Condition-dependence of male sexual signalling in the drumming wolf spider Hygrolycosa rubrofasciata

Abstract

Jari Ahtiaisen väitöskirjatyön tutkimusten perusteella voidaan todeta, että H. rubrofasciata susihämähäkkinaaraat voivat käyttää koiraan seksuaalista kuntoa mittana arvioidessaan koiraan fenotyyppistä ja geneettistä laatua. Näyttäisi myös siltä, että seksuaalivalinnalla on merkittäviä populaatiotason vaikutuksia. Eläinten seksuaaliominaisuudet voivatkin tulevaisuudessa tarjota uusia keinoja populaatioiden sukupuuttouhan ja elinkyvyn arvioimiseen.

The ´good genes´ or ´handicap´ model of sexual selection focuses on the evolution of female preferences for male sexual traits by using condition-dependence as the mediating mechanism. As the ´size´ of the preferred sexual trait increases, male fitness increases in terms of mating success, but this is traded-off with viability which decreases as the ornament size increases. Males in poor condition have lower optimum level of the ornament expression than males in good condition. Ultimately, females that base their choice on the size of condition-dependent male sexual traits benefit in terms of increased offspring viability (i.e. `good genes´). In the wolf spider Hygrolycosa rubrofasciata, males have a costly and condition-dependent acoustic signal, courtship drumming, which is the target of female choice. Males with higher drumming rate have considerably higher viability than males with lower drumming rate, and females that mate with the more actively drumming males gain genetic benefits in terms of increased offspring viability. Also, male mobility is positively associated with male mating success. In this thesis, I tested if higher immunocompetence level can partly explain the better viability of males with high sexual performance. I also investigated if males with high sexual activity can compensate costs of increased risk of predation. Further, I examined if fluctuating symmetry can be used as a short-cut measure of male quality. Finally, I studied if sexual selection for viable males has any population-level consequences. My results show that males with higher drumming rate had higher encapsulation rate than males with lower drumming rate, and males with higher mobility had higher lytic activity than males with lower mobility. I also found that males with increased investment in drumming rate had lower lytic enzyme activity than control males. However, encapsulation rate was higher among males with increased investment in drumming rate. This might be due to males investing their immune effort to components of the body where it is likely to be most useful. My results suggest that conspicuous sexual signalling increases the risk of predation, but males with high sexual activity might be better in escaping from predation than males with low sexual activity. My results indicate that fluctuating asymmetry was negatively, but weakly related to male sexual performance at the population level. Finally, my results show that male survival and drumming rate were substantially reduced only in the smallest wolf spider populations. This suggests that the indirect benefits of female choice in terms of `good genes´ might cover the reduction in viability in small populations. The results of this thesis suggest that, in the drumming wolf spider H. rubrofasciata, sexual traits are related to several measures of individual quality through ´good genes´, and that sexual selection may have significant consequences at the population level.