Female Sexual Signaling in a Capital Breeder, the European Glow-Worm Lampyris noctiluca

J Insect Behav https://doi.org/10.1007/s10905-020-09763-9 Female Sexual Signaling in a Capital Breeder, the European Glow-Worm Lampyris noctiluca Gautier Baudry & Juhani Hopkins & Phillip C. Watts & Arja Kaitala Received: 3 February 2020 / Revised: 29 November 2020 / Accepted: 22 December 2020 # The Author(s) 2021 Abstract 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 glowworm 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. G. Baudry (*) : J. Hopkins : A. Kaitala Ecology and Genetics Research Unit, University of Oulu, PO Box 3000, FI-90014 Oulu, Finland e-mail: P. C. Watts Department of Biological and Environmental Science, University of Jyväskylä, 40014 Jyväskylä, Finland Keywords Sexual signaling costs . Female sexual selection . Female sexual signaling . Capital breeding . Lampyridae . Lampyris noctiluca Introduction The processes that promote or constrain sexual signaling by females remains poorly understood compared with our understanding of the drivers of male ornamentation (Rosenqvist and Berglund 1992; Amundsen 2000; Clutton-Brock 2009; Edward and Chapman 2011; Tobias et al. 2012). While male reproductive success is usually limited by the quantity of mates, female reproductive success has been thought to be limited by access to resources or mate quality. This has led to the acceptance of the Bateman gradient as an explanation to the differences in sexual signaling between males and females, whereby males increase their reproductive success with sexual signaling, while females make their choice based on the quality of the male that can provide the best resources (Bateman 1948; Cunningham and Birkhead 1998). However, the Bateman gradient fails to explain recent evidence that female mate attraction is more common than traditionally recognized (CluttonBrock 2009), which highlights the need to develop an understanding of sexual signaling in females. Female sexual ornaments are expected to evolve when females have limited access to males (Levitan 2004; Rhainds 2010). However, investment into sexual ornaments is often traded off against investment into other components of female fitness such as J Insect Behav fecundity. Thus, females that use sexual signals are expected to evolve strategies to minimize the associated cost of signaling. For example, female moths can attract males by emitting pheromones and balance the costs and benefits of sexual signaling by increasing the signaling effort as they age (Umbers et al. 2015). Signaling females are also expected to gather information about whether their reproductive success is likely to be sperm limited and to adjust their signaling effort accordingly (Umbers et al. 2015). Thus, female signaling effort is expected to be lower when male density is high, and, conversely, higher when male density is low. When signaling females have no information about male density, the only cue on male density is the time a female spends signaling unsuccessfully (Umbers et al. 2015). Consequently, females are predicted to start their signaling effort at low levels to minimize signaling costs and increase their signaling effort with time until a mating is secured (Rhainds 2010, 2019; Umbers et al. 2015). Life-history constraints may affect the trade-offs between signaling, fecundity and maintenance. Many species can continually acquire resources that are then used either for maintenance or reproduction, whereas capital breeders finance their reproduction with resources acquired before their reproductive bout, for example during the larval stage in many capital breeding insects (Stearns 1989, 1992; Bonnet et al. 1999, 2002; Jervis et al. 2005). In capital breeders, this resource limitation may constrain signaling, fecundity and self-maintenance until successful reproduction. Hence, capital breeding females should balance their resource allocation into sexual signaling in relation to the resources they had been able to gather prior to reproduction. Given the earlier findings (Rhainds 2010, 2019; Umbers et al. 2015) we hypothesized that female signaling effort (measured as glowing intensity and duration) in a capital breeder is initially low and then increases with time, as long as the female has not mated. We tested this hypothesis using the European common glow-worm, a capital breeding species in which wingless and sedentary females glow at night to attract flying males. For this purpose, we (1) measured glowing of unmated females in the laboratory and (2) modified day/ night cycles to manipulate female glowing effort. We quantified variation in signaling (glowing) duration and intensity for each female and each treatment during five nights. Materials and Methods Study Species The common glow-worm is a lampyrid beetle in which the predatory larvae mostly feed on snails, whereas adults do not feed and are principally concerned by reproduction (Tyler 2002). Wingless adult females glow at night to attract males, while males fly in search of glowing females (Tyler 2002). Females usually stop glowing soon after mating and lay their eggs within a day (Tyler 2002). In the wild, females glow 1–3 h per night until they mate, with about half of the females glowing for one night only and most females (95%) glowing for four nights or less (Dreisig 1978; Tyler 2002; Hickmott and Tyler 2011). Females that fail to mate on a given night will glow every successive night until they mate. Therefore, we can assume that the amount of time a female needs to glow is determined by her success in mate attraction. Because the common glow-worm is a capital breeder (adults do not feed Grassé 1949; Tyler 2002), glow-worm reproduction is fueled with a finite amount of stored energy reserves that cannot be replenished. The exact costs of continuous glowing are not known, but in related Photinus (Lampyridae) species, the metab (...truncated)