He found that only the dominant male of a team produced offspring. These dominant males produced, on average, seven offspring over the course of the study, compared to an average solitary male operating on its own, which produced less than one offspring. Because the coalition males were brothers or half brothers, those seven offspring contained a significant proportion of the subordinate male's genes, even though he never bred. (Genetically, because males share half their genes with their brother, two of your brother's offspring contain the same proportion of your DNA as one of your own offspring.)
Krakauer noted that subordinate males probably could have had some breeding success on their own. But this solitary breeding would not offset the advantage of helping a dominant brother. A subordinate male's inclusive fitness - in other words, the net benefit of cooperation - works out to about 1.7 offspring.
"Basically, since subordinate males and dominant males share some proportion of their DNA, the subordinate males benefit indirectly by helping their relative breed," Krakauer said. "A subordinate partner gains more by giving help than it would by going off on its own and trying to breed by itself."
He also looked for other possible benefits to the subordinate males, since in the case of two other birds thought to exemplify kin selection, other benefits were found to outweigh kin selection. In a group of New World tropical birds called manakins, subordinate males were found to help dominant males attract mates because they would eventually inherit the mating territory, which is jealously guarded. And in a Eurasian sandpiper called the ruff, helpers bring in additional females, but when the dominant male is distracted, they are able to sneak copulations, directly passing on their own genes by mating with females.
Krakauer found that neither of
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Contact: Robert Sanders
rsanders@berkeley.edu
510-643-6998
University of California - Berkeley
2-Mar-2005