January 26, 2011

Hormones Dictate Breeding Success In Birdsï»¿

Max Planck researchers can predict the number of offspring a bird will have based on its prolactin and corticosterone levels

Some animals produce more offspring than others do. Hormones like prolactin and corticosterone can exercise a crucial influence on the behavior of birds in the breeding season and therefore on their reproductive success. Scientists from the Max Planck Institute for Ornithology in Radolfzell and their colleagues at the universities of Princeton and Edinburgh have now demonstrated that hormone levels not only play a key role during the breeding season, but already dictate, long in advance, how many eggs a breeding pair will lay, when they will lay them and how often. An animal's hormonal constitution is thus of major significance for its reproductive success, and is possibly an important driving force of evolution. (Proceedings of the Royal Society B, January 19, 2011)



The scientists counted the number of eggs, clutches and fledglings for each breeding pair. They also took regular blood samples three weeks before the beginning of breeding and during the rearing of the first clutch, both in natural situations and under artificially generated stress, to determine the corresponding concentrations of the hormones corticosterone and prolactin in the birds' blood. "We were surprised that we could predict how many offspring a breeding pair would have based on hormone levels three weeks before the breeding season," reports Jenny Ouyang. "Sparrows that had low corticosterone values before the breeding season raised the most offspring. Especially birds with low values before, but high values during the season had the highest reproductive success. They apparently invested a lot of energy in the brood." In contrast, the animals that had a very strong hormonal reaction to stress fed their young less and produced fewer offspring.

According to the scientists, prolactin plays a crucial role in the timing of laying the first egg: females with higher prolactin levels started laying earlier and had more offspring as a result. "The fact that the two parents have very similar hormonal values is also fascinating for us," adds Jenny Ouyang. "It is not yet clear whether the couples influence each other's hormonal status or whether they select partners with similar hormone levels."

The results of this study enhance knowledge about the physiological mechanisms that can dictate when a bird breeds, how many eggs it produces per clutch and how often it breeds. The hormones prolactin and corticosterone, in particular, play a more important role in the regulation of individual investments before reproduction begins than was previously assumed. If an individual's hormonal constitution can be inherited, this could provide a fundamental explanation as to why some individuals produce large numbers of offspring and are more successful than others from an evolutionary perspective.


On the Net: