Nuptial feeding in the scorpionfly <i>Panorpa vulgaris</i> : ultimate and proximate causes

In my thesis I was concerned with ultimate as well as with proximate aspects of nuptial feeding in the scorpionfly Panorpa vulgaris (Mecoptera: Panorpidae). Chapter I begins with the attempt to verify the ultimate function of nuptial gifts in this species. Nuptial feeding is a common strategy shown by males of many insect species. These gifts presented during or after courtship and/or copulation are mostly considered to represent a form of mating effort. During copulations males of P. vulgaris produce salivary secretions which are then consumed by the females. Since females adjust copulation duration and thereby the number of received sperm to the number of salivary masses they receive from a male, the gifts' function as mating effort is unquestionable. Here I present data which indicate that nuptial feeding in P. vulgaris also represents paternal investment. Receiving a high number of salivary masses causes females to lay significantly more eggs compared to females receiving few or no salivary secretions. Thus, in P. vulgaris the nuptial gift increases the reproductive output of females and hence must not only be considered as mating effort but also as paternal investment. However, I cannot decide whether the increase in egg production is caused by an incorporation of additional nutrients or by allohormones. In chapter II I leave the ultimate level to deal with proximate factors affecting salivary gland development and nuptial gift production during matings. In mating systems that are characterised by resource dependent male behaviour like nuptial feeding, food limitation obviously plays a major role in male performance. In P. vulgaris the ability to produce nuptial gifts, however, implies major fitness consequences, as the number of gifts decides about copulation duration. Since the number of transferred sperm increases with ongoing mating duration and sperm of different males are utilised according to the fair raffle principle, nuptial gift production determines male mating success. The results presented in chapter II show that males of P. vulgaris are limited in their production of salivary secretions. The number of saliva secretions males are able to produce declines in successive matings. Moreover, males of nutritionally high status produce more gifts than those of nutritionally low status. The degree of male mating effort corresponds to the size of the salivary gland, yet while absolute investment increases with gland size, the relative investment decreases. Thus, the marginal costs of saliva production are differential for males of different nutritional status. This result provides further evidence of the nuptial gift's function as a Zahavian quality indicator. Furthermore, I found evidence that males of low nutritional status seem to allocate their mating effort strategically according to the female's fecundity. Therefore, cryptic male choice may exist in P. vulgaris, but only below a certain quality threshold of males. In the last experiment, presented in chapter III, I manipulated food availability throughout the animals' entire life history. Here I am concerned with the question whether environmental conditions met during an early phase of life history that is dominated by the natural component of selection will affect traits and behaviour in a sexual selection context later on in life. I once again found strong effects of nutrition during adulthood on various fitness relevant traits (e.g. salivary gland development, saliva investment in copulations). But moreover, I am able to show that food availability during larval development affects male body weight, salivary gland weight before and after a copulation and the proportion of saliva resources that is invested in a mating. Therefore, larval feeding history implies considerable long time consequences and affects several traits that become important in a sexual selection context later on in male life history

Nutrition- and sex-dependent utilization of body resources in relation to reproduction in a scorpionfly.

Engqvist L, Sauer KP, Engels S. Nutrition- and sex-dependent utilization of body resources in relation to reproduction in a scorpionfly. Biological Journal of the Linnean Society. 2011;102(1):199-207.Reproduction often comes at a cost of a reduction in body functions. In order to enhance their reproductive output, some insect species degenerate their thoracic muscles, typically resulting in reduced flight ability. From a life-history trade-off perspective, we expect the importance of body resource utilization to be amplified both with increased reproductive expenditure and with increased resource limitation. In this study, we measured age-related changes in thorax weight, as a measure of flight muscle size, during a major part of the adult lifespan in males and females of the scorpionfly Panorpa vulgaris. The aim of the study was twofold: first to investigate whether scorpionflies have the potential to degenerate their flight muscles; second, and more importantly, to determine whether the magnitude of flight muscle degeneration is a plastic response in relation to resource availability, and if it differs between the sexes. The results clearly demonstrate that food availability does influence investment in flight muscle development. The build-up of the thoracic muscles was strongly influenced by nutrient availability. Furthermore, the age-related decrease in thorax weight was significantly different for males and females. Only females showed a strong age-dependent decrease in thorax weight, indicative of muscle degeneration, yet no difference between food treatments was detected. For males, there was no significant directional change in thorax weight. Nevertheless, with increasing age, the difference in thorax weight between food treatments increased significantly

Sexual coercion in Panorpa scorpionflies?--The function of the notal organ reconsidered

Conflict between the sexes over the occurrence and timing of copulation is a common feature of sexual reproduction. In Panorpa scorpionflies (Mecoptera, Panorpidae), sexual conflict occurs over mating duration, and it has been suggested that the notal organ--a clamp-like structure on the males' abdomen--is an adaptation to coerce mating duration, enabling males to prolong mating against female interests. However, as recent studies suggest that female scorpionflies have considerable control over mating decisions, we question this hypothesis and reexamined the function of the notal organ with respect to prolonged mating. We manipulated the notal organs of male Panorpa vulgaris and compared copulation durations achieved by males with functional and nonfunctional notal organs. There were no effects on the overall copulation duration or the period of time a copulation continues after the last nuptial gift has been delivered. We therefore reject the hypothesis that the notal organ is a male adaptation to extend copulation duration against female interests. Copyright 2009, Oxford University Press.