The division of labor in ants over 100 million years ago

Over 100 million years ago, the ants shared the work. This suggests a well-preserved amber fossil in which three wingless adult ants and a chrysalis are surrounded together. Apparently, one of the barren females was carrying a chrysalis in a classic worker task. High-resolution microcomputer tomography images also allowed for a close look at the anatomy of the insects. It turned out that the two workers came from a previously unknown species of ants.

Ants are among the most famous colony-forming insects. In an ant colony, which can sometimes consist of millions of individuals, there are three castes, each with different tasks: The queen, a fertile female, lays eggs fertilized by males. The sterile females, workers, take care of the offspring as well as take care of food and build nests. This division of tasks is reflected not only in behavior, but also in animal morphology: fertile queens are larger and winged, sterile workers are smaller and have no wings. But when did this division of labor arise in the course of the ant evolution?

altruistic care for brood

A team led by Brendon Boudinot of the Friedrich Schiller University in Jena has now found new clues. In amber, they found four well-preserved ant fossils, including three wingless adult females and one pupa. Amber comes from Kachin in northern Burma, a well-known place of amber fossils. It probably dates from the transition from the Lower Cretaceous to the Upper Cretaceous over 100 million years ago. The chrysalis is the first found in chalk amber.

“Since ant pupae cannot move, it is fair to say that an adult animal carries them,” says Boudinot. “This so-called brood transport is a unique feature of the coexistence of ants. Thus, the fossil provides the first physical evidence of Cretaceous cooperative behavior: these ants shared care of their young, foraged together, and had separate castes of queens and workers. “

Micro-CT enables species identification

Using high-resolution microcomputer tomography images, scientists examined the bodies of the ants in greater detail. “It turned out that the soft tissue of the insects was perfectly preserved,” says Boudinot. “We were able to study the structure of the brain in detail, the structure of the nervous system and the transverse muscle bands, and thus compare four specimens with each other.” It also allowed scientists to draw conclusions about the species. they belong to. The chrysalis and the closest adult ant to it belonged to the well-known species Gerontoformica gracilis, now extinct. By using detailed images from microtomography, Boudinot and his colleagues were able to describe the species more accurately than had been possible with previous studies.

The other two workers also belonged to the genus Gerontoformica, but differed in certain physical characteristics from all previously known species of the genus. “So we assume that these two people come from a previously unknown species,” the researchers write. Based on the unique shape of the new species’ breast and body, Boudinot and his colleagues named it Gerontoformica sternorhabda, from the Greek words sternon for breast and rhabdos for tail. A closer examination of the ant paws in amber revealed that G. sternorhabda was adapted to move on steeply sloping surfaces, while G. gracilis probably had particularly good grip on smooth surfaces. ‘So G. sternorhabda and G. gracilis may have preferred different surfaces and occupying different niches in the same environment,’ the researchers suspect.

It also applies to other amber finds

According to the researchers, their method of studying insects preserved in amber in detail using micro-CT images is also important for future research. “We can now gain new insight into the development of the internal anatomy of fossil insects and elucidate the relationship between fossil species and species living today in much more detail,” says Boudinot. For example, with the right fossils, it would be possible to understand how two different female forms of ants – queens and workers – came into being.

Source: Brendon Boudinot (Friedrich Schiller University Jena) et al., Zoological Journal of the Linnean Society, doi: 10.1093 / zoolinnean / zlab097

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