More than 100 million years ago, ants shared labor. This suggests a well-preserved amber fossil in which three wingless adult ants and a pupa are encased together. Apparently one of the infertile females had carried the pupa, performing a classic worker task. High-resolution micro-computed tomography images also allowed a close look at the anatomy of the insects. It turned out that two of the workers are from a previously unknown ant species.
Ants are among the best-known colony-forming insects. Within 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 that are fertilized by the males. Infertile females, the workers, take care of the offspring and also take care of food and nest building. This division of tasks is reflected not only in the behavior but also in the morphology of the animals: the fertile queens are larger and winged, the sterile workers are smaller and have no wings. But when in the course of the evolution of the ants did this division of labor arise?
altruistic brood care
A team led by Brendon Boudinot from the Friedrich Schiller University in Jena has now found new clues. In an amber they found four well-preserved ant fossils, including three wingless adult females and one pupa. The amber comes from Kachin in northern Myanmar, a well-known site for amber fossils. It probably originated at the time of the transition from the Lower Cretaceous to the Upper Cretaceous more than 100 million years ago. The pupa is the first ever found in Cretaceous amber.
“Since ant pupae cannot move, it is reasonable to conclude that the adult animal carried them,” says Boudinot. “This so-called brood transport is a unique feature of the collaborative coexistence of ants. The fossil thus provides the first physical evidence of Cretaceous cooperative behavior: these ants cared for their young together, foraged together, and had distinct queen and worker castes.”
Micro-CT enables species identification
With the help of high-resolution micro-computed tomography images, the researchers examined the bodies of the ants more closely. “It turned out that the soft tissue of the insects was excellently preserved,” says Boudinot. “We were able to examine the structure of the brain, the structure of the nervous system and the transverse strands of the muscles in detail, and thus compare the four specimens with each other.” This also enabled the researchers to draw conclusions about the species they belong to. The pupa and the adult ant closest to it belonged to the well-known species Gerontoformica gracilis, which is now extinct. Using the detailed micro-CT images, Boudinot and his colleagues were able to describe this species more precisely than was possible based on previous studies.
The other two workers also belonged to the genus Gerontoformica, but differed in some physical features from all previously known species of this genus. “We therefore assume that the two individuals are 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, after the Greek words sternon for breast and rhabdos for tail. Close examination of the ants’ feet in the amber revealed that G. sternorhabda was adapted to locomotion on steeply sloping surfaces, while G. gracilis probably had a particularly good grip on smooth surfaces. “Thus, G. sternorhabda and G. gracilis may have preferred different surfaces and occupied different niches in the same habitat,” the researchers suspect.
Relevant also for other amber finds
According to the researchers, their method of examining insects preserved in amber in detail using micro-CT images is also relevant for future studies. “We can now derive new insights into the development of the internal anatomy of fossil insects and clarify the relationships between fossil species with each other and with the species living today,” says Boudinot. With suitable fossils, for example, it would be possible to understand how the two different female forms of ants – queens and workers – came about.
Source: Brendon Boudinot (Friedrich Schiller University Jena) et al., Zoological Journal of the Linnean Society, doi: 10.1093/zoolinnean/zlab097