Fossils in Amber Show Social Behavior in Ancient Insects

by AMNH on

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Fighting ants, giant solider termites, and foraging worker ants recently discovered in 100-million-year-old amber provide direct evidence for advanced social behavior in ancient ants and termites—two groups that are immensely successful because of their ability to organize in hierarchies.

The new work, led by scientists at the American Museum of Natural History and the University of Kansas, and published today in two papers in the journal Current Biology, proves that advanced sociality in ants and termites was present tens of millions of years earlier than indicated by the previous fossil record.

Workers of two different ant species fighting in a polished oval amber fossil.
Workers of two different ant species fighting in an amber fossil.
© AMNH/D. Grimaldi, P. Barden

“Ecologically, advanced sociality is one of the most important adaptive features for animals,” said co-author Dave Grimaldi, a curator in the Museum’s Division of Invertebrate Zoology. “All ants and termites are social, and they are ubiquitous across terrestrial landscapes, with thousands of described species and probably even more that we haven’t yet found.”

Advanced sociality, or eusociality, a hallmark of which is reproductive specialization into worker and queen castes, is essentially a phenomenon of the phylum of invertebrates known as arthropods. Queens and reproductive males take the roles as the sole reproducers while the soldiers and workers defend and care for the colony.

The trait is thought to have appeared first in termites in the Late Jurassic, about 150–160 million years ago, but the earliest termites that could definitively be tied to a caste system dated from the Miocene, a mere 20 to 17 million years ago.

An ant and terminite preserved in amber.
A unique Cretaceous co-occurrence of a worker ant and worker termite in the same piece of amber.
© AMNH/D. Grimaldi, P. Barden

A number of spectacular pieces of amber recently recovered from Myanmar provided a clear answer: Eusociality was going strong in both groups during the Cretaceous, between 65 and 145 million years ago. 

In termites, the researchers made this determination based on the diverse anatomy of the animals, indicating the presence of castes. They found six different termite species preserved in the amber, two of which are new to science: Krishnatermes yoddha, comprising workers, reproductives, and soldiers; and Gigantotermes rex, based on one of the largest soldier termites ever found—about an inch in length, half of that length being a head with scissor-like jaws.

Line illustrations of two termite species: Gigantotermes rex (left) and Krishnatermes yoddha (right).
Reconstructions of two new species: Gigantotermes rex (left) and Krishnatermes yoddha(right).
© AMNH/D. Grimaldi

The amber ant fossils also froze a number of eusocial behaviors in time. Those include: the presence of different castes, including queen ants and workers; groups of worker ants in single pieces of amber, probably foraging on behalf of nestmates; and two workers of different ant species engaging in combat.

“We know that wingless solitary relatives of ants don’t fight or defend territories against other species,” said co-author Phillip Barden, a recent graduate of the comparative biology doctoral program at the Museum’s Richard Gilder Graduate School and a National Science Foundation Postdoctoral Fellow at Rutgers University. “But modern ants war all of the time. The behavior of these fossil ants, frozen for 100 million years, resolves any ambiguity regarding sociality and diversity in the earliest ants.”