Wendy R. Hood

A multifactorial test of the effects of carotenoid access, food intake and parasite load on the production of ornamental feathers and bill coloration in American goldfinches.

SUMMARY It has been well established that carotenoid and melanin pigmentation are often condition-dependent traits in vertebrates. Expression of carotenoid coloration in birds has been shown to reflect pigment intake, food access and parasite load; however, the relative importance of and the potential interactions among these factors have not been previously considered. Moreover, carotenoid and melanin pigmentation have been proposed to signal fundamentally different aspects of individual condition but few data exist to test this idea. We simultaneously manipulated three environmental conditions under which American goldfinches (Cardeulis tristis) grew colorful feathers and developed carotenoid pigmentation of their bills. Male goldfinches were held with either high or low carotenoid supplementation, pulsed or continuous antimicrobial drug treatment, or restricted or unlimited access to food. Carotenoid supplementation had an overriding effect on yellow feather coloration. Males given more lutein and zeaxanthin grew yellow feathers with hue shifted toward orange and with higher yellow chroma than males supplemented with fewer carotenoids. Parasites and food access did not significantly affect yellow feather coloration, and there were only minor interaction effects for the three treatments. By contrast, bill coloration was significantly affected by all three treatments. Carotenoid supplementation had a significant effect on yellow chroma of bills, drug treatment and food access both had a significant effect on bill hue, and food access had a significant effect on the yellow brightness of bills. Neither the size nor blackness of the black caps of male goldfinches was affected by any treatment. These results indicate that pigment intake, food access and parasite load can have complex and variable effects on color displays, and that feather and bill coloration signal different aspects of male condition.

Testing the predictions of energy allocation decisions in the evolution of life‐history trade‐offs

Summary 1. Allocating a greater amount of limited resources, such as energy, to current reproduction can reduce the amount of energy available for somatic maintenance and can ultimately impair future breeding success or maternal survival (i.e. cost of reproduction hypothesis). Although there is some support for the cost of reproduction hypothesis in birds, few empirical studies of mammals have demonstrated a trade-off between current and future reproduction. More importantly, most studies testing ultimate costs have neglected to confirm that the proximate costs of reproduction are high. 2. We experimentally manipulated litter size in a wild population of Columbian ground squirrels for 2 years to examine both the proximate energetic and ultimate fitness (i.e. survival and breeding) costs of reproduction. We predicted that females raising augmented litters would have the highest rates of daily energy expenditure and as a result would experience lower survival rates or future fecundity. 3. Females raising augmented litters weaned more pups, had the highest litter masses at weaning, and had field metabolic rates that were almost 15 times greater than females raising control or reduced litters. Contrary to our prediction, there were no negative impacts of greater maternal investment and higher energy expenditure on the probability of maternal survival or future reproduction. 4. Pups from augmented litters grew more slowly during the lactation period were smaller at weaning and had a lower probability of survival over-winter. Thus, although females were capable of raising more young than they gave birth to without short-term costs of reduced survival or fecundity, our observations suggest that limitations to litter size are not due to a trade-off in the allocation of energy, but rather due to the reduced survival of offspring from larger litters. 5. Examining the proximate mechanisms hypothesized to underlie life-history trade-offs can be challenging but is critical for a comprehensive understanding of the evolution of life histories.

Current versus future reproduction and longevity: a re-evaluation of predictions and mechanisms

ABSTRACT Oxidative damage is predicted to be a mediator of trade-offs between current reproduction and future reproduction or survival, but most studies fail to support such predictions. We suggest that two factors underlie the equivocal nature of these findings: (1) investigators typically assume a negative linear relationship between current reproduction and future reproduction or survival, even though this is not consistently shown by empirical studies; and (2) studies often fail to target mechanisms that could link interactions between sequential life-history events. Here, we review common patterns of reproduction, focusing on the relationships between reproductive performance, survival and parity in females. Observations in a range of species show that performance between sequential reproductive events can decline, remain consistent or increase. We describe likely bioenergetic consequences of reproduction that could underlie these changes in fitness, including mechanisms that could be responsible for negative effects being ephemeral, persistent or delayed. Finally, we make recommendations for designing future studies. We encourage investigators to carefully consider additional or alternative measures of bioenergetic function in studies of life-history trade-offs. Such measures include reactive oxygen species production, oxidative repair, mitochondrial biogenesis, cell proliferation, mitochondrial DNA mutation and replication error and, importantly, a measure of the respiratory function to determine whether measured differences in bioenergetic state are associated with a change in the energetic capacity of tissues that could feasibly affect future reproduction or lifespan. More careful consideration of the life-history context and bioenergetic variables will improve our understanding of the mechanisms that underlie the life-history patterns of animals. Summary: This paper critically reviews the effects and physiological mechanisms that reproduction could have on bioenergetic capacity to better understand how a reproductive event could affect future reproduction and/or survival.

