The Cost of Sex: Quantifying Energetic Investment in Gamete Production by Males and Females

The relative energetic investment in reproduction between the sexes forms the basis of sexual selection and life history theories in evolutionary biology. It is often assumed that males invest considerably less in gametes than females, but quantifying the energetic cost of gamete production in both sexes has remained a difficult challenge. For a broad diversity of species (invertebrates, reptiles, amphibians, fishes, birds, and mammals), we compared the cost of gamete production between the sexes in terms of the investment in gonad tissue and the rate of gamete biomass production. Investment in gonad biomass was nearly proportional to body mass in both sexes, but gamete biomass production rate was approximately two to four orders of magnitude higher in females. In both males and females, gamete biomass production rate increased with organism mass as a power law, much like individual metabolic rate. This suggests that whole-organism energetics may act as a primary constraint on gamete production among species. Residual variation in sperm production rate was positively correlated with relative testes size. Together, these results suggest that understanding the heterogeneity in rates of gamete production among species requires joint consideration of the effects of gonad mass and metabolism

Elevated Plasma Corticosterone Decreases Yolk Testosterone and Progesterone in Chickens: Linking Maternal Stress and Hormone-Mediated Maternal Effects

Despite considerable research on hormone-mediated maternal effects in birds, the underlying physiology remains poorly understood. This study investigated a potential regulation mechanism for differential accumulation of gonadal hormones in bird eggs. Across vertebrates, glucocorticoids can suppress reproduction by downregulating gonadal hormones. Using the chicken as a model species, we therefore tested whether elevated levels of plasma corticosterone in female birds influence the production of gonadal steroids by the ovarian follicles and thus the amount of reproductive hormones in the egg yolk. Adult laying hens of two different strains (ISA brown and white Leghorn) were implanted subcutaneously with corticosterone pellets that elevated plasma corticosterone concentrations over a period of nine days. Steroid hormones were subsequently quantified in plasma and yolk. Corticosterone-implanted hens of both strains had lower plasma progesterone and testosterone levels and their yolks contained less progesterone and testosterone. The treatment also reduced egg and yolk mass. Plasma estrogen concentrations decreased in white Leghorns only whereas in both strains yolk estrogens were unaffected. Our results demonstrate for the first time that maternal plasma corticosterone levels influence reproductive hormone concentrations in the yolk. Maternal corticosterone could therefore mediate environmentally induced changes in yolk gonadal hormone concentrations. In addition, stressful situations experienced by the bird mother might affect the offspring via reduced amounts of reproductive hormones present in the egg as well as available nutrients for the embryo

Differential Effects of Early- and Late-Life Access to Carotenoids on Adult Immune Function and Ornamentation in Mallard Ducks (Anas platyrhynchos)

Environmental conditions early in life can affect an organism’s phenotype at adulthood, which may be tuned to perform optimally in conditions that mimic those experienced during development (Environmental Matching hypothesis), or may be generally superior when conditions during development were of higher quality (Silver Spoon hypothesis). Here, we tested these hypotheses by examining how diet during development interacted with diet during adulthood to affect adult sexually selected ornamentation and immune function in male mallard ducks (Anas platyrhynchos). Mallards have yellow, carotenoid-pigmented beaks that are used in mate choice, and the degree of beak coloration has been linked to adult immune function. Using a 2×2 factorial experimental design, we reared mallards on diets containing either low or high levels of carotenoids (nutrients that cannot be synthesized de novo) throughout the period of growth, and then provided adults with one of these two diets while simultaneously quantifying beak coloration and response to a variety of immune challenges. We found that both developmental and adult carotenoid supplementation increased circulating carotenoid levels during dietary treatment, but that birds that received low-carotenoid diets during development maintained relatively higher circulating carotenoid levels during an adult immune challenge. Individuals that received low levels of carotenoids during development had larger phytohemagglutinin (PHA)-induced cutaneous immune responses at adulthood; however, dietary treatment during development and adulthood did not affect antibody response to a novel antigen, nitric oxide production, natural antibody levels, hemolytic capacity of the plasma, or beak coloration. However, beak coloration prior to immune challenges positively predicted PHA response, and strong PHA responses were correlated with losses in carotenoid-pigmented coloration. In sum, we did not find consistent support for either the Environmental Matching or Silver Spoon hypotheses. We then describe a new hypothesis that should be tested in future studies examining developmental plasticity

The Case of the Missing Mechanism:How Does Temperature Influence Seasonal Timing in Endotherms?

<p>Temperature has a strong effect on the seasonal timing of life-history stages in both mammals and birds, even though these species can regulate their body temperature under a wide range of ambient temperatures. Correlational studies showing this effect have recently been supported by experiments demonstrating a direct, causal relationship between ambient temperature and seasonal timing. Predicting how endotherms will respond to global warming requires an understanding of the physiological mechanisms by which temperature affects the seasonal timing of life histories. These mechanisms, however, remain obscure. We outline a road map for research aimed at identifying the pathways through which temperature is translated into seasonal timing.</p>