In the race of life, some animals are hares, running at top speed, producing lots of young and dying quickly. Others are tortoises, maturing slowly, producing few offspring, and living a long time. There must be physiological mechanisms that regulate these differing life strategies, and researchers studying bird hormones have found a clue.

Corticosteroids

Corticosteroids, found in many animals, are stress hormones released in the blood. If an animal is threatened by a predator or rival, the hormones allow the individual to survive by fighting or fleeing.

The strange thing about corticosteroids is that, just like life strategies, they vary dramatically from one species to the next. The question is: Why?

Why A Variety?

Researchers wondered if corticosteroid variability was somehow related to life strategies. Would birds like the short lived great Tit, which lays twelve eggs at a time, have different hormone levels than the Spotted Ant bird which lays only two eggs and lives as long as eighteen years?

Researchers collected blood samples from many bird species in North America and Panama, and used additional research data to compare corticosteroid levels to life strategies.

They measured both baseline and stress induced hormone levels, and found that species with long life spans had lower baseline hormone levels, but higher stress induced levels than short lived species.

What Did Scientists Learn?

Scientists think there is a physiological reason for this. Hormone levels are costly to maintain. The "hares" of the bird world pay that cost because they have few chances to have offspring. They can't afford to make a mistake.

The "tortoises", on the other hand, have more breeding time and don't need to keep their hormones on alert continuously. Instead, they wait until the threat is present and then ramp up their hormone levels. It's all a matter of strategy.