Periodic Cooling during Incubation Alters the Adrenocortical Response and Posthatch Growth in Zebra Finches

Heat-induced maternal effects shape avian eggshell traits and embryo development and phenotype at high incubation temperatures

Phenotypic plasticity is an important avenue by which organisms may persist in the face of rapid environmental change. Environmental cues experienced by the mother can also influence the phenotype of offspring, a form of plasticity called maternal effects. Maternal effects can adaptively prepare offspring for the environmental conditions they will likely experience; however, their ability to buffer offspring against environmental stressors as embryos is understudied. Using captive zebra finches, we performed a maternal-offspring environmental match-mismatch experiment utilizing a 2 × 2 × 2 factorial design. Mothers were exposed to a mild heat conditioning (38°C) or control (22°C) treatment as juveniles, an acute high heat (42°C) or control (22°C) treatment as adults, then paired for breeding. The eggs produced by those females were incubated at a hyperthermic (38.5°C) or optimal temperature (37.2°C). We found that when mothers were exposed to a mild heat conditioning as juveniles, their embryos exhibited reduced water loss, longer development times, and produced hatchlings with heavier pectoralis muscles when incubated at high incubation temperatures, compared to embryos from control mothers. Mothers exposed to both the mild heat conditioning as juveniles and a high heat stressor as adults produced eggs with a higher density of shell pores and embryos with lower heart rates during development. However, there was a cost when there was a mismatch between maternal and embryo environment. Embryos from these conditioned and heat-stressed mothers had reduced survival at control incubation temperatures, indicating the importance of offspring environment when interpreting potential adaptive effects.

Suboptimal Embryonic Incubation Temperature Has Long-Term, Sex-Specific Consequences on Beak Coloration and the Behavioral Stress Response in Zebra Finches

Secondary sex characteristics, like beak color in some avian species, have indirect impacts on reproductive success, as they are considered to be honest indicators of condition, immunocompetence, and developmental history. However, little is known about the long-term effects of environmental perturbations on the production and maintenance of these secondary sex characteristics in avian species. In zebra finches (Taeniopygia guttata), redder beaks indicate increased carotenoid expression and implantation into beak tissue, and female zebra finches prefer males with pronounced bright red beaks as a mate. The present study examines the long-term effects of embryonic incubation temperature on the maturation of beak color in zebra finches. We also investigated the effects of embryonic incubation temperature on sensitivity to a handling and restraint stressor in adulthood. Specifically, the aims of this study were to examine: (1) whether suboptimal incubation temperatures affect the timing of beak color development and color characteristics before and after sexual maturity, (2) if repeated handling causes short-term changes in beak color and whether color changes are related to embryonic thermal environment, and (3) how thermal stress during incubation alters future responses to a repeated handling stressor. Zebra finch eggs were randomly assigned to one of three incubators: “Control,” “Low,” or “Periodic Cooling.” Beak color (hue, saturation, and value) was quantified before [45, 60, 75 days post-hatch (dph)] and after sexual maturity (95 dph), as well as after repeated handling stress later in adulthood (avg of 386 dph). We found that there were age- and sex- specific effects of incubation treatment on beak hue, where females from periodically cooled eggs had decreased hues (redder) in adulthood. Additionally, eggs laid later in a clutch had decreased beak saturation levels throughout life regardless of incubation environment. We found that females had lower beak hue and saturation following a capture and restraint stressor, while males showed increased beak saturation. Lastly, males subjected to the Low incubation treatment had relatively higher activity levels during restraint than those in the Control group. Overall, these findings suggest that fluctuating incubation temperatures combined with repeated, short-term stressors can have significant, sex-specific effects on sexual ornamentation and behavior.

Incubation temperature and physiological aging in the zebra finch

In birds, incubation temperature has received increased attention as an important source of phenotypic variability in offspring. A lower than optimal incubation temperature may negatively affect aspects of nestling physiology, such as body growth and energy metabolism. However, the long-term effects of sub-optimal incubation temperature on morphology and physiology are not well understood. In a previous study, we showed that zebra finches from eggs incubated at a low temperature (35.9°C) for 2/3 of the total incubation time suffered a lower post-fledging survival compared to individuals that had been incubated at higher temperatures (37.0 and 37.9°C). In the present study, we investigated whether these variations in incubation temperature could cause permanent long-lasting differences in body mass, body size, or basal metabolic rate. Furthermore, we tested whether the observed differences in survival between treatment groups would be reflected in the rate of physiological deterioration, assessed through oxidative damage and decreased metabolic rate with age (i.e. 'metabolic aging'). Incubation temperature did not significantly affect embryonic or nestling body growth and did not influence final adult body mass or body size. Nor was there any long-term effect on basal metabolic rate. Birds from eggs incubated at the lowest temperature experienced an accumulation of oxidative damage with age, although this was not accompanied by an accelerated rate of metabolic aging. The present results suggest that the low survival in these birds was possibly mediated by increased oxidative stress, but independent of body growth and the basal metabolic rate.

Detrimental or beneficial? Untangling the literature on developmental stress studies in birds

Developing animals display a tremendous ability to change the course of their developmental path in response to the environment they experience, a concept referred to as developmental plasticity. This change in behavior, physiology or cellular processes is primarily thought to allow animals to better accommodate themselves to the surrounding environment. However, existing data on developmental stress and whether it brings about beneficial or detrimental outcomes show conflicting results. There are several well-referred hypotheses related to developmental stress in the current literature, such as the environmental matching, silver spoon and thrifty phenotype hypotheses. These hypotheses speculate that the early-life environment defines the capacity of the physiological functions and behavioral tendencies and that this change is permanent and impacts the fitness of the individual. These hypotheses also postulate there is a trade-off among organ systems and physiological functions when resources are insufficient. Published data on avian taxa show that some effects of developmental nutritional and thermal stressors are long lasting, such as the effects on body mass and birdsong. Although hypotheses on developmental stress are based on fitness components, data on reproduction and survival are scarce, making it difficult to determine which hypothesis these data support. Furthermore, most physiological and performance measures are collected only once; thus, the physiological mechanisms remain undertested. Here, we offer potential avenues of research to identify reasons behind the contrasting results in developmental stress research and possible ways to determine whether developmental programming due to stressors is beneficial or detrimental, including quantifying reproduction and survival in multiple environments, measuring temporal changes in physiological variables and testing for stress resistance later in life.