Reproduction does not adversely affect liver mitochondrial respiratory function but results in lipid peroxidation and increased antioxidants in house mice

Reproduction is thought to come at a cost to longevity. Based on the assumption that increased energy expenditure during reproduction is associated with increased free-radical production by mitochondria, oxidative damage has been suggested to drive this trade-off. We examined the impact of reproduction on liver mitochondrial function by utilizing post-reproductive and non-reproductive house mice (Mus musculus) living under semi-natural conditions. The age-matched post-reproductive and non-reproductive groups were compared after the reproductive females returned to a non-reproductive state, so that both groups were in the same physiological state at the time the liver was collected. Despite increased oxidative damage (p = 0.05) and elevated CuZnSOD (p = 0.002) and catalase (p = 0.04) protein levels, reproduction had no negative impacts on the respiratory function of liver mitochondria. Specifically, in a post-reproductive, maintenance state the mitochondrial coupling (i.e., respiratory control ratio) of mouse livers show no negative impacts of reproduction. In fact, there was a trend (p = 0.059) to suggest increased maximal oxygen consumption by liver mitochondria during the ADP stimulated state (i.e., state 3) in post-reproduction. These findings suggest that oxidative damage may not impair mitochondrial respiratory function and question the role of mitochondria in the trade-off between reproduction and longevity. In addition, the findings highlight the importance of quantifying the respiratory function of mitochondria in addition to measuring oxidative damage.

Milk composition in a hibernating rodent, the Columbian ground squirrel (Urocitellus columbianus)

Milk is essential to a mammalian mothers reproductive strategy and is necessary for offspring growth and development. In hibernators with a short duration between weaning and winter immergence, milk synthesis is likely constrained by time and trade-offs between maternal and offspring condition, thus influencing milk composition. We characterized the proximate and mineral composition of milk produced by a hibernating rodent, the Columbian ground squirrel (Urocitellus columbianus). The concentration of all milk components varied across lactation; the concentration of most constituents peaked between days 14 and 19 postpartum. Columbian ground squirrel milk was relatively low in lipids but high in protein and calcium. At peak lactation, milk was composed of 10.71% ± 0.46% SE protein, 9.15% ± 0.47% lipids, 3.39% ± 0.13% sugar, and 0.47% ± 0.02% calcium (wet mass basis). High protein, energy from protein, and calcium in milk corroborate earlier reports of the importance of fast growth rates of juveniles to overwinter survival, whereas the low lipid content of milk may reflect fat conservation for adults. Production of high-calcium milk also may be a preventive mechanism enabling offspring to cope with bone mineral loss during hibernation.

Calcium Availability Influences Litter Size and Sex Ratio in White-Footed Mice (Peromyscus leucopus)

The production of offspring typically requires investment of resources derived from both the environment and maternal somatic reserves. As such, the availability of either of these types of resources has the potential to limit the degree to which resources are allocated to reproduction. Theory and empirical studies have argued that mothers modify reproductive performance relative to exogenous resource availability and maternal condition by adjusting size, number or sex of offspring produced. These relationships have classically been defined relative to availability of energy sources; however, in vertebrates, calcium also plays a critical role in offspring production, as a considerable amount of calcium is required to support the development of offspring skeleton(s). We tested whether the availability of calcium influences reproductive output by providing female white-footed mice with a low-calcium or standard diet from reproductive maturity to senescence. We then compared maternal skeletal condition and reproductive output, based on offspring mass, offspring number and litter sex ratio, between dietary treatments. Mothers on the low-calcium diet exhibited diminished skeletal condition at senescence and produced smaller and strongly female-biased litters. We show that skeletal condition and calcium intake can influence sex ratio and reproductive output following general theoretical models of resource partitioning during reproduction.

Relationship between maternal environment and DNA methylation patterns of estrogen receptor alpha in wild Eastern Bluebird (Sialia sialis) nestlings: a pilot study.

