Over the past twenty years there has been an exponential increase in the investigation of maternal effects. Understanding the adaptive function of maternal allocation strategies is integral to interpreting the evolutionary outcomes of sexual selection. Thus, model animal systems that facilitate experimental manipulation and controlled investigation of the physiological and behavioural mechanisms underlying maternal effects are important to evolutionary biologists. The zebra finch (Taeniopygia guttata) has been used as a model to investigate avian life-history, signalling behaviour, neurophysiology, mate choice, and more recently, maternal effects. However, a potentially influential and rarely addressed problem with this species is the process of domestication. Within this thesis we aimed to both test current predominant maternal allocation hypotheses, but for the first time in both domesticated and wild zebra finches. Chapter 2 develops on earlier work using domesticated zebra finches that has demonstrated differential allocation of maternally derived yolk androgens and antioxidants in eggs dependent on paternal attractiveness. This chapter specifically tests the ratio of these two yolk resources within individual eggs and shows that the balance of androgens to antioxidants varies by offspring sex and paternal attractiveness. Specifically, we found that mothers allocated a smaller androgen to antioxidant ratio to daughters when paired to green ringed (unattractive) males compared to red ringed (attractive) males. This pattern was reversed for sons, where mothers allocated a larger ratio of androgen to antioxidant when paired to red ringed (attractive) compared to green ringed (unattractive) males. We also show that brood sex ratio depended on both female condition and male attractiveness. It is concluded that investigating female allocation of individual resources within egg yolks may lead to incorrect assumptions on offspring fitness consequences, and that individual female state is an important consideration when predicting a resource allocation strategy. Throughout this thesis colour bands are used as a method to manipulate male attractiveness. In chapter 3 the influence of these bands was further tested to elucidate whether they affect male behaviour or quality. Wild birds were used for this chapter as preferences for bands based on colour have only once been demonstrated in wild birds and it was felt this should also be replicated. We confirmed a female preference for males based on colour bands worn in mate choice trials, with red bands preferred over green. Interestingly, we also found that colour of bands worn by males for an extended period in the single sex aviary influenced both their song rate and condition. Males that had worn red bands sang more in mate choice trials than both green banded or un-banded males. In addition red banded males were found to be in significantly better physical condition. These data suggest that earlier experiments in which it has been assumed that colour bands do not manipulate any form of intrinsic male quality should be re-evaluated. The final two data chapters, 4 and 5, return to investigating maternal allocation in response to male attractiveness, but for the first time in wild birds. Chapter 4 presents an experiment that was conducted on a wild, nest box breeding population of birds. Maternal resources allocation was investigated in both an experimental manipulation of male attractiveness, and also by correlating resource allocation with paternal phenotypic traits. A limited sample size meant few conclusions could be drawn from the experimental study, but significant positive correlations were found between both egg size and yolk testosterone (T) concentration and male phenotypic traits. This suggested that wild zebra finches may follow a positive investment strategy but requires further investigation. In chapter 5 experiments were repeated on wild birds that had been brought into captivity, to allow both an improved sample size and further control of influential environmental features. Again, female allocation strategies are tested using colour bands to manipulate male attractiveness, to allow direct comparisons with work on domesticated zebra finches. We found that females laid significantly heavier eggs for attractive compared to unattractive males, supporting the positive investment hypothesis. In addition we found an interaction between offspring size and paternal attractiveness treatment, with daughters of red banded (attractive) males being smaller than sons. This experiment is the first to demonstrate the influence of colour bands on maternal allocation in wild zebra finches and also provides further support for the positive investment hypothesis in this species. The final chapter discusses how overall patterns of female allocation were shown to be similar among wild and domesticated populations. It is concluded that demonstrated variations between populations and/or contexts reported in these studies cannot be explained by inherent differences between wild and domesticated individuals. Thus, the zebra finch remains a robust and reliable model for testing the evolution of avian maternal allocation strategies.

