Proximate and Evolutionary Causes of Sexual Size Dimorphism in the Crab Spider \u3ci\u3eMecaphesa celer\u3c/i\u3e

Animal species’ body sizes result from the balance between selection for survival and selection for reproduction. In species with sexual size dimorphism (SSD), this balance differs between females and males, resulting in distinct sizes despite similar constraints. I used an integrative approach to understand how sexual section, and differences in developmental trajectories and metabolic physiology, resulted in the female biased SSD of the crab spider Mecaphesa celer (Thomisidae). SSD in spiders is often assumed to be a consequence of selection for early male maturation, which should provide males with additional mating opportunities. My results allow us to discard mate choice and differential fitness benefits as sexually selected drivers of M. celer’s SSD. Interestingly, I found evidence that M. celer females may mate with multiple males, and that, in such instances, eggs are fertilized by a mix of the males’ sperm. Such sperm mixing contradicts the hypothesis that M. celer males benefit from early-male maturation, as early-matured males do not necessarily fertilize most of a females’ eggs. To gain a better understanding of the relation between M. celer’s SSD and early male maturation, I identified the proximate mechanisms underlying differences in size between females and males, as well as the effects of the environment on the degree of SSD. Female M. celer reach larger sizes by growing faster and for longer than males, although both sexes have the same metabolism at rest. Also, female, but not male size, may be influenced by the interaction between diet and temperature. Integrating all of these results, I followed the reproductive season of a population of M. celer in the wild and found evidences that early maturation does indeed grant males with increased mating opportunities. Interestingly, I also found that the timing of male maturation is not proportional to female and male size, challenging the relationship between early maturation and SSD. My research offers a new perspective to the study of sexual dimorphism evolution, highlighting the importance of studying both sexes from an integrative perspective and shedding light on the developmental processes underlying SSD. Advisor: Eileen A. Hebet

Proximate and Evolutionary Causes of Sexual Size Dimorphism in the Crab Spider \u3ci\u3eMecaphesa celer\u3c/i\u3e

Animal species’ body sizes result from the balance between selection for survival and selection for reproduction. In species with sexual size dimorphism (SSD), this balance differs between females and males, resulting in distinct sizes despite similar constraints. I used an integrative approach to understand how sexual section, and differences in developmental trajectories and metabolic physiology, resulted in the female biased SSD of the crab spider Mecaphesa celer (Thomisidae). SSD in spiders is often assumed to be a consequence of selection for early male maturation, which should provide males with additional mating opportunities. My results allow us to discard mate choice and differential fitness benefits as sexually selected drivers of M. celer’s SSD. Interestingly, I found evidence that M. celer females may mate with multiple males, and that, in such instances, eggs are fertilized by a mix of the males’ sperm. Such sperm mixing contradicts the hypothesis that M. celer males benefit from early-male maturation, as early-matured males do not necessarily fertilize most of a females’ eggs. To gain a better understanding of the relation between M. celer’s SSD and early male maturation, I identified the proximate mechanisms underlying differences in size between females and males, as well as the effects of the environment on the degree of SSD. Female M. celer reach larger sizes by growing faster and for longer than males, although both sexes have the same metabolism at rest. Also, female, but not male size, may be influenced by the interaction between diet and temperature. Integrating all of these results, I followed the reproductive season of a population of M. celer in the wild and found evidences that early maturation does indeed grant males with increased mating opportunities. Interestingly, I also found that the timing of male maturation is not proportional to female and male size, challenging the relationship between early maturation and SSD. My research offers a new perspective to the study of sexual dimorphism evolution, highlighting the importance of studying both sexes from an integrative perspective and shedding light on the developmental processes underlying SSD. Advisor: Eileen A. Hebet

Absence of Mate Choice and Postcopulatory Benefits in a Species with Extreme Sexual Size Dimorphism

