They conducted their experiment on laboratory colonies with two mother queens. The colonies were set up in the early spring, just before the queens lay their brood. At the time the colonies were established, they consisted of only the queens and adult worker ants.
"When the queens started to lay eggs in our laboratory colonies, we collected a sample of about 50 eggs and left about the same amount in the colony. The eggs left in the colony were allowed to mature into adults," said Sundström.
The researchers then screened all the queens and eggs and a sample of mature individuals from each colony. Based on their genetic analysis, the researchers determined the proportion of eggs laid by each queen and also the proportion of the adult brood that belonged to each queen. The researchers then calculated how the proportions had changed.
"We predicted that if the workers on average are more closely related to one queen than the other they could gain genetic benefits by favoring the offspring laid by this queen," said Sundström. "Conversely, if the workers are equally related to both queens they would have no incentive to favor the offspring of one queen over the other."
If the researcher's theory is correct, in colonies where there is a high proportion of worker ants more closely related to one queen than the other, the brood belonging to the queen with all the close working relatives will get a higher proportion of surviving offspring.
The results from Sundström and Hannonen's experiment were in line with this prediction, implying that "the workers manipulated brood composition to fit their interests," said Sundström.
Ross says there is little reason to question the methodology of Sundström and Hannonen, but their result does raise the question of why nepotism was found in this species of ant when it has not been found in so many others.
"You can't argue that the earlier studies were systematically flawed, so why is this one different?" he said.
While the researchers do not yet have direct evidence that workers remove eggs or larvae to achieve their nepotistic goals, they say that such action is the most plausible explanation for the observed nepotistic behavior among Formica fusca ants.
"The rationale behind this argument builds on kin selection theory," said Sundström. "In social insects such as ants and bees, workers are sterile females, which help their mother, or some other female relative to reproduce."
Since the workers share half their genes with their mothers, they also share half their genes in common with their siblings. Since siblings have more genes in common than do cousins, if a worker wants to see its genes passed on to the next generation, it's to the ant's advantage to favor its close relatives, assuming it can tell which ants are its closest relatives.
"Accurate kin discrimination abilities are a prerequisite for nepotism. Because we found evidence for nepotism, we can conclude that workers have the necessary ability to discriminate between closer and more distant relatives," said Sundström
That nepotism is now known to exist at least among one species of ant in one situation, Ross says it puts ant nepotism research back 15 years, posing such questions as, "How common is it? How universal is it? What sort of conditions should we expect to see it in?" he said.
Sundström finds such questions to be the most rewarding aspect of her research. "Every time one resolves one question, there will be ten new ones emerging."
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