Abstract There is mounting evidence that, across taxa, females breeding in competitive environments tend to allocate more testosterone to their offspring prenatally and these offspring typically have more aggressive and faster‐growing phenotypes. To date, no study has determined the mechanisms mediating this maternal effects influence on offspring phenotype. However, levels of estrogen receptor alpha (ER α) gene expression are linked to differences in early growth and aggression; thus, maternal hormones may alter gene regulation, perhaps via DNA methylation, of ER α in offspring during prenatal development. We performed a pilot study to examine natural variation in testosterone allocation to offspring through egg yolks in wild Eastern Bluebirds (Sialia sialis) in varying breeding densities and percent DNA methylation of CG dinucleotides in the ER α promoter in offspring brain regions associated with growth and behavior. We hypothesized that breeding density would be positively correlated with yolk testosterone, and prenatal exposure to maternal‐derived yolk testosterone would be associated with greater offspring growth and decreased ER α promoter methylation. Yolk testosterone concentration was positively correlated with breeding density, nestling growth rate, and percent DNA methylation of one out of five investigated CpG sites (site 3) in the diencephalon ER α promoter, but none in the telencephalon (n = 10). Percent DNA methylation of diencephalon CpG site 3 was positively correlated with growth rate. These data suggest a possible role for epigenetics in mediating the effects of the maternal environment on offspring phenotype. Experimentally examining this mechanism with a larger sample size in future studies may help elucidate a prominent way in which animals respond to their environment. Further, by determining the mechanisms that mediate maternal effects, we can begin to understand the potential for the heritability of these mechanisms and the impact that maternal effects are capable of producing at an evolutionary scale.

A Test of Bone Mobilization Relative to Reproductive Demand: Skeletal Quality Is Improved in Cannibalistic Females with Large Litters

In species with repeated bouts of reproduction, a female’s ability to retain sufficient tissue for self-maintenance is essential to her survival and capacity for future reproduction. Loss of bone mineral content results in bone fragility and the possibility of reduced survival, so females should guard against the overuse of their bone mineral during reproduction. Given these constraints, I predicted that bone mobilization would increase with litter size in mice but plateau before maximum litter size was reached. To test this idea, I manipulated the litter sizes of house mice on the day of parturition to 3, 8, 13, and 18 offspring. At weaning, I euthanized the females and calculated whole-body and bone mineral composition. The total mineral content of females’ femurs dropped as litter size increased to the average litter size for this strain of mouse (13) but surprisingly, femoral mineral content was higher for females assigned the largest litter sizes (18). Seven of the nine females assigned 18 young cannibalized some of their offspring. For females assigned to these larger litters, femoral ash content was not correlated with number of young consumed, suggesting that mineral recycling had little effect on final bone mineral content. However, nursing effort (accounting for young lost to cannibalism) was correlated with maternal femoral ash at weaning. These finding suggest that the high bone mineral content of females assigned the largest litters was associated with a reduction in endogenous mineral allocated to the litter.

Milk matters: offspring survival in Columbian ground squirrels is affected by nutrient composition of mother's milk

Through maternal effects, information about environmental conditions experienced in the maternal generation can be transmitted to subsequent generations. Although maternal effects have been described and quantified in many mammalian species, the underlying causal links are often under-studied. The close association between mother and neonate during the extended period of lactation in mammals provides a unique opportunity for mothers to influence offspring phenotype through nutrient provisioning of milk. The purpose of this study was to examine sources of inter-individual variation in milk composition and impacts on offspring phenotype and survival. Variation in fat content, protein content, and energy density of mother’s milk was associated with timing of reproduction. Mothers with intermediate birthdates produced milk that was higher in fat and energy density, but lower in protein than females with early or late birthdates. In turn, the pattern of change in milk composition affected juvenile overwinter survival. The protein content of Columbian ground squirrel milk typically increases to peak lactation and then declines before weaning. Pups consuming milk that was lower in protein in early lactation but then relatively high in protein at peak and late lactation had a higher probability of survival overwinter. Our results indicate that the interplay between the timing of reproduction and lactation performance has consequences for maternal and offspring fitness.

No evidence that carotenoid pigments boost either immune or antioxidant defenses in a songbird

Dietary carotenoids have been proposed to boost immune system and antioxidant functions in vertebrate animals, but studies aimed at testing these physiological functions of carotenoids have often failed to find support. Here we subject yellow canaries (Serinus canaria), which possess high levels of carotenoids in their tissue, and white recessive canaries, which possess a knockdown mutation that results in very low levels of tissue carotenoids, to oxidative and pathogen challenges. Across diverse measures of physiological performance, we detect no differences between carotenoid-rich yellow and carotenoid-deficient white canaries. These results add further challenge to the assumption that carotenoids are directly involved in supporting physiological function in vertebrate animals. While some dietary carotenoids provide indirect benefits as retinoid precursors, our observations suggest that carotenoids themselves may play little to no direct role in key physiological processes in birds.Dietary carotenoids have been proposed to have physiological benefits in addition to contributing to coloration. Here, Koch et al. compare immune and antioxidant functions in yellow, carotenoid-rich vs. white, carotenoid-deficient canaries and find no difference, suggesting a limited physiological role of carotenoids.