Biologists from George Washington University and Rockefeller University have found that during evolution, turtle ants have repeatedly changed their head shape, which can be used to judge the "specialization" of the insect. An article about the discovery was published in the PNAS journal.
Turtle ants are interesting in that they have disproportionately large heads, which they use to block the entrances to their nests, acting as living doors. But not all insects' heads have the same shape: for some, they resemble manhole covers and ideally cover the entrances to the tunnels; others have square heads, allowing several individuals to fold them like shields, blocking the passage to the tunnel.
This variety in head shapes is not just an interesting evolutionary adaptation. It can tell a lot about the development of species during evolution and the associated filling of ecological niches.
The new work of biologists shows that such evolution can move not only in one direction, towards increasing specialization, but also in the opposite direction. The authors have shown that representatives of this species of ants over time have become "broad-based specialists."
Like many other social insects living in colonies, turtle ants specialize in various functions - they evolve to evolve traits that are suitable for their job. For the soldiers, this process resulted in large heads of various shapes.
"There is a whopping fourfold difference between the smallest and largest turtle ant heads," said Scott Powell, lead author of the new study, biologist at George Washington University. “To help people imagine this, I often say that the smallest specimen can sit comfortably on the head of the largest specimen.”
The shape and size of the head of a Cephalotes goniodontes soldier is determined by the type of tunnel the species occupies. Ants themselves do not dig tunnels, but move along those dug by borers. And since the tunnel through which they move may be too large or too small, the ants quickly diversify to be able to occupy it. Thus, the relationship between the heads of this species and the tunnels may provide a unique insight into the natural selection in the population of these insects. Researchers can easily compare a trait - head circumference - with the ecological trait that caused it to evolve.
To study the evolutionary development of various forms of heads, the researchers grouped 89 types of turtle ants for this trait. They identified in insects a square, domed, disc or bowl-shaped head. The authors then explored the evolutionary relationships between these groups using previously collected genetic information.
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If evolution were a one-way path, then the first turtle ants, which appeared about 45 million years ago, would have to completely lose their soldiers, and then gradually evolve towards specialization - from square-headed soldiers to individuals with heads that perfectly cover the entrances to the tunnels.
But new analysis shows that things were different. The oldest common ancestor that researchers were able to locate probably had a square head. A number of ants with different levels of specialization evolved from it. In some cases, more specialized species changed direction over time, evolving back into more universal head shapes. The work perfectly shows how amazingly flexible nature can be in adapting to the conditions of the ecological niche that they occupy.