Negatively condition dependent predation cost of a positively condition dependent sexual signalling.

Predation is considered as an important factor constraining the expression of sexual signals. Nevertheless, direct quantitative evidence for predation provoking significant viability costs on individuals signalling at high rates is scarce. Moreover, it is unclear whether high rate signallers are able to balance presumably increased predation costs. We examined whether a condition dependent audible sexual signal, drumming, makes Hygrolycosa rubrofasciata male spiders more prone to predation by pied flycatchers (Ficedula hypoleuca), and whether sexual signalling rate is related to escaping ability once attacked. When birds were given a choice between two spider males manipulated to drum either one or three bouts per minute using playbacks, naïve birds attacked the males randomly regardless of the drumming rate. However, experienced birds chose significantly more often the males with high signalling rate. When spiders were allowed to escape, males with high sexual signalling rate tended to be better at escaping attacks than males with low sexual signalling rate. This study provides evidence that high signalling rate increases the risk of predation, but simultaneously males with high mobility, which correlates positively with signalling rate seemed to be better at compensating this cost.

A trade-off between sexual signalling and immune function in a natural population of the drumming wolf spider Hygrolycosa rubrofasciata.

The field of ecological immunology is ultimately seeking to address the question 'Why is there variation in immune function?' Here, we provide experimental evidence that costs of ubiquitous sexual signals are a significant source of variation in immune function. In the mating season, males of the wolf spider Hygrolycosa rubrofasciata drum against dry leaves while wandering around the habitat searching for receptive females. According to a previous study, the male metabolic rate during the drumming increases 22-fold compared to the resting metabolic rate. In the present study, we examined whether investment in costly courtship drumming decreases male immune function in a wild population of H. rubrofasciata. We induced males to increase their drumming rate by introducing females in proximity. As estimates of male immune function, we used lytic activity and encapsulation rate. Lytic activity estimates the concentration of antimicrobial peptides in haemolymph, which have been shown to play an important role in defence against bacteria, viruses and fungi. Encapsulation is an important defence mechanism against nematodes and insect parasitoids, but it also plays a role in defence against viruses. Our results show that males with nonarbitrarily increased investment in drumming rate had considerably lower lytic activities than control males. Also, there was a tendency for males with nonarbitrarily increased investment in drumming rate to have lower encapsulation rates than control males. This study provides experimental evidence for the first time, to our knowledge, that there are direct immunological costs of sexual signalling in natural populations. Therefore, immunological costs of sexual signals may provide significant phenotypic variation to parasite-mediated sexual selection.