Human beings, fish, reptiles and birds have the same hormones in their blood with very similar functions. But why does one find hormone values in some species that are ten times higher than in others? A team of international scientists headed by Dr. Michaela Hau from the Max Planck Institute for Ornithology in Radolfzell has now discovered that the differing concentrations in birds of the stress hormone, corticosterone, and the reproductive hormone, testosterone, are correlated with the "pace of life." They control whether energy is invested into reproduction, i.e. the number of eggs laid and the breeding attempts in a given season, or more into longevity, i.e. immune function or the flight response in times of danger.
The results show that the hormone titres vary with the animal's life history.
Corticosterone and testosterone play a significant role in human beings and animals where controlling reproduction and reacting to stressful situations are concerned. The sudden appearance of a predator or rival causes the stress hormone, corticosterone, to increase rapidly in an animal's blood. In this way the organism is prepared for suitable reactions to overcome the situation such as fleeing or fighting. The hormone, testosterone, is responsible during the reproductive season for exhibiting the song, for successful courtship and territorial behaviour, and for copulation frequency. However, it has not been known until now why large fluctuations in the blood concentrations of these hormones are found in different species of birds. Should not all species of songbird have the same hormonal values?
"Two of my co-authors, Bob Ricklefs and Martin Wikelski, proposed in a publication in 2002 that the "pace of life" of animals was based on certain physiological mechanisms, including hormonal effects," explains Michaela Hau.
There are indeed large differences. For example, a great tit produces a large number of offspring within a short time, laying up to 12 eggs per breeding attempt, but on average only lives for a few years -- this species "lives fast." On the other hand, the spotted antbird of about the same size is a species of bird found in the Panamanian rainforest that only lays 2 eggs per attempt, but can live for over 18 years -- it "lives slowly."
The scientists examined the hormone concentrations in 24 species of songbirds in North and Central America during the breeding season. A small blood sample was taken from the wing vein of the freshly caught male birds. Then they were kept in a small linen sack for about 30 minutes. The scientists took a second blood sample and then set the bird free, unharmed. Since corticosterone is not released until after three minutes, values were thereby obtained for a stress-free situation and a danger situation.
Using the data on annual survival rates, the duration of the annual reproduction phase and the body size of each species which the co-author Jeff Brawn from the University of Illinois included in the study, the researchers discovered that if one ignores the influence of the body size on the corticosterone values, the species of birds that live a long life have a greater amount of stress hormone in their blood after the stress phase than the species living a short life. "These results suggest that the increased release of corticosterone during a stressful situation does indeed support the processes securing survival. This might be the reason that species with a "slow" life speed and a long life span have higher corticosterone values," explains Michaela Hau. On the other hand, the blood values of testosterone were higher in species from more northerly regions with a short breeding season, that have to invest strongly in the temporally limited reproduction phase -- the "fast life" birds. "I wouldn't have expected being able to correlate the large variation in hormone concentrations with the life history of the species. The results have a far-reaching significance for understanding the evolution of hormones and physiological processes that enable life history to be adapted to environmental conditions."
The research team would now like to combine these results with the findings of other colleagues who have been examining the correlation between the immune function and life history, and energy metabolism and life history, and continue their experimental investigations. Michaela Hau believes that "hormones, as organismal messenger substances, certainly play an essential role in the coordination of physiological systems."
- Michaela Hau, Robert E. Rickleffs, Martin Wikelski, Kelly A. Lee and Jeffrey D. Brawn. Corticosterone, testosterone and life-history strategies of birds. Proceedings of the Royal Society, June 16th 2010 DOI: 10.1098/rspb.0673
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