Effects of social and visual environments on female sexual signaling

Abstract Animals pass information about their sexual availability and quality as a mate with sexual signals. The evolution of sexual signals is also affected by the preference of choosing mates and the properties of the signaling environment. Under the effects of these factors, signalers often have to compete over the limited number of potential mates, leading to the most attractive individuals to be more likely selected than others. However, those with lower mate attraction success may take advantage on other signalers or the signaling environment to increase their mate attraction probabilities. Although sexual signals and competition are more prominent in males than in females, females have a wide range of sexual signals they may use in competition over mates. In my thesis, I study how the social and visual environment affects female sexual signaling. I use field and laboratory experiments, as well as field measurements, to investigate how the light environment and the relative signal intensity of a competitor affect the use and properties of a visual female sexual signal. As my study species, I use the European common glow-worm (Lampyris noctiluca) whose flightless females glow at night to attract flying males. Large females produce brighter glow, which is preferred by males. The results show that female sexual signals are shaped by both social and visual environments, but also by the characteristics of signalers. A rival affected the signaling location, as females increased the distance to a brighter competitor. A dimmer rival did not affect signaling location, but the onset of glowing was significantly later when the distance to the rival at the start of the experiment was shorter. Moreover, with a competitor, the onset of glowing depended on body size so that large females started to glow earlier. Body size and glow intensity increased towards the north, which likely maintains the female visibility to males in the bright summer nights of higher latitudes. My results suggest that females may be able to assess their attractiveness in relation to others and change their signaling behavior accordingly. Signaling environments, in turn, may cause selection pressure to increase visibility to males, causing a latitudinal cline in signal intensity and body size.Tiivistelmä Eläimet viestivät seksuaalisten signaalien avulla lisääntymisvalmiudestaan ja laadustaan parittelukumppanina. Signaalien kehittymiseen vaikuttavat kuitenkin myös valikoivien kumppaniehdokkaiden mieltymykset sekä signalointiympäristön ominaisuudet. Näiden vaikuttavien tekijöiden lomassa signaloijien on usein kilpailtava niukemmasta määrästä kumppaneita, jolloin houkuttelevimmat yksilöt valikoituvat muita herkemmin. Heikommin menestyvät voivat kuitenkin käyttää kilpailijoita tai ympäristöä hyväkseen lisääntyäkseen. Vaikka seksuaalisignaalit ja -kilpailu ovat huomattavampia koirailla kuin naarailla, naarailla on monipuolinen joukko signaaleja, joita ne voivat käyttää kilpaillakseen parittelukumppaneista. Väitöskirjassani tutkin, kuinka sosiaalinen ja visuaalinen ympäristö vaikuttavat naaraan seksuaaliseen signalointiin. Tutkin kenttä- ja laboratoriokokein sekä kenttämittauksin, kuinka kilpailijoiden kirkkaus ja valoympäristön ominaisuudet vaikuttavat naaraan visuaalisen seksuaalisignaalin käyttöön ja ominaisuuksiin. Käytän tutkimuslajina kiiltomatoa (Lampyris noctiluca), jonka naaraat loistavat öisin houkutellakseen lentäviä koiraita. Suuret naaraat tuottavat kirkkaamman loisteen, jota koiraat suosivat. Tulokseni osoittavat, että naaraiden seksuaaliseen signalointiin vaikuttavat sekä sosiaalinen että visuaalinen ympäristö, mutta myös signaloijien ominaisuudet. Kilpailija vaikutti signalointipaikan valintaan, kun naaraat kasvattivat etäisyyttä kirkkaampaan kilpailijaan. Himmeämpi kilpailija vaikutti signalointipaikan sijasta -aikaan naaraiden loistaessa myöhemmin, kun etäisyys kilpailijaan kokeen alussa oli lyhyempi. Lisäksi signalointiajankohta kilpailijan läsnä ollessa riippui ruumiinkoosta niin, että suuret naaraat loistivat aiemmin. Loisteen kirkkaus sekä naaraan koko kasvoivat pohjoista kohti, mikä todennäköisesti ylläpitää naaraan näkyvyyttä koiraille kirkkaissa kesäöissä. Tulokseni antavat ymmärtää, että naaraat saattavat kyetä tunnistamaan houkuttelevuutensa suhteessa muihin naaraisiin ja muuttamaan signalointikäyttäytymistään sen mukaan, niiden ruumiin koon myös vaikuttaessa signalointiin. Signalointiympäristö voi puolestaan aiheuttaa valintapaineen olla näkyvämpi koiraille, synnyttäen pohjois-eteläsuuntaisen trendin signaalin voimakkuudessa ja ruumiin koossa

Pale by comparison:competitive interactions between signaling female glow-worms

Abstract When individuals differ in their abilities to compete for a mate, weaker competitors may evolve tactics to increase their mating success. Exploiting attractiveness of others to get mates is a common tactic in many taxa, although examples of this behavior in females are scarce. In glow-worms (Lampyris noctiluca L., Coleoptera: Lampyridae), females attract males by glowing and males prefer the brightest female. How unattractive females succeed in competition for mates is largely understudied. We hypothesize that less attractive female glow-worms may succeed in competition over mates by parasitizing glow of more attractive competitors. We tested our hypothesis with a combination of field and laboratory experiments. Contrary to our expectations, we found that females move away from brighter competitors. This behavior may explain our field observation that females are often more than 1 m apart from each other. Increasing distance to a brighter female may make comparison on brightness difficult for males and increase attractiveness of dimmer females. Our study provides evidence of behavior by which less attractive competitors may actively avoid competition and therefore affect female distribution in nature. This behavior may explain maintenance of variation in attractiveness of sexual signals, even in species where mates are selected by ornaments. We conclude that sexual competition may play a crucial role in spatial distribution. Spatial distribution of competing sex affects choosing individuals’ ability to compare mates and thus affects mate choice

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

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

Abstract Animals often disperse from one habitat to another to access mates or suitable breeding sites. The costs and benefits of such movements depend, in part, on the dispersing individuals’ phenotypes, including their sex and age. Here we investigated dispersal and road-related mortality in larvae of a bioluminescent beetle, the European common glow-worm, Lampyris noctiluca, in relation to habitat, sex and proximity of pupation. We expected these variables to be relevant to larval dispersal because adult females are wingless, whereas adult males fly when searching for glowing females. We found that dispersing glow-worm larvae were almost exclusively females and close to pupation. The larvae were often found on a road, where they were able to move at relatively high speeds, with a tendency to uphill orientation. However, each passing vehicle caused a high mortality risk, and we found large numbers of larvae run over by cars, especially close to covered, forest-like habitat patches. In contrast, adult females in the same area were most often found glowing in more open rocky and grassy habitats. These findings demonstrate an underappreciated ecological strategy, sex-biased dispersal at larval phase, motivated by different habitat needs of larvae and wingless adult females. The results are also consistent with roads being an ecological trap, facilitating dispersal and presumably females’ signal visibility but causing severe larval mortality just before the reproductive stage. Hence, in addition to the previously recognised threats of urbanisation, even low traffic volumes have a high potential to negatively affect especially females of this iconic beetle