This thesis describes an investigation into the effects of habitat composition, principally the composition of algae in a wrack bed, on the life history of the seaweed fly, Coelopa frigida. The mating system of C. frigida is dominated by intense sexual conflict characterised by frequent harassment by males leading to a vigorous pre-mating struggle. This response leads to sexual selection for large male size and sexual dimorphism. The mating behaviour of C. frigida is affected by their environment, with exposure to brown algae inducing harassment in males and oviposition in females. Despite more than two decades of research into coelopid reproduction little is known about how habitat composition alters the patterns and processes of sexual conflict. Studies contained in this thesis consider environmental influences that both directly and indirectly influence sexual conflict. Direct effects of the environment are measured by conducting mating trials following culture of C. frigida on different species of algae and by exposing males to different species of algae. This work is accompanied by studies of larval development and adult survival on different algae. In recent years it has been observed that the distribution of European coelopids has undergone a northward range shift. An investigation into the current distribution of European coelopid species and a discussion of the effects of climate change that may have caused this change is included. Finally, the use of stable isotope analysis to determine the diet of wild coelopids and alternative statistical methods to analyse mating trials are described.

Social dominance relationships are formed within numerous animal species and reduce costly fights over resources. Dominant individuals often enjoy greater access to important resources such as food and mating partners, and are generally more aggressive, bold, active and explorative compared to subdominant individuals. These behavioural traits can differ among individuals, but they can also be consistent within the individual, thereby describing the individual’s personality type. However, the causal direction of the observed correlation between dominance and personality is not well studied. One possibility is that some personality types have higher chances of obtaining a dominant social position. This would suggest that personality has consequences for fitness. Another possible explanation is that possessing different social positions gives rise to consistent behavioural differences among individuals on various timescales. If social status has a lasting effect on behaviour, social status would constitute a ‘stable state’ that explains some of the diversity of personality types that has been observed in a multitude of animal species. Dominance and personality may also share underlying proximate factors. In this thesis, I investigate the relationship between social dominance and personality using male domestic fowl, Gallus gallus domesticus. The species is group-living with pronounced dominance hierarchies, and dominance increases male access to mating partners. I show that some aspects of personality, exploration, vigilance and in particular aggressiveness, increased a male’s chances of obtaining dominance (paper III, IV, V), and that aggressiveness can be even more important than body weight and ornament size (comb size, paper V) or recent experience of winning or losing (paper IV). Winning a social interaction resulted in an increase in aggressiveness, while a decrease was seen in males that experienced a loss (paper IV). By observing behaviour before and after changes in male dominance relationships, I further show that a recent (2 days earlier) change in social status induced behavioural adjustments to the new social status in activity, exploration and vigilance (paper I). By extending the time of the new social relationship to 3 weeks, I show that such behavioural changes did not continue (paper II). Finally, I show that the social environment during juvenile development had little impact on adult male competitiveness (paper V). Social interactions appear to have several short-term effects on behaviour, but did not contribute significantly to variation and long-term consistency of personality in male fowl. Instead, a male's personality, and in particular his aggressiveness, affected the outcome of dominance interactions. Overall, my studies reveal important consequences of individual differences in behaviour, and contribute to the highly sought-after empirical testing of hypotheses explaining variation in animal personality. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 5: Manuscript.</p>