Most hypotheses related to the evolution of female-biased extreme sexual size dimorphism (SSD) attribute the differences in the size of each sex to selection for reproduction, either through selection for increased female fecundity or selection for male increased mobility and faster development. Very few studies, however, have tested for direct fitness benefits associated with the latter – small male size. Mecaphesa celer is a crab spider with extreme SSD, whose males are less than half the size of females and often weigh 10 times less. Here, we test the hypotheses that larger size in females and smaller size in males are sexually selected through differential preand postcopulatory reproductive benefits. To do so, we tested the following predictions: matings between small males and large females are more likely to occur due to mate choice; females mated to small males are less likely to accept second copulation attempts; and matings between small males and large females will result in larger clutches of longer-lived offspring. Following staged mating trials in the laboratory, we found no support for any of our predictions, suggesting that SSD in M. celer may not be driven by pre- or post-reproductive fitness benefits to small males

Absence of Mate Choice and Postcopulatory Benefits in a Species with Extreme Sexual Size Dimorphism

Most hypotheses related to the evolution of female-biased extreme sexual size dimorphism (SSD) attribute the differences in the size of each sex to selection for reproduction, either through selection for increased female fecundity or selection for male increased mobility and faster development. Very few studies, however, have tested for direct fitness benefits associated with the latter – small male size. Mecaphesa celer is a crab spider with extreme SSD, whose males are less than half the size of females and often weigh 10 times less. Here, we test the hypotheses that larger size in females and smaller size in males are sexually selected through differential pre- and postcopulatory reproductive benefits. To do so, we tested the following predictions: matings between small males and large females are more likely to occur due to mate choice; females mated to small males are less likely to accept second copulation attempts; and matings between small males and large females will result in larger clutches of longer-lived offspring. Following staged mating trials in the laboratory, we found no support for any of our predictions, suggesting that SSD in M. celer may not be driven by pre- or post-reproductive fitness benefits to small males

Ecophysiological Determinants of Sexual Size Dimorphism: Integrating Growth Trajectories, Environmental Conditions, and Metabolic Rates

Sexual size dimorphism (SSD) often results in dramatic differences in body size between females and males. Despite its ecological importance, little is known about the relationship between developmental, physiological, and energetic mechanisms underlying SSD. We take an integrative approach to understand the relationship between developmental trajectories, metabolism, and environmental conditions resulting in extreme female-biased SSD in the crab spider Mecaphesa celer (Thomisidae). We tested for sexual differences in growth trajectories, as well as in the energetics of growth, hypothesizing that female M. celer have lower metabolic rates than males or higher energy assimilation. We also hypothesized that the environment in which spiderlings develop influences the degree of SSD of a population. We tracked growth and resting metabolic rates of female and male spiderlings throughout their ontogeny and quantified the adult size of individuals raised in a combination of two diet and two temperature treatments. We show that M. celer’s SSD results from differences in the shape of female and male growth trajectories. While female and male resting metabolic rates did not differ, diet, temperature, and their interaction influenced body size through an interactive effect with sex, with females being more sensitive to the environment than males. We demonstrate that the shape of the growth curve is an important but often overlooked determinant of SSD and that females may achieve larger sizes through a combination of high food ingestion and low activity levels. Our results highlight the need for new models of SSD based on ontogeny, ecology, and behavior

Field evidence challenges the often-presumed relationship between early male maturation and female-biased sexual size dimorphism

Female-biased sexual size dimorphism (SSD) is often considered an epiphenomenon of selection for the increased mating opportunities provided by early male maturation (i.e., protandry). Empirical evidence of the adaptive significance of protandry remains nonetheless fairly scarce. We use field data collected throughout the reproductive season of an SSD crab spider, Mecaphesa celer, to test two hypotheses: Protandry provides fitness benefits to males, leading to female-biased SSD, or protandry is an indirect consequence of selection for small male size/large female size. Using field-collected data, we modeled the probability of mating success for females and males according to their timing of maturation. We found that males matured earlier than females and the proportion of virgin females decreased abruptly early in the season, but unexpectedly increased afterward. Timing of female maturation was not related to clutch size, but large females tended to have more offspring than small females. Timing of female and male maturation was inversely related to size at adulthood, as early-maturing individuals were larger than late-maturing ones, suggesting that both sexes exhibit some plasticity in their developmental trajectories. Such plasticity indicates that protandry could co-occur with any degree and direction of SSD. Our calculation of the probability of mating success along the season shows multiple male maturation time points with similar predicted mating success. This suggests that males follow multiple strategies with equal success, trading-off access to virgin females with intensity of male–male competition. Our results challenge classic hypotheses linking protandry and female-biased SSD, and emphasize the importance of directly testing the often-assumed relationships between co-occurring animal traits

