A wide variety of factors are predicted to influence the intensity and outcome of agonistic interactions in animals, including the resource holding potential of the opponents and the nature and value of the resource over which the individuals are competing. Field crickets (Orthoptera: Gryllidae) have been used extensively as model organisms with which to study animal contests, but relatively few studies have examined the effect of mandible size or structure, or the level of contact with females on the intensity and outcome of agonistic interactions. To do so was the aim of the present thesis, using Gryllus bimaculatus as the study species. The first finding of this study was that there is a significant degree of sexual dimorphism for anterior components of the anatomy in G. bimaculatus. The mandibles, head and pronotum of male crickets were all relatively larger than those of females. This indicates that these traits may be acted upon by intrasexual selection. In many animal species that show sexual dimorphism, a trade-off in development sees enhanced weapon growth at the expense of testes size, but no such relationship was seen in this species. A comparison of the mandible structure of males that either won or lost at flaring and or wrestling showed that a relatively wider mandible span was a significant predictor of success during mandible displays. It was also found that specific components of tooth structure, namely the length of the incisor and length to distal tip, were significantly associated with victory at the jaw flaring stage. This is the first time that mandible shape has been shown to affect fight outcome in the Gryllidae, and also the first confirmed identification of a visual cue component of fighting behaviour. Despite the effectiveness of their weapons in fighting, body mass is a primary predictor of victory in combat between G. bimaculatus males, with the greater the degree of asymmetry in weight the more likely the heavier fighter will win. However, a study of fighting behaviour between asymmetrically matched opponents found that even males who were out-weighed by 40% were still likely to escalate the fight to grappling. Furthermore, males who were able to fend off their larger opponent in their first clash were significantly more likely to win their overall encounter. This hyper-aggressive response may therefore represent an adaptive mechanism to extreme odds and is worthy of further study. Female contact is known to be a significant promoter of male aggression and fighting enthusiasm, and mate guarding aggression is well documented in G. bimaculatus. A recreation of two contradictory studies, including one which concluded that mating makes males lose fights, highlighted that female contact after spermatophore transfer can overcome the loser effect and cause a male to re-engage with a previously dominant opponent. Fighting behaviour in this species is therefore highly flexible and factors affecting the outcome of contests are complex. There is much scope for further studies on this topic.

De acordo com o paradigma Darwin-Bateman, fêmeas não ganham benefícios em copular com múltiplos machos (i.e., poliandria), dado que seu sucesso reprodutivo depende principalmente da qualidade dos seus parceiros sexuais. Porém, a poliandria é um comportamento amplamente encontrado nas espécies com reprodução sexuada. O objetivo central desta tese foi explorar as possíveis causas e consequências da poliandria. No Capítulo 1, utilizamos experimentos em laboratório para investigar como a experiência social antes e depois da maturação influencia o grau de poliandria. A espécie estudada foi o grilo australiano Teleogryllus commodus, para a qual o ambiente acústico antes da maturação determina o comportamento reprodutivo das fêmeas após a maturação. Nossos resultados mostram que o grau de poliandria não varia entre fêmeas criadas em dois ambientes acústicos distintos, um composto por machos de qualidade variada (QV) e outro composto por machos de alta qualidade (AQ). As fêmeas do grupo AQ foram menos responsivas aos machos de alta qualidade e aceitaram machos independentemente de sua qualidade, um padrão diferente daquele encontrado para as fêmeas do grupo QV. Quando a qualidade média dos machos encontrados pelas fêmeas do grupo QV foi baixa, o número total de machos aceitos foi baixo, porém quando a qualidade média dos machos foi alta, o número de machos aceitos foi alto. Portanto, há uma interação entre a experiência social antes e após a maturação na determinação do grau de poliandria. No Capítulo 2, utilizamos métodos comparativos para testar se o risco de competição espermática promovido pela poliandria determina a evolução de características masculinas relacionadas à monopolização de fêmeas. Nosso modelo de estudo foram rãs da subfamília Leptodactylinae, cujas espécies podem depositar ovos na água (onde o risco de competição espermática é alto) ou dentro de tocas na terra (onde o risco de competição espermática é baixo). Encontramos que há correlação entre o tipo de local de reprodução (aquático x terrestre) e quatro características masculinas relacionadas à monopolização de fêmeas. Nossos resultados provêem uma relação causal entre características dos machos e a reprodução terrestre, mas os padrões encontrados nem sempre seguiram as previsões propostas pela hipótese de que o risco de competição espermática é menor em espécies com reprodução terrestre. Tais resultados sugerem que as pressões seletivas que agem nas quatro características masculinas exploradas aqui não são as mesmas, provavelmente devido a diferenças nas funções que cada característica tem durante as interações inter- e intra-sexuais. Ao fim desta tese, fornecemos evidências de uma nova causa da poliandria, além de mostrarmos as consequências da poliandria sobre a evolução de características dos machos / According to the Darwin-Bateman paradigm, females derive no benefit from multiple mating (i.e., polyandry) and their reproductive success is mainly influenced by the quality of their partners. However, there is increasing evidence that polyandry is ubiquitous across many animal groups. The main goal of this thesis was to explore possible causes and consequences of polyandry. In Chapter 1, we conducted an experiment to investigate how the social experience pre- and post-maturity influences the degree of polyandry. We studied the Australian cricket Teleogryllus commodus, for which we know that the acoustic environment prior to maturity alters female reproductive behavior after maturity. We found that the degree of polyandry did not differ between females reared in two acoustic environments, one composed of varied-quality (VQ) males and other composed of only high-quality (HQ) males. However, whereas females from the HQ group were less responsive to high quality male calls, accepting males regardless of their quality, females from the VQ group showed a different response. When the mean male quality experienced by a female from the VQ group was low, the total number of accepted males was low, and when the mean male quality was high, the total number of accepted males was high. Thus, the social experience pre- and post-maturation interact to determine the degree of polyandry in females of the Australian cricket. In Chapter 2, we used comparative methods to test if the risk of sperm competition promoted by polyandry drives the evolution of male traits related to female monopolization. We studied frogs of the subfamily Leptodactylinae, which lay eggs either on the water (where sperm competition risk is high) or inside terrestrial burrows (where sperm competition risk is low). We found evidence of an evolutionary correlation between the type breeding site (aquatic x terrestrial) and four male traits related to female monopolization. Our study creates a causal link between male traits and terrestrial reproduction, but the patterns not always followed the hypothesis that sperm competition risk is lower in terrestrial breeders. This finding suggests that the selective pressure acting on male traits is not the same, probably because they have different functions during inter- and intrasexual interactions. In conclusion, we provide evidence of a new cause of polyandry and show that polyandry may have consequences for the evolution of weaponry traits in males