La théorie de la sélection sexuelle a été largement élaborée à partir du constat de dimorphisme sexuel chez les espèces à sexes séparés. Une de ses caractéristiques générales est une sélection plus forte pour l'augmentation du nombre de partenaires sexuels chez les mâles que chez les femelles qui résulterait d'un investissement différentiel dans les descendants entre les deux sexes (anisogamie). Si hermaphrodisme et sélection sexuelle sont considérés comme compatibles depuis les travaux de Charnov (1979), les études sur le sujet restent rares que ce soit chez des animaux ou des plantes. Une raison importante est que la méthodologie disponible pour quantifier la sélection sexuelle ne prend pas en compte les particularités des hermaphrodites (par ex., corrélations ou effets croisés entre les deux fonctions sexuelles d'un même individu, autofécondation). Le premier objectif de cette thèse est de combler cette lacune méthodologique en proposant un cadre travail adapté aux hermaphrodites, que nous appliquons dans une étude empirique chez Physa acuta, un gastéropode hermaphrodite d'eau douce. Nous observons que la sélection sexuelle est plus intense sur la fonction mâle, comme généralement chez les espèces gonochoriques. Par ailleurs, nous ne détectons aucun effet des particularités des hermaphrodites dans cette expérience. Dans un deuxième temps, nous nous intéressons de manière plus détaillée aux composantes du succès reproducteur mâle (RSm). Nous montrons que chez P. acuta il existe une priorité spermatique au premier partenaire mâle lorsque plusieurs individus sont en compétition. Enfin, nous proposons une décomposition de la variance de RSm en ses composantes pré- et post-copulatoires, qui représentent respectivement 60 et 40% de la variance. Dans la troisième partie, nous intégrons la sélection sexuelle à l'étude de l'évolution de l'allocation sexuelle d'un hermaphrodite, via une approche d'évolution expérimentale chez P. acuta. Menée sur plus de 10 générations, elle vise à faire évoluer l'allocation sexuelle de manière disruptive (lignées mâle ou femelle) en sélectionnant les composantes mâle et femelle du succès reproducteur. Les résultats préliminaires suggèrent qu'il est possible de manipuler l'allocation sexuelle chez un hermaphrodite simultané en sélectionnant sur son régime d'appariement. Nous concluons que l'anisogamie suffit à justifier l'existence de la sélection sexuelle sans avoir à supposer un dimorphisme sexuel. Son étude chez les hermaphrodites simultanés ouvre des perspectives pour la compréhension du rôle de l'allocation sexuelle dans l'évolution des systèmes de reproduction. / A cornerstone of the theory of sexual selection in gonochoric species is sexual dimorphism. A very general result is stronger selection on males than on females for increasing mating success, and this fundamentally relies on differential investment in offspring between the sexes (anisogamy). Although sexual selection does operate in hermaphroditic species as well, few empirical studies have been performed whether in animals or in plants. The main reason is that the current framework for studying sexual selection does not incorporate the particularities of hermaphrodites, including correlations or cross-sex effects between sex functions and self-fertilization. The first goal of this thesis is to fill this gap by proposing an appropriate framework for hermaphrodites (generalizing that available for gonochoric species). It was applied to approach sexual selection in the hermaphroditic freshwater gastropod Physa acuta. Sexual selection turns out to be stronger on the male than on the female function, as classically observed in gonochorists. Moreover, we do not detect any effect in relation to hermaphrodites' particularities. We then focus on the components of male reproductive success (RSm) in more details. We detect a pattern of sperm precedence in conditions of sperm competition. We develop a new method for decomposing the variance in RSm into pre- and post-copulatory components (representing 60 and 40% of the variance respectively). The third section aims at integrating sexual selection in studies of sex allocation and its evolution. It relies on a protocol of experimental evolution in P. acuta. Conducted over more than 10 generations. Its aim is to observe the evolution of sex allocation by disruptively selecting male and female components of reproductive success. Preliminary results indeed suggest that it is possible to manipulate sex allocation in a simultaneous hermaphrodite by manipulating its mating system. We conclude that anisogamy alone is a sufficient condition for sexual selection to proceed, and that sexual dimorphism is not required. Study sexual selection in simultaneous hermaphrodites gives insights for understanding the role of sex allocation in the evolution of mating systems.