Absence of Mate Choice and Postcopulatory Benefits in a Species with Extreme Sexual Size Dimorphism

Most hypotheses related to the evolution of female-biased extreme sexual size dimorphism (SSD) attribute the differences in the size of each sex to selection for reproduction, either through selection for increased female fecundity or selection for male increased mobility and faster development. Very few studies, however, have tested for direct fitness benefits associated with the latter – small male size. Mecaphesa celer is a crab spider with extreme SSD, whose males are less than half the size of females and often weigh 10 times less. Here, we test the hypotheses that larger size in females and smaller size in males are sexually selected through differential preand postcopulatory reproductive benefits. To do so, we tested the following predictions: matings between small males and large females are more likely to occur due to mate choice; females mated to small males are less likely to accept second copulation attempts; and matings between small males and large females will result in larger clutches of longer-lived offspring. Following staged mating trials in the laboratory, we found no support for any of our predictions, suggesting that SSD in M. celer may not be driven by pre- or post-reproductive fitness benefits to small males

Maternal care in the harvestman Neosadocus maximus (Arachnida: Opiliones)

Muitas formas de cuidado parental aumentam a sobrevivência da prole, ao custo de uma diminuição na capacidade dos pais em investir em proles futuras. Espera-se, portanto, que indivíduos parentais adotem estratégias de cuidado que lhes permitam balancear benefícios imediatos e custos futuros, otimizando seu sucesso reprodutivo total. Nesta dissertação, testamos um potencial custo e um benefício do cuidado maternal do opilião Neosadocus maximus, espécie cujas fêmeas desertam suas desovas periodicamente e acrescentam ovos a elas por um período de até duas semanas características únicas entre opiliões com cuidado maternal. Exploramos também quais fatores permitem a deserção temporária das fêmeas de N. maximus. Mais especificamente, testamos as seguintes hipóteses: (1) o cuidado maternal protege os ovos contra predadores, que são mais comuns no período noturno; (2) o cuidado maternal diminui a fecundidade imediata da fêmea; (3) a camada de muco que as fêmeas depositam sobre seus ovos mantém a prole protegida nos momentos de deserção temporária. Nossos resultados apontam que o cuidado maternal é uma proteção eficaz contra o ataque de predadores, sem afetar negativamente a fecundidade imediata das fêmeas. Demonstramos também que a camada de muco mantém os ovos relativamente protegidos na ausência da fêmea guardiã, permitindo que estas se ausentem de suas desovas sem deixar a prole totalmente vulnerável. Sendo assim, a camada de muco que recobre os ovos de N. maximus permite que estas fêmeas minimizem os custos fisiológicos associados ao cuidado maternal sem, entretanto, aumentar os custos da deserção em termos de redução de prole provocada por predação.Many forms of parental care increase offspring survival, at the cost of a decrease in the parental individuals capacity to invest in a future brood. It is expected, therefore, that parental individuals adopt parental strategies that allow them to balance immediate benefits and future costs, optimizing their total reproductive success. In this thesis, we aimed to identify costs and benefits of maternal care in the harvestman Neosadocus maximus, whose females periodically desert their broods and add eggs to their clutches for up to two weeks unique characteristics among harvestmen with maternal care. We also explored which factors allow N. maximus females to desert their clutches frequently. We tested the following hypothesis: (1) maternal care protects the eggs against predators, especially at night; (2) maternal care decreases the current female fecundity; (3) the mucus coat covering the eggs protects them against predators even in the absence of the guarding female. Our results indicate that maternal care is an effective protection against egg predators, and does not decrease current female fecundity. We also demonstrated that the mucus coat provides effective protection to the eggs in the absence of the guarding female, allowing them to abandon periodically their clutches without leaving the offspring completely vulnerable to predators. The mucus coat covering N. maximus eggs allows these females to minimize the physiological costs of caring, with no severe increase in the cost of deserting in terms of brood reduction by predation

