Everybody gains when larvae ‘follow the leader’

"The next stage of our research is to find out how certain larvae become the leaders in a group and how they are communicating directions and encouragement to their followers," says Mark Elgar. (Credit: Lisa Hodgkin)

If insect larvae follow a leader while foraging for food, both the leaders and their followers grow much faster than if the group is all followers or all leaders.

The work gives new insight into why such social relationships evolve in insects, and why they are maintained.

The study looked at the larvae of the Australian steel-blue sawfly Perga affinis, which are often known as “spitfires.” These sawfly larvae can grow to 7cm (2.7 inches) long and forage nocturnally in Australian Eucalyptus trees, forming large groups that can strip all the leaves from a tree in a few days.

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Sawfly societies operate democratically, with leaders and followers cooperating to decide on group movements. This contrasts with other animal societies, such as baboons and wolves, where leaders are despotic and dominate their followers.

Lisa Hodgkin, a PhD candidate at the University of Melbourne, says the team wanted to understand why the larvae followers allow others to determine the group’s movements.

“In many types of animals, the dominant leaders in a group are larger and stronger because when they forage or hunt, they take more of the food resources. But we found no difference in the weight gain between sawfly leaders and followers.

“Sawflies live in social groups that can have hundreds of individuals and they stay together for their seven-month larval stage. We wanted to know why this distinction of leaders and followers works and persists for so long,” says Hodgkin.

Equal benefit

The team observed the insects’ natural behavior for two weeks, individually marking each sawfly to understand which ones behaved as leaders or followers when foraging for food, noting their weight at the start and end of the period.

Once leaders were established by their position at the front of the group, the scientists could generate experimental groups of larvae—all leaders, all followers, or a mixture of the two—and again measured their weight over two weeks.

“Our field experiments revealed no clear individual benefit to being a leader, but all individuals in groups with a mixture of leaders and followers gained more weight than those in groups of only followers or only leaders,” says Hodgkin.

“We see that leaders only benefit from being leaders if they have followers, and that followers only benefit if they have leaders. There is no use being a shepherd without sheep or sheep without a shepherd.”

Study coauthor Professor Mark Elgar says that while leaders do not differ in growth rates or weight, they may acquire other benefits such as lower predation or enhanced immune function.

“The next stage of our research is to find out how certain larvae become the leaders in a group and how they are communicating directions and encouragement to their followers,” adds Elgar.

The findings appear in the Proceedings of the Royal Society B.

Source: University of Melbourne