Chez les espèces où mâles et femelles portent des traits voyants et élaborés, les traits observés chez les femelles peuvent être des sous-produits non-fonctionnels de la sélection sexuelle exercée sur les traits mâles ou alors être directement soumis à la sélection. Cette thèse a eu pour objectif de tester l'hypothèse de sélection sexuelle chez les femelles de Mésange bleue (Cyanistes caeruleus) sur trois traits présents chez le mâle et la femelle : (1) la coloration structurelle UV/bleue de la tête, (2) la coloration jaune basée sur les caroténoïdes de la poitrine et (3) l'agressivité. Pour cela, nous avons testé expérimentalement (1) le lien entre les traits colorés et le transfert de composants potentiellement bénéfiques dans les ufs, (2) la condition-dépendance des traits colorés, (3) l'utilisation de la coloration du plumage dans les interactions femelle-femelle et (4) le lien entre l'agressivité des femelles et leur investissement dans la reproduction. Enfin, nous avons exploré le rôle potentiel des couleurs femelles dans le choix de partenaires mâle en testant le lien entre la couleur UV/bleue des femelles et le nombre de jeunes issus de copulations hors couple et l'appariement selon la couleur bleue et jaune dans notre population. Nos résultats suggèrent (1) qu'il existe un lien entre la qualité maternelle et la coloration de leur plumage, (2) que les couleurs UV/bleues et jaune du plumage sont conditions-dépendants, (3) que les UV/bleus de la tête sont utilisés comme badge de statut dans les interactions femelle-femelle, (4) qu'il pourrait exister un compromis entre l'agressivité femelle et son investissement dans la reproduction et (5) qu'il existe un potentiel choix mutuel de partenaires basés sur les couleurs. Au final, cette thèse a ainsi permis de mettre en évidence que chez une espèce où mâles et femelles sont ornementés, les traits colorés femelles ont le potentiel d'évoluer sous l'effet direct de la sélection sexuelle. / In mutually ornamented species, female conspicuous traits could be non-functional by-products of sexual selection acting on male traits or could be directly under selection. The aim of this PhD was to test the hypothesis of sexual selection in Blue tit (Cyanistes caeruleus) females on three traits present in both males and females: (1) the structural coloration of the UV/blue crown, (2) the yellow chest coloration based on carotenoids and (3) aggressiveness. To do so, we experimentally tested (1) the links between plumage coloration and the transfer of potentially beneficial components in egg yolks, (2) the condition-dependence of the plumage coloration, (3) the use of the plumage coloration in female-female interactions and (4) the link between female aggressiveness and investment in reproduction. Finally, we explored the role of female plumage coloration in male mate choice by testing the link between female UV/blue crown coloration and the n umber of extra-pair young in the nest and by estimating whether the individuals were assortatively mated in respect of their yellow and blue coloration in the studied population. Our results suggest that (1) there is a link between female plumage coloration and maternal quality, (2) plumage UV/blue and yellow coloration is condition-dependant, (3) the UV/blue crown is used as a badge of status in female-female interactions, (4) there could be a trade-off between female aggressiveness and female investment in reproduction and (5) there is a potential mutual mate choice based on both coloration. So, this PhD supports the hypothesis that in a mutually ornamented species, female ornaments are potentially under direct sexual selection.

Rodents exhibit diverse biology, mating systems and diverse sperm morphology. Different levels of sperm competition have therefore been found in this group. Generally, it is accepted that sperm competition in animals causes the prolongation of sperm tails. However, what other sperm morphological adaptations in rodents are affected by sperm competition? Species-specific sexual selection likely optimizes sperm morphology. Sperm physiology, morphology and competition may be influenced, ultimately, by environmental factors. This Ph.D. thesis focuses on sperm competition in murine rodents. The thesis contains three research articles (including one as a submitted manuscript). In addition, another published article, partialy related to the topic of this thesis, is represented in the Appendix. The sperm morphology in rodents plays an important role in sperm competition. Longer tails and apical hooks are usually found in the species with relatively larger testes and with higher percentage of multiple paternity (i.e. rate of promiscuity), that is in species with the higher risk of sperm competition. The goal of the first study was to investigate the relationships between relative testis weight and sperm traits (apical hook and tail lengths and variance in this traits). The apical hook length was positively...

