"We were able to pick apart what was going on there," McGlothlin said. "By splitting reproduction into two different components, we were able to see how successful male birds were at home versus how they did away from home."
After tallying within-pair and extra-pair matings, the scientists produced two more unexpected results. Higher testosterone was associated with more offspring from within-pair matings, and intermediate levels of testosterone were associated with more extra-pair offspring.
"Which may seem counterintuitive," McGlothlin said. "We think what's going on is a time trade-off. For a male to make sure he is fertilizing the offspring in his home nest, he must spend a fair amount of time near his nest and perhaps follow his paired female around to make sure no other males approach her. Testosterone makes males more territorial and more likely to engage in that behavior."
Low testosterone levels were shown by McGlothin, Ketterson, and others in 2007 to cause males to spend more time parenting offspring in their nests, but also to be less territorial. McGlothlin said that might explain why his group saw a balancing effect for testosterone on extra-pair offspring. Males with low testosterone may make fewer forays to other nests, reducing their likelihood of encountering other males, while those with very high testosterone may be staying home for a different reason -- to guard their territory against potential intruders. Alternatively, these high-testosterone males may be seeking extra-pair mates, but are unsuccessful, perhaps because the behavior they produce is unattractive to females.
In evolutionary biology, the fitness of an individual organism is dependent on its capacity for survival and reproduction, but it is also a relative thing -- for any given character, fitness depends on how one set of genes or attributes performs
|Contact: David Bricker|