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Dissertation zugänglich unter
URN: urn:nbn:de:gbv:18-96424
URL: http://ediss.sub.uni-hamburg.de/volltexte/2019/9642/

Phenotypic plasticity of life history and mating strategies in African golden-silk spiders, Nephila senegalensis and N. fenestrata

Neumann, Rainer

 Dokument 1.pdf (1.785 KB) 

SWD-Schlagwörter: Evolutionsbiologie , Zoologie
Freie Schlagwörter (Deutsch): Verhaltensbiologie , Entomologie , Arachnologie , Araneae , Spinnen
Freie Schlagwörter (Englisch): Evolutionary Biology , Zoology , Behavioural Ecology , Sexual selection , Arachnology , Araneae, Spiders
Basisklassifikation: 42.21
Institut: Biologie
DDC-Sachgruppe: Biowissenschaften, Biologie
Dokumentart: Dissertation
Hauptberichter: Schneider, Jutta (Prof. Dr.)
Sprache: Deutsch
Tag der mündlichen Prüfung: 15.03.2019
Erstellungsjahr: 2018
Publikationsdatum: 25.04.2019
Kurzfassung auf Englisch: Golden-silk spiders (genus Nephila) represent an established model lineage to study extreme female-biased sexual size dimorphism (SSD). Several species also show an extraordinary within-sex variation of body size, particularly in males. Low mating rates are characteristic of the tiny males, whereas females are often polyandrous. Male-male competition is intense in most species, and several studies have reported benefits of large size in contest competition. However, consistent large male advantages should generate directional selection, which would narrow the existing variation. The evolution and maintenance of size variation and extreme reversed SSD remains challenging to explain, and is therefore subject to ongoing research. Natural environments are rarely constant, and phenotypic plasticity enables animals to adjust fitness-relevant traits to the prevalent conditions. Adaptive plastic responses can be used to adjust morphology and life history, but also the behavioural phenotype. I investigated the role of phenotypic plasticity in shaping mating strategies and significant life history traits in African golden-silk spiders, N. senegalensis and N. fenestrata. The study animals were expected to adjust these traits to different reproductive environments. I used N. senegalensis to evaluate paternity success in dyads of size-mismatched males competing for a single female and show that differently-sized phenotypes achieve equivalent paternity success. Behavioural plasticity in mating strategies may thus explain how principal large size benefits can be dissolved. I studied socially cued anticipatory plasticity (SCAP), using a comparative approach. Male N. fenestrata follow a highly specialized terminal investment strategy, whereas male N. senegalensis are less specialized to monopolize females. Particularly short lived males with terminal mating investment strategies are expected to adjust the timing of maturation to social cues indicating the availability of receptive females. While I expected a distinct developmental response in male N. fenestrata, I found that only male N. senegalensis presented with female cues matured several days earlier than control males. Furthermore, I studied sex-specific developmental plasticity in N. senegalensis. Animal growth is often constrained by unfavourable conditions, but growth restriction in early juvenile stages can later be compensated by adaptive catch-up growth. Natural selection should favour optimization of fecundity in growth-restricted females, whereas sexually selected benefits of early maturation should generate a stronger trade-off between size-related benefits and costs of a delayed maturation in males. Consistent with my predictions, efficient compensation in growth-restricted females resulted in equivalent lifetime fecundity compared to unrestricted females, while male growth compensation remained incomplete. I also expected behavioural plasticity in male N. fenestrata in which I studied the unique behaviour of copulatory male leg ejection in the context of male mating investment and sexual conflict. Males in this species often attempt to monopolize paternity with a single female by mate plugging and postcopulatory mate guarding. Females often attack and sometimes cannibalize males during copulation. However, females that are busy with prey capture and feeding rarely cannibalize the male. Attacked males regularly eject (autotomize) legs during copulation, which the female then feeds on. Thus, male copulatory leg ejection may serve to pacify females and prevent further attacks and sexual cannibalism, but the loss of legs reduces males´ mate guarding ability. To verify the adaptive function of male copulatory leg ejection, I tested whether experimentally simulated leg ejection (i.e. females were offered a male foreleg during mating) reduces the risk of female attacks and sexual cannibalism. Females were less likely to attack males when they had been offered a male leg, but the treatment did not affect the probability of cannibalism. Males copulated longer with females of higher body mass and female attacks were correlated with prolonged copulation duration and a higher number of male legs ejected. This finding probably reflects sexual conflict between male attempts to monopolize fertilization and females counteracting this restriction by forcing males to sacrifice legs, thereby reducing the efficiency of postcopulatory mate guarding. In conclusion, I show that adaptive phenotypic plasticity in Nephila spiders enables individuals to modify general patterns of life history and behavioural strategies; thus optimizing the adaptive value of the respective traits in response to specific environmental conditions. These mechanisms likely facilitate size variation and extreme female-biased SSD over evolutionary time.


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