O cuidado paternal exclusivo à prole é a forma mais rara de investimento parental pós-zigótico na natureza, tendo evoluído independentemente em não mais do que 14 linhagens de artrópodes. Embora muitas dessas espécies sejam facilmente observadas e manipuladas, apenas recentemente os pesquisadores começaram a prestar atenção e a testar hipóteses sobre investimento parental e evolução de papéis sexuais nesses interessantes sistemas biológicos. O principal objetivo desta tese foi investigar características individuais e populacionais que poderiam afetar o sucesso reprodutivo de machos de opiliões (Arachnida: Opiliones) com cuidado paternal. No primeiro capítulo, fazemos uma revisão teórica detalhada, desde trabalhos clássicos de papéis sexuais até os mais recentes avanços, abordando também de maneira sistemática o conhecimento atual acerca de custos e benefícios do cuidado paternal em artrópodes, assim como o papel da seleção sexual sobre a evolução do comportamento parental e dos papéis sexuais. Em seguida, nos capítulos 2 e 3, acessamos os custos energéticos e em termos de sobrevivência do cuidado paternal nos opiliões Iporangaia pustulosa e Zygopachylus albomarginis. Comparando essas duas espécies, que apresentam diferentes níveis de investimento paternal, testamos previsões direcionadas de que os machos que investem mais na prole deveriam pagar maiores custos do cuidado. No quarto capítulo, por meio de informações da história natural e parâmetros populacionais estimados para I. pustulosa, fazemos previsões sobre qual dos sexos deveria ser criterioso ao escolher parceiros e as testamos utilizando observações comportamentais de interações entre machos e fêmeas. Por fim, no capítulo 5, avaliamos a importância relativa do sítio de oviposição, assim como do tamanho e do estado parental dos machos sobre a sua atratividade. Ao final, integramos os resultados obtidos em um corpo teórico sobre a inter-relação entre aparentes demandas conflitantes entre esforço parental e esforço reprodutivo, cuidado paternal e papéis sexuais, abrangendo não apenas as espécies estudadas, mas com um escopo bem mais amplo / Exclusive paternal care is the rarest form of post-zygotic parental investment in nature, which has independently evolved in no more than 14 arthropod lineages. Although many of those species are easily observed and manipulated, only recently researchers have started to pay attention and test hypotheses on parental investment and evolution of sex roles in such interesting biological systems. The main goal of this thesis was investigating individual and population characteristics that affect the reproductive success of males of harvestmen (Arachnida: Opilioes) with paternal care. In the first chapter, we revise the theory in details, since seminal works on sex roles until the most recent advances in the area, sistematically approaching the current knowledge abot costs and benefits of paternal care in arthropods, as well as the role of sexual selection on the evolution of parental behavior and sex roles. Then, in chapters 2 and 3, we access energetic and survival costs of paternal care in the harvestmen Iporangaia pustulosa and Zygopachylus albomarginis. Comparing these two species, that show different levels of parental investment, we test directional predictions that males which invest more should also pay more intense costs rekated to care. In the fourth chapter, based on natural history information and population parameters for I. pustulosa, we predict which sex should be choosier when selecting sexual partners, and test such predictions using behavioral data on male-female interactions. Finally, in chapter 5, we evaluate the relative importance of oviposition site, males\' body size and males\' parental state on their attractiveness. At the end, we integrate all the results obtained during the five chapters into a theoretical framework about the inter-dependence among trade-offs between parental and mating efforts, paternal care and sex roles, not only regarding the pair od species studied, but in a broad way.

