The link between a mother and its offspring goes much deeper than simply sharing genetic material. The condition of the mother and the environment she lives in also influence the development of her offspring, through mechanisms such as exposure to maternal stress hormone levels during development. In birds, the levels of stress hormones in the mother's blood are reflected in her eggs and exposure to stress hormones during development can have detrimental effects on her young. However, it has also been proposed that stress hormone exposure might prepare the offspring to face its new environment. The evolutionary and ecological consequences of these `maternal effects' have recently been considered by Eunice Chin, Oliver Love and their colleagues from Trent and Simon Fraser Universities in a paper published in the Proceedings of the Royal Society B. To study the impact that maternal effects can have on fitness, the researchers studied flight performance of European starlings. They hypothesized that exposure to higher levels of the stress hormone corticosterone during development would improve fledgling flight performance.
Using nest boxes located in the field, the team injected newly laid European starling eggs with corticosterone hormone. They then marked individual hatchlings, tracked them until they fledged at 21 days and took various physical measurements. Taking a flight performance chamber to the field the team filmed the fledglings during a short flight so that they could calculate the mechanical energy gain during take-off. They also measured the birds' body mass and wing morphology and were able to calculate the animals'wing loading, which is the body mass supported by a given surface area of wing. After the flight trials, the team measured the mass and composition of the fledglings' pectoral flight muscles, as well as their metabolic enzyme activity, which is an indicator of the metabolic energy production potential of the muscle. Having quantified the fledglings' physical condition, the research group looked to see whether exposure to elevated corticosterone levels had affected the chicks' development.
Initially they found no differences in hatching success, brood size, sex ratio, fledging success, body mass or size between birds that had been exposed to corticosterone and those that had not. However, exposure to elevated levels of stress hormone did improve the fledgling's flight performance. Their energy gain after take-off (an indicator of the bird's take-off velocity and height gained after take-off) was greater in both male and female fledglings treated with hormone. Their improved flight performance was associated with physical differences between the treated and untreated birds. The corticosterone-treated birds had larger pectoral flight muscles and a larger wing surface area, which reduced their wing loading during flight. In addition to the physical differences, the properties of the flight muscle were also different. Two enzymes involved in metabolic energy production, citrate synthase and creatine phosphokinase, had higher activity per unit muscle mass in the corticosterone-treated group.
Chin, Love and their colleagues have shown that fledglings exposed to maternal stress hormone during embryonic development have improved flight performance due to differences in wing size and muscle tissue, adding maternal effects to the role call of phenotype fine-tuning for future consideration.
