When night never falls: female sexual signalling in a nocturnal insect along a latitudinal gradient

Abstract

The environment can play an important role in animal communication by affecting signal transmission and detection. Variation in the signalling environment is expected to be especially pronounced in widely distributed species, potentially affecting how their signals are detected. Such environmental variability is presumably relevant for sedentary females of a nocturnal capital breeder, the European common glow-worm (Lampyris noctiluca), which produce green light during the night to attract flying males to mate. Being widely distributed in Europe, glow-worm populations are exposed to both rapidly descending, darker summer nights in the south, and slowly dimming, brighter summer nights further north, with the latter potentially posing challenges to the visibility of the female glow. To test how female signalling is affected by latitude, we sampled glowing females during summer nights along a latitudinal gradient in Finland, Northern Europe, and used a novel apparatus to measure the intensity and peak wavelength (hue/colour) of their glow. Surprisingly, females at higher latitudes, similar to those at lower latitudes, were commonly glowing during the brightest (and hence the shortest) nights of the year. Females also glowed brighter in more northern areas, partly due to their larger body size, whereas the colour of their glow was not associated with latitude. Since females glow even during midsummer, independent of latitude, the increase in glow intensity at higher latitudes presumably serves to maintain signal visibility in brighter signalling conditions. Overall, these findings highlight the influence of environmental conditions on the evolution of sexual signals, especially in the context of species distribution range. Significance statement When environmental conditions impact signal transmission and perception, local conditions can have a crucial role in shaping animal communication and signal evolution. To analyse how dark-dependant common glow-worm females cope with variable nocturnal light environments, we used a novel apparatus, presumably not applied to living animals before, to measure female glow intensity at various latitudes along a latitudinal gradient. Interestingly, females did not avoid signalling during the brightest summer nights, but instead, their glow intensity and body size both increased with latitude. These findings suggest that females can ensure visibility to mate-searching males over a range of local conditions. Our study therefore shows how females can adapt to environmental constraints on signal visibility, and how the expression of sexual signals is shaped not only by social interactions but also by the signalling environment.

The duration of artificial light defines sexual signalling in the common glow-worm

“…These results could be explained by larger females being brighter (Hopkins et al 2015) and hence having a higher probability to attract males under artificial light than smaller females (Elgert et al 2021). Conversely, small females may be able to attract mates only during the darkest hours of the night (Borshagovski et al 2020). Given that small females start their glow later, and cease it more easily under light, artificial light shortens the time available for mate attraction relatively more in small than large females.…”

“…Given that small females start their glow later, and cease it more easily under light, artificial light shortens the time available for mate attraction relatively more in small than large females. Similarly, the effects of artificial light on the glow and its initiation may differ across latitudes (Dreisig 1978), with females presumably needing a larger size and brighter glow to attain an adequate visibility in higher latitudes (Borshagovski et al 2020), providing an interesting topic for further research.…”

“…As capital breeders, adult glow-worms have a limited amount of resources to allocate to survival and reproduction, with the fecundity of unmated females quickly decreasing over time (Tyler 2002;Hopkins et al 2021). In Finland, at the northern limit of their range (Borshagovski et al 2020), glow-worm peak reproductive season is in June-July (Borshagovski et al 2020; personal observations), when nights are short. Interestingly, previous research suggests that both female size and the intensity of female glow increase with latitude, presumably as local adaptations to the higher levels of ambient light during summer nights (Borshagovski et al 2020).…”

“…In Finland, at the northern limit of their range (Borshagovski et al 2020), glow-worm peak reproductive season is in June-July (Borshagovski et al 2020; personal observations), when nights are short. Interestingly, previous research suggests that both female size and the intensity of female glow increase with latitude, presumably as local adaptations to the higher levels of ambient light during summer nights (Borshagovski et al 2020).…”

High road mortality during female-biased larval dispersal in an iconic beetle

“…Aside from buildings belonging to the research station at the end of the road, there is a low number (< 10) of houses along the road, and only a short stretch of it has streetlights (~100 m, on one side). The study site was selected due to the logistic convenience of it being close to the research station and because previous studies had showed that glow-worms inhabit the area (Borshagovski et al 2019(Borshagovski et al , 2020Elgert et al 2020Elgert et al , 2021Lehtonen and Kaitala 2020). More generally, the European glow-worm has a wide range from Iran (N32°) to central Finland (N64°) (Samin et al 2018;Borshagovski et al 2020), with the species being on the decline across Europe, as are many other Lampyrid species globally (Lewis et al 2020).…”