Proximate and evolutionary causes of sexual size dimorphism in the crab spider Mecaphesa celer

Animal species’ body sizes result from the balance between selection for survival and selection for reproduction. In species with sexual size dimorphism (SSD), this balance differs between females and males, resulting in distinct sizes despite similar constraints. I used an integrative approach to understand how sexual section, and differences in developmental trajectories and metabolic physiology, resulted in the female biased SSD of the crab spider Mecaphesa celer (Thomisidae). SSD in spiders is often assumed to be a consequence of selection for early male maturation, which should provide males with additional mating opportunities. My results allow us to discard mate choice and differential fitness benefits as sexually selected drivers of M. celer’s SSD. Interestingly, I found evidence that M. celer females may mate with multiple males, and that, in such instances, eggs are fertilized by a mix of the males’ sperm. Such sperm mixing contradicts the hypothesis that M. celer males benefit from early-male maturation, as early-matured males do not necessarily fertilize most of a females’ eggs. To gain a better understanding of the relation between M. celer’s SSD and early male maturation, I identified the proximate mechanisms underlying differences in size between females and males, as well as the effects of the environment on the degree of SSD. Female M. celer reach larger sizes by growing faster and for longer than males, although both sexes have the same metabolism at rest. Also, female, but not male size, may be influenced by the interaction between diet and temperature. Integrating all of these results, I followed the reproductive season of a population of M. celer in the wild and found evidences that early maturation does indeed grant males with increased mating opportunities. Interestingly, I also found that the timing of male maturation is not proportional to female and male size, challenging the relationship between early maturation and SSD. My research offers a new perspective to the study of sexual dimorphism evolution, highlighting the importance of studying both sexes from an integrative perspective and shedding light on the developmental processes underlying SSD

Proximate and evolutionary causes of sexual size dimorphism in the crab spider Mecaphesa celer

Animal species’ body sizes result from the balance between selection for survival and selection for reproduction. In species with sexual size dimorphism (SSD), this balance differs between females and males, resulting in distinct sizes despite similar constraints. I used an integrative approach to understand how sexual section, and differences in developmental trajectories and metabolic physiology, resulted in the female biased SSD of the crab spider Mecaphesa celer (Thomisidae). SSD in spiders is often assumed to be a consequence of selection for early male maturation, which should provide males with additional mating opportunities. My results allow us to discard mate choice and differential fitness benefits as sexually selected drivers of M. celer’s SSD. Interestingly, I found evidence that M. celer females may mate with multiple males, and that, in such instances, eggs are fertilized by a mix of the males’ sperm. Such sperm mixing contradicts the hypothesis that M. celer males benefit from early-male maturation, as early-matured males do not necessarily fertilize most of a females’ eggs. To gain a better understanding of the relation between M. celer’s SSD and early male maturation, I identified the proximate mechanisms underlying differences in size between females and males, as well as the effects of the environment on the degree of SSD. Female M. celer reach larger sizes by growing faster and for longer than males, although both sexes have the same metabolism at rest. Also, female, but not male size, may be influenced by the interaction between diet and temperature. Integrating all of these results, I followed the reproductive season of a population of M. celer in the wild and found evidences that early maturation does indeed grant males with increased mating opportunities. Interestingly, I also found that the timing of male maturation is not proportional to female and male size, challenging the relationship between early maturation and SSD. My research offers a new perspective to the study of sexual dimorphism evolution, highlighting the importance of studying both sexes from an integrative perspective and shedding light on the developmental processes underlying SSD