O objetivo geral desta tese foi investigar como a distribuição espacial dos indivíduos influencia a organização dos sistemas de acasalamento e a intensidade da seleção sexual. A tese contém dois capítulos em que analisamos dados empíricos e dois capítulo em que abordamos questões mais teóricas mais gerais. No primeiro capítulo, avaliamos como a distribuição espacial de haréns defendidos por machos territoriais influencia a rede de competição espermática que existe entre machos do opilião Serracutisoma proximum. No segundo capítulo, propusemos um modelo probabilístico de escolha de parceiros que leva em consideração o fato de que as fêmeas estão restritas a amostrar apenas alguns machos da população, e que essa restrição é imposta principalmente pela distribuição espacial dos indivíduos. Nosso modelo foi mais acurado do que modelos alternativos que ignoram o espaço. No terceiro capítulo, investigamos os movimentos de busca de parceiros do besouro Leptinotarsa undecimlineata. Encontramos que tanto machos quanto fêmeas se movimentam estrategicamente, saindo de onde estão quando não obtém cópulas buscando áreas próximas e com muitos parceiros em potencial. Finalmente, no quarto capítulo, voltamos a explorar o efeito de restrições espaciais sobre a escolha de parceiros. Usamos simulações baseadas em indivíduos para investigar como a restrição espacial influencia a seleção sexual e a evolução de ornamentos sexualmente selecionados. Encontramos que quanto maior o número de parceiros que as fêmeas podem amostrar durante a escolha, mais intensa é a seleção sexual, o que permite a evolução de ornamentos mais exagerados nos machos. Além disso, analisamos um conjunto de dados publicados sobre a intensidade de seleção sexual e observamos que estes se ajustam às previsões do modelo teórico. Concluímos que o espaço é muito mais importante para a organização de sistemas de acasalamento e para a intensidade da seleção sexual do que se acreditava previamente. Esperamos que as idéias propostas aqui encontrem terreno fértil na mente do leitor e que gerem novos e estimulantes desdobramentos no campo teórico e empírico / The main goal of this thesis was to investigate how the spatial distribution of individuals influences the organization of the mating systems and the intensity of sexual selection. The thesis contains two empirical chapters and two chapters with a theoretical scope. In the first chapter, we investigate how the spatial distribution of harems defended by territorial males influences the sperm competition network among males of the harvestman Serracutisoma proximum. In the second chapter, we proposed a probabilistic model of mate choice that includes the spatial constraint in the analyses of mate choice. This model takes into account the fact that females are restricted to sample only some males available in the population, and that this restriction is imposed primarily by the spatial distribution of individuals. In the third chapter, we investigate the mate searching movements of the leaf beetle Leptinotarsa undecimlineata. We found that both males and females move strategically, leaving the host plant when they do not obtain copulations and seeking neighboring areas with many potential mates. Finally, in the fourth chapter, we explore the effect of spatial restrictions on mate choice. We used individual based simulations to investigate how spatial restrictions influence sexual selection and the evolution of sexually selected ornaments. We found that the higher the number of potential partners female can sample during mate choice, the more intense is sexual selection, which leads to the evolution of more exaggerated male ornaments. In addition, we analyzed a published dataset of intensity of sexual selection and found that the data adjust to the predictions made by our simulations. We conclude that the space is much more important than previously thought to mating systems organization and the intensity of sexual selection. We hope the ideas proposed here can flourish in the minds of the readers, stimulating both empirical and theoretical follow ups

Para avaliar a quantidade ótima de energia e recursos a ser investida em reprodução, um animal deve não só acessar informações sobre sua própria condição corporal, como acessar também informações sobre coespecíficos presentes no ambiente em que vive. Este trabalho investigou essas duas vias (endógena e exógena) de obtenção de informação. Utilizando como organismo modelo a aranha Paratrechalea ornata (Trechaleidae), uma espécie em que os machos produzem presentes nupciais, este estudo investigou: (a) se machos investem mais tempo de busca quando na presença de pistas químicas de fêmeas virgens; (b) se machos aumentam ou diminuem o investimento na produção do presente nupcial quando há pistas da presença de machos competidores; (c) se restrições alimentares impõem efeitos de curto e longo prazo sobre o comportamento de construção de presentes nupciais. Machos mostraram preferência por locais com pistas da presença de fêmeas, mas não diferenciaram pistas de fêmeas virgens e copuladas. Diante de pistas da presença de um macho competidor, machos produziram presentes de menor qualidade, com menos seda. Por fim, restrições alimentares provocam efeitos de curto e longo prazo no comportamento reprodutivo dos machos, diminuindo a frequência de produção do presente nupcial e o tamanho do presente construído. Conjuntamente, os três experimentos realizados mostram que machos de P. ornata utilizam tanto