The costs and consequences of female sexual signals

Abstract Sexual ornaments have developed in a very wide variety of animal taxa to increase fitness by improving mating success. How increased mating rate improves fitness is obvious in the case of most males: each mating provides more offspring. Whether more matings benefit females, whose fecundity is limited by resources and not mates, is unclear. Mate choice is linked to ornamentation, when individuals of one sex choose who to mate with based on the ornamentation in members of the other sex. Ornaments may work as a basis for mate choice due to conveying information about the quality of the mate. Mate choice may then lead to intrasexual competition for mates in the chosen sex as animals try to outcompete their neighbours and attract more mates. My aim in this thesis is to study the purpose and costs of female ornamentation as well as female competition for males. The main questions revolve around understanding what information female ornaments provide about the bearer and how males choose between females. I also examine how females compete against each other and what the costs of being ornamented are for a female. To study these questions I use the common glow-worm (Lampyris noctiluca Linnaeus, Lampyridae), whose females glow at night to attract flying males. According to my results, a female’s glowing provides accurate information about fecundity and males base their choice of mate on the intensity of female glowing. I showed that the perceived strength of an ornament in comparison to others close by is more important than the actual strength of an ornament. In the glow-worm, the perceived strength of an ornament depends on distance to the observer, making the system open to exploitation. A mathematical model I developed suggested that dull females could outcompete brighter ones by choosing optimal locations, and experimentation showed this to be the case. Finally, my results show that glow-worm females lose eggs each day they remain unmated. This suggests that the evolution of female ornamentation may have been driven in part by the need to find a mate as soon as possible after eclosion. Which in turn means that female and male ornamentation may have fundamentally different purposes: for females mating soon may be important, where for males the total number of offspring fertilized is critical.Tiivistelmä Seksuaalisia ornamentteja on kehittynyt hyvin laajaan kirjoon eläinlajeja parantamaan yksilöiden kelpoisuutta nostamalla niiden parittelutodennäköisyyttä. Koirailla parittelutodennäköisyyden ja kelpoisuuden välinen yhteys on yleensä selvä: jokainen parittelu nostaa jälkeläismäärää. Naarailla yhteys ei ole yhtä selvä, sillä niillä jälkeläismäärä yleensä riippuu resurssien saatavuudesta eikä kumppanien määrästä. Parinvalinta on kiinteästi yhteydessä ornamentaatioon: Yhden sukupuolen yksilöt päättävät kenen kanssa paritella toisen sukupuolen yksilöiden ornamenttien laadun perusteella. Ornamentit toimivat valinnan perusteena välittämällä tietoa parin laadusta. Parinvalinta voi johtaa yksilöiden väliseen kilpailuun kumppaneista valitun sukupuolen sisällä. Tässä väitöskirjassa tarkoituksenani on tutkia naaraiden ornamenttien tarkoitusta ja kustannuksia sekä tutkia miten naaraat voivat kilpailla toisiaan vastaan koiraista. Pääkysymykset liittyvät ornamenttien välittämään informaatioon ja siihen miten koiraat valitsevat kumppaninsa. Lisäksi tarkastelen miten naaraat voivat kilpailla keskenään ja mitä kustannuksia ornamenteista on naaraille. Kysymysten selvittämiseksi käytän tutkimuslajina kiiltomatoa (Lampyris noctiluca Linnaeus, Lampyridae), lajia jonka naaraat loistavat öisin houkutellakseen lentäviä koiraita. Tulosteni mukaan kiiltomatonaaraan loiste välittää tarkkaa informaatiota niiden munamäärästä ja koiraat osaavat valita parinsa tämän perusteella. Osoitin, että ornamentin havaittu vahvuus verrattuna ympärillä oleviin on tärkeämpää kuin ornamentin todellinen vahvuus. Kiiltomadolla havaittu kirkkaus riippuu etäisyydestä havaitsijaan, mikä tekee signallointijärjestelmästä avoimen väärinkäytölle. Laatimani matemaattisen mallin mukaan himmeät naaraat voisivat päihittää kilpailussa kirkkaampia valitsemalla parempia loistamispaikkoja. Käytännön kokeet osoittivat tämän pitävän paikkansa. Viimeinen tulokseni oli, että kiiltomadolla naaras menettää osan munistaan joka päivä jonka se joutuu odottamaan parittelua. Tästä voi päätellä, että naaraan ornamentin evoluutiota on voinut osaltaan ajaa tarve paritella mahdollisimman pian aikuistumisen jälkeen. Naaraiden ja koiraiden ornamenttien syyt voivat olla hyvin erilaiset: naarailla parittelu mahdollisimman pian voi olla tärkeintä, kun koiraalla parittelukumppanien määrä on tärkein

Reproduction under light pollution:maladaptive response to spatial variation in artificial light in a glow-worm

Abstract 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

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

Female sexual signaling in a capital breeder, the European glow-worm Lampyris noctiluca

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 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