These beetles are successfully loaded since 100 million years ago

Almost 100 million years ago, a tiny and unfortunate beetle died after getting lost in a resin sticky globule that was escaping from a nearby tree. from Southeast Asia. Fossilized in amber, this beetle eventually went to the office of entomologist Joe Parker, an assistant professor of biology and biological engineering at Caltech. Parker and his colleagues have now determined that the perfectly preserved beetle fossil is the oldest known animal example in a symbiotic behavioral relationship.

An article describing the work appears on April 16 in the newspaper eLife.

Symbiotic relationships between two species have appeared several times during the evolution of the animal. These relationships range from mutually beneficial associations, such as humans and their pet dogs, to the parasite, such as the tapeworm and its host.

Some of the most complex examples of behavioral symbiosis occur between ants and other types of small insects called myrmecophiles, which means "ant lovers." Thanks to the ability of ants to form complex social colonies, they are able to repel predators and hoard food resources, making ant nests a highly desirable habitat. Myrrhophiles exhibit complex social behaviors and chemical adaptations to fool ants and live among them, reaping the benefits of a safe environment and abundant food.

The social behaviors of ants appear for the first time in the fossil record 99 million years ago, during the Cretaceous of the Mesozoic Era, and would have evolved shortly before, at the beginning of the Cretaceous. Now, the discovery of a myrmecophilic Cretaceous fossil implies that free-loading insects are already benefiting from the oldest ant societies. This finding means that myrmecophiles are a constant presence among ant colonies from their earliest origins and that this socially parasitic way of life may persist during vast periods of evolution.

"This beetle-ant relationship is the oldest behavioral symbiosis known to date in the animal kingdom," says Parker. "This fossil shows us that symbiosis can be a very effective long-term survival strategy for animal lines."

The fossilized beetle, called Promyrmister kistneri, belongs to a sub-family of "clown" beetles (Haeteriinae), all modern species of which are myrmecophilous. These modern beetles are so specialized in the life of ants that they will die without their hosts and have developed extreme adaptations for infiltrating colonies. Coleoptera are well protected by a thick body plan and robust appendages. They can mimic the pheromones of the nest of the host ant, allowing them to disguise themselves in the colony. They also secrete compounds that are known to be soothing or attractive to ants, helping beetles to be accepted by their aggressive hosts. The fossilized Promyrmister is an equally robust insect, with thick legs, an armored head and glandular orifices that the researchers theorized as exuded chemical substances to soothe its primitive ant host.