In many species, males rely on sexual ornaments to attract females. Females, by contrast, rarely produce ornaments. The glow-worm ( Lampyris noctiluca ) is an exception where wingless females glow to attract males that fly in search of females. However, little is known about the factors that promote the evolution of female ornaments in a sexual selection context. Here, we investigated if the female ornament of the glow-worm is a signal of fecundity used in male mate choice. In support of this, we found brightness to correlate with female fecundity, and males to prefer brighter dummy females. Thus, the glow emitted by females is a reliable sexual signal of female fecundity. It is likely that male preference for the fecundity-indicating ornament has evolved because of large variation among females in fecundity, and because nocturnal males cannot directly assess female size and fecundity. These results indicate that female ornamentation may evolve in capital breeders (i.e. those in which stored resources are invested in reproduction) when females vary significantly in fecundity and this variation cannot be assessed directly by males.

When individuals differ in attractiveness, less attractive ones may fail to mate. In the common glow-worm, females glow to attract flying males, which prefer brighter females. We show that females move away from more attractive competitors, possibly to avoid comparison. In nature, females glowed far from each other. As distance between females may affect how males perceive female attractiveness, dim females may appear bright if close by and if comparison is not possible for males.

Because male mating success usually varies more than that of females, males typically gain more from investment into sexual ornaments (Clutton-Brock, 2007;Kokko & Jennions, 2008;Tobias et al., 2012). Sexual ornamentation is also thought to be much rarer in females because of their lower potential reproduction rates and typically higher overall costs of reproduction (Bateman, 1948;Fitzpatrick et al., 1995;Kokko & Monaghan, 2001), meaning that selection on male and female ornaments can fundamentally differ. In this respect, ornamentation in females is expected to evolve only if its costs are low (Tobias et al.

Theory predicts that because costs constrain female sexual signaling, females are expected to have a low signaling effort that is increased with passing time until mating is secured. This pattern of signaling is expected to result from females balancing the costs associated with a higher than optimal signaling effort and those costs associated with a low signaling effort that increase the likelihood of delayed mating. We tested whether this prediction applies in the common glow-worm Lampyris noctiluca (Coleoptera, Lampyridae), a capital breeding species in which females glow at night to attract males. Contrary to predictions, we found that the duration of female sexual signaling significantly decreased with time. Moreover, when females experienced multiple light/dark cycles within 24 h, both signaling duration and intensity significantly decreased. These results imply that females attempt to signal as much as possible at first, with the decrease in signaling duration and intensity likely being due to female resource depletion. Because in capital breeding females the costs of a delayed mating are likely greater than the costs of sexual signaling, females should mate as soon as possible and thus always invest into signaling as much as possible.