Selection on male size, leg length and condition during mate search in a sexually highly dimorphic orb-weaving spider

Abstract

Mate search plays a central role in hypotheses for the adaptive significance of extreme female-biased sexual size dimorphism (SSD) in animals. Spiders (Araneae) are the only free-living terrestrial taxon where extreme SSD is common. The "gravity hypothesis" states that small body size in males is favoured during mate search in species where males have to climb to reach females, because body length is inversely proportional to achievable speed on vertical structures. However, locomotive performance of males may also depend on relative leg length. Here we examine selection on male body size and leg length during mate search in the highly dimorphic orb-weaving spider Argiope aurantia, using a multivariate approach to distinguish selection targeted at different components of size. Further, we investigate the scaling relationships between male size and energy reserves, and the differential loss of reserves. Adult males do not feed while roving, and a size-dependent differential energy storage capacity may thus affect male performance during mate search. Contrary to predictions, large body size was favoured in one of two populations, and this was due to selection for longer legs. Male size was not under selection in the second population, but we detected direct selection for longer third legs. Males lost energy reserves during mate search, but this was independent of male size and storage capacity scaled isometrically with size. Thus, mate search is unlikely to lead to selection for small male size, but the hypothesis that relatively longer legs in male spiders reflect a search-adapted morphology is supported.

Effects of male size and female dispersion on male mate-locating success in Nephila clavipes

“…In our study, there is some suggestion that smaller males lose a higher percentage of their body mass while searching for females, which could be rapidly regained by pilfering prey from the female's web. This trend is consistent with the view that larger males have a lower size-specific metabolic rate (see Foellmer and Fairbairn 2005b).…”

“…There may be stronger selection for N. clavipes, but not necessarily favouring larger males; the mean size of males arriving from distances of 5 and 10 m was smaller than that of males arriving from 1 m, and males that take longer to arrive at the web are generally smaller than those arriving more quickly. Perhaps smaller males are better at escaping predators while in transit (Foellmer and Fairbairn 2005b), or have an energetic advantage because their lower metabolic requirements allow them to spend more time searching for females (see Blanckenhorn 2000). In our study, there is some suggestion that smaller males lose a higher percentage of their body mass while searching for females, which could be rapidly regained by pilfering prey from the female's web.…”

Fecundity and sexual size dimorphism of wolf spiders (Araneae: Lycosidae) along an elevational gradient in the Arctic

“…Furthermore, scramble competition may select for a morphology that begets finding females more efficiently and earlier than other males (Foellmer and Moya-Laraño 2007), although this could also lead to larger males having an advantage (Foellmer and Fairbairn 2005).…”

Do Female Spiders Select Heavier Males for the Genes for Behavioral Aggressiveness They Offer Their Offspring?

“…Foellmer & Fairbairn (2005) found no evidence for the hypothesis that small male size should be selected for during mate search. On the contrary, large males were most successful in one of two populations of Argiope aurantia Lucas 1833 studied.…”