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.

The amount of artificial light at night is growing worldwide, impacting the behaviour of nocturnal organisms. Yet, we know little about the consequences of these behavioural responses for individual fitness and population viability. We investigated if females of the common glow-worm Lampyris noctiluca —which glow in the night to attract males—mitigate negative effects of artificial light on mate attraction by adjusting the timing and location of glowing to spatial variation in light conditions. We found females do not move away from light when exposed to a gradient of artificial light, but delay or even refrain from glowing. Further, we demonstrate that this response is maladaptive, as our field study showed that staying still when exposed to artificial light from a simulated streetlight decreases mate attraction success, while moving only a short distance from the light source can markedly improve mate attraction. These results indicate that glow-worms are unable to respond to spatial variation in artificial light, which may be a factor in their global decline. Consequently, our results support the hypothesis that animals often lack adaptive behavioural responses to anthropogenic environmental changes and underlines the importance of considering behavioural responses when investigating the effects of human activities on wildlife.

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.