Male mate choice as differential investment in contest competition is affected by female ornament expression

Risky business: males choose more receptive adults over safer subadults in a cannibalistic spider

Understanding factors affecting male mate choice can be important for tracking the dynamics of sexual selection in nature. Male brown widow spiders (Latrodectus geometricus) mate with adult as well as immature (subadult) females. Mating with adults involves costly courtship with a repertoire of signaling behaviors, and typically ends with cannibalism ("self-sacrifice" initiated by male somersault). Mating with subadults involves brief courtship with behavioral repertoire reduced to one component (vibration) and no cannibalism. We examined male mate choice as a function of risks associated with different types of mates and the cues available to courting males. Previous studies showed male preference for adults based on air-borne pheromones, but it was unclear whether that preference is maintained after males reach female's webs. We show that males prefer adults also based on silk-borne contact cues. To determine which types of cues trigger different courtship components, we swapped adults and subadults between webs. We showed that contact with adult females' webs triggers two courtship behaviors from the repertoire, with adult female's bodies triggering additional behaviors. However, vibrational signals occur regardless of the web origin or female developmental stage. We conclude that males recognize subadult females as potential mates, but are more likely to invest in costly courtship behaviors and mating attempts with adults. In our experiments, subadults were less likely to mate than adults. We conclude that mating with adults could be the preferred option for males because of the higher likelihood of copulation, even at the cost of a higher risk of cannibalism.

Mate-guarding duration is mainly influenced by the risk of sperm competition and not by female quality in a golden orb-weaver spider

Males are expected to mate with as many females as possible, but can maximize their reproductive success through strategic mating decisions. For instance, males can increase their own fitness by mating with high quality females that produce more offspring. Additionally, males can adjust mating effort based on the relative distribution of females and male competitors. To test factors that influence male mate choice, we assessed male mating decisions in the golden silk orb-weaver spider, Trichonephila clavipes (Nephilidae), a species in which females are polyandrous, males guard females before and after copulation occurs and large males are the most successful at guarding mates. We tested the hypothesis that males spend more time guarding high quality females that are spatially isolated, and when the risk of sperm competition is higher. We also hypothesized that this effect increases with male body size. We assessed solitary and aggregated female webs in the field and quantified female quality (i.e., female body condition), male size (i.e., male body size), the risk of sperm competition (i.e., number of males in each female web), and mate-guarding duration (i.e., number of days each male spent in each web). We found that mate-guarding behaviour is largely influenced by the presence of male competitors. In addition, male body size seems to moderately influence male guarding decisions, with larger males guarding for a longer time. Finally, female body condition and type of web (i.e., solitary or aggregated) seem to play small roles in mate-guarding behaviour. As mate-guarding duration increased by 0.718 day per each additional male competitor in the web, and guarding behaviour prevents males from seeking additional mates, it seems that guarding females can be considerably costly. We conclude that failing to guard a sexual partner promotes high costs derived from sperm competition, and a male cannot recover his relative loss in fertilization success by seeking and fertilizing more females. In addition, the search for more sexual partners can be constrained by possible high costs imposed by weight loss and fights against other males, which may explain why the type of web only moderately influenced male mate choice. Following the same rationale, if high-quality females are not easy to find and/or mating with a high-quality female demands much effort, males may search females and guard them regardless of female quality. In conclusion, the factor that most influences male mate-guarding behaviour among T. clavipes in the field is the risk of sperm competition.

Web wars: males of the golden orb-web spider invest more in fights for mated females

In addition to resource value, the cost of finding mates may affect how much males invest in fights for females. The cost of finding females may be imposed through natural factors extrinsic to males, such as female spatial distribution and predation pressure, which can be challenging to simulate in laboratory conditions. Therefore, studies under natural conditions may be suitable for understanding how the costs of finding mating partners affect male investment in fights. We used the spider Trichonephila clavipes to evaluate the hypotheses that males in field conditions invest more in contests for access to 1) unmated and 2) more fecund females and 3) when access to females is harder. To test these hypotheses, we recorded the occurrence, duration, and escalation of induced contests between males located in webs of females that differed in reproductive status (estimated by female life stage), fecundity (estimated by female abdominal area), and spatial distribution (i.e., isolated or aggregated with webs of other females). The occurrence and duration of contests were unrelated to female value or search costs. However, the probability for escalation was higher when males were fighting for adult (and probably mated) females. We also found that males tended to start a contest more often in aggregated webs. These results indicate that males of T. clavipes adjust investment in contests but contrarily to what we expected. We suggest that males invest more in contests for adult females because they are defending females that they previously fertilized to avoid sperm competition.

Wildfire as a natural stressor and its effect on female phenotype and ornament development

Controlled low-intensity fires are commonly used in ecosystem management for both habitat restoration and wildfire management. Animals in those ecosystems may respond to fire by shifting energy allocation away from reproduction and growth, and toward maintenance. Stress-induced shifts in energy allocation may affect the expression of condition-dependent sexual signals, which are sensitive to energetic and physiological trade-offs mediated by glucocorticoids. Here, we examine the effect of fire on ornament expression, corticosterone, and other phenotypic traits in a population of striped plateau lizards, Sceloporus virgatus, affected by the Horseshoe 2 Fire in the Chiricahua Mountains, Arizona, USA. The condition-dependent female ornament was significantly smaller the month following the fire than 2 years prior and was both smaller and less orange on the burned site relative to a nearby unburned site. These patterns are similar to those found in a previous experimental study examining the response of the ornament to corticosterone manipulations. Yet, in the current study, corticosterone levels were not different in lizards on the burned and unburned sites. Perhaps glucocorticoid levels already returned to baseline, or do not adequately track environmental change. Females tended to be smaller and lighter on the burned site than the unburned site; however, the year after the fire, body condition was higher for females on the burned site, indicating a rapid recovery and potential long-term benefits in response to low-intensity fires in this fire-adapted ecosystem. We found that the lizards adjusted energy allocation away from sexual signaling and growth in response to low-intensity fires. As fires and fire management are likely to increase in response to changing fire regimes across the globe, it will be important to consider behavioral and physiological responses of impacted species, as well as population-, community-, and ecosystem-level responses.