informações exógenas quanto endógenas para ajustar seu investimento em reprodução. A combinação das informações obtidas deve aumentar a eficiência no gasto de energia, maximizando o sucesso reprodutivo dos machos sem que isso prejudique a manutenção de seu organismo / To evaluate the optimal amount of energy and resources to be invested in reproduction, animals should not only obtain information about their own body condition, but they should also obtain information about conspecifics in the environment where they live. This study investigated these two ways (endogenous and exogenous) of acquiring information. Using as model organism the spider Paratrechalea ornata (Trechaleidae), a species in which males produce nuptial gifts, this study investigated: (a) if males invest more time searching for sexual partners in the presence of chemical cues of virgin females; (b) if males increase or decrease the investment in nuptial gift construction in the presence of chemical cues of competitor males; (c) if food deprivation imposes short- and long-term effects on nuptial gift construction. Males showed a preference for sites with cues of females, but they did not discriminate cues of virgin from cues of copulated females. When males detected cues of a male competitor, they produced lower quality gifts, with less silk. Finally, food deprivation imposed both short- and long-term negative effects on male reproductive behavior, decreasing the frequency of nuptial gift construction and also the size of the gift. Together, the three experiments show that males of P. ornata use both exogenous as endogenous information to adjust their investment in reproduction. The combination of information obtained may increase the efficiency in energy use, maximizing male reproductive success without compromising self maintenance

Cavidades naturais constituem um recurso reprodutivo monopolizável, cuja disponibilidade determina a intensidade da competição macho-macho que, por sua vez, pode influenciar os custos do cuidado paternal, pois quanto maior o risco de perda do recurso, menor deve ser a freqüência de forrageio dos machos. Fêmeas do opilião Magnispina neptunus utilizam cavidades naturais como sítios de nidificação, que são monopolizados por machos em um sistema de poliginia por defesa de recursos. Após a oviposição, as fêmeas abandonam os ninhos e todo o cuidado parental é exercido pelos machos. O objetivo deste trabalho foi avaliar o efeito da disponibilidade de ninhos sobre a competição intrassexual e os custos do cuidado paternal em M. neptunus. Dois grupos experimentais foram estabelecidos em laboratório: alta (8 ninhos) e baixa (4 ninhos) disponibilidade de sítios reprodutivos. Cada grupo era composto por sete terrários contendo 12 machos e 12 fêmeas. Apesar da redução dos sítios de nidificação ter reduzido o número de machos detentores de ninhos, o tamanho dos indivíduos não exerceu nenhum efeito sobre o sucesso de monopolização de ninhos. Aparentemente, o pequeno diâmetro de entrada dos ninhos experimentais reduz a importância do tamanho dos machos sobre sua probabilidade de monopolizar um sítio de nidificação. Apenas a residência prévia parece determinar o resultado das disputas entre machos e, portanto, os custos de abandonar os ninhos devem ser altos. De fato, machos guardiões permanecem quase todo o tempo dentro dos seus ninhos em ambos os grupos experimentais, o que aumenta os custos do cuidado. Como conseqüência, o canibalismo filial é freqüente, pois os custos em termos de redução no tamanho da desova são menores do que os benefícios de manter a posse do ninho e dos ovos, que sabidamente aumentam a atratividade dos machos e suas chances de obterem novas desovas / Natural cavities are a reproductive resource that can be monopolized, and their availability determines the intensity of male-male competition, which in turn may influence the costs of paternal care since the higher the risk of loosing the resource, the low should be male foraging frequency. Females of the harvestman Magnispina neptunus use natural cavities as nest site, which are monopolized by males in typical resource defense polygyny mating system. After oviposition, females leave the nests and all parental activities are in charge of males. The goal of this study was to evaluate the effect of nest site availability on the intrasexual competition and the costs of paternal care in M. neptunus. Two experimental groups were established in the laboratory: high (8 nests) and low (4 nests) nest site availability. Each group was composed of seven terraria containing 12 males and 12 females. Although the reduction of in the number of nests sites has indeed reduced the number of males holding nests, male size did not influence the probability of acquiring and maintaining a nest. Apparently, the small diameter of entrance in the experimental nests reduces the importance of male size on their resource holding power. Only previous residence seems to influence the output of agonistic interactions between two males and, therefore, the costs of leaving the nest to forage are likely to be high. In fact, guarding males remain almost all the time inside their nests, which increases the costs of paternal care. As a consequence, filial cannibalism is frequent in both experimental groups because the costs in terms of reduction in clutch size are likely to lower than the benefits of holding a nest containing eggs, which is known to increase male attractiveness and his chance of acquiring additional eggs