Development Time?

A prominent interspecific pattern of sexual size dimorphism (SSD) is Rensch’s rule, according to which male body size is more variable or evolutionarily divergent than female body size. Assuming equal growth rates of males and females, SSD would be entirely mediated, and Rensch’s rule proximately caused, by sexual differences in development times, or sexual bimaturism (SBM), with the larger sex developing for a proportionately longer time. Only a subset of the seven arthropod groups investigated in this study exhibits Rensch’s rule. Furthermore, we found only a weak positive relationship between SSD and SBM overall, suggesting that growth rate differences between the sexes are more important than development time differences in proximately mediating SSD in a wide but by no means comprehensive range of arthropod taxa. Except when protandry is of selective advantage (as in many butterflies, Hymenoptera, and spiders), male development time was equal to (in water striders and beetles) or even longer than (in drosophilid and sepsid flies) that of females. Because all taxa show female‐biased SSD, this implies faster growth of females in general, a pattern markedly different from that of primates and birds (analyzed here for comparison). We discuss three potential explanations for this pattern based on life‐history trade‐offs and sexual selection.

Generalist vs. specialist oviposition behaviour in Papilio machaon (Lepidoptera) and functional aspects on the hierarchy of oviposition preferences

Oviposition choice experiments were performed with females from a southern Swedish population of Papilio machaon L. The results show that the females had a hierarchy of oviposition preferences which were in accordance with the suitability of the plants as food for the larvae. There was little variation between females in hierarchical ranking of host plants. However, variation between females was profound with respect to differences in thresholds of acceptance of alternative plants. Thus some females exhibited a generalist oviposition choice strategy and laid eggs on several plants even when the optimal host plant was available, whereas others exhibited a specialist strategy and restricted their egglaying to the optimal host plant as long as it was available. Generalist females also had a tendency to lay eggs on plants that were unsuitable as food for the larvae, whereas specialist females avoided to lay eggs on such plants. It is suggested that the hierarchical arrangement of oviposition preferences may have a dual function, viz. both to ensure that most eggs are laid on the optimal host plant as long as it is present, and make possible the deposition of eggs on sub-optimal host plants when the optimal plant is absent. Thus, the preferential hierarchy may be the mechanism which effects conservatism in host plant choice in habitats to which the butterflies are adapted, and also makes possible innovative change in host plant utilization in novel habitats where the familiar host plants are absent.

Sexual size dimorphism in relation to female polygamy and protandry in butterflies: a comparative study of Swedish Pieridae and Satyridae

In a number of insects male size has been shown to be positively correlated with mating success. Since variance in male mating success should increase with the degree of female polygamy, sexual selection for large male size may be stronger under polyandry than under monandry. To test this hypothesis we correlated male/female sexual size dimorphism, defined as male/female forewing length, with the degree of female polygamy, assessed by counting the number of spermatophores in wild-caught females. Both sets of data were assessed from 23 species of butterflies, 11 in the Pieridae and 12 in the Satyridae. There was a positive correlation between male/ female sexual size dimorphism and female polygamy in both families. Although size is affected by many factors, we contend that this result is best explained by sperm competition; in butterflies, the mass of the ejaculate delivered by a male is positively correlated with male size, and large ejaculates induce a longer period of female unreceptivity, resulting in sexual selection favouring large male size in species where females mate repeatedly. Although there is an obvious conflict between having a short development time and acquiring large size, the expected negative relationship between sexual size dimorphism and protandry was not statistically significant under direct development in the 8 species tested. This suggests that males under direct development achieve both large size and short development time simultaneously by increasing their growth rate. In diapause generations of 14 species (including overwintering generations of the 8 bivoltine species above) there was, however, such a negative relationship. Since protandry in diapause generations is mainly the result of morphogenesis of the adult during postdiapause pupal development, this is consistent with the tendency for small individuals, males, to require shorter morphogenetic development times than larger individuals, females. We found a weak negative correlation between polyandry and protandry, statistically significant only under diapause development. This suggests that mating with virgin females is more beneficial than mating with non-virgins also under female polygamy, and indicates that selection for early male emergence is only weakly relaxed in species where females mate repeatedly.