First fossil ant-exploiting beetles

The film Jurassic Park introduced many people to the idea that insects from the era of the dinosaurs occasionally ended up trapped in tree resin that became fossilised amber around them, and preserved them to an incredible degree. The film then took the storyline off into the realms of science fiction, but what researchers can learn from these sorts of specimens is very much science fact, and, speaking to Chris Smith, Joe Parker thinks he’s stumbled upon the 100 million year old remains of a beetle that has a very special relationship with ants... 

Joe - What we have here is an imposter of the earliest known ant societies. This is a beetle that targeted the first colonies formed by ants that we know about in the fossil record, and used what we think are chemical and behavioural strategies to infiltrate colonies of these earliest ants, and forge a kind of socially parasitic relationship with them. This is a relationship where the beetle is essentially tricking the ants into gaining acceptance inside their colonies; and once it's inside, it feeds on the resources - in some cases, we think these beetles are being fed mouth-to-mouth by the ants. And so they're essentially these stealth organisms that assimilate into the social fabric of ant colonies for their own selfish needs.

Chris - Because there are examples of this sort of relationship existing today aren't there? There are caterpillars that do this, there are beetles that do this...

Joe - There's about 10,000 species that we know about that use ant colonies as a resource in this way. And these organisms have the amazing name myrmecophiles, which means ant lovers. And across the arthropod tree of life, we see myrmecophiles evolving kind of sporadically. So there are examples of crickets that live inside ant colonies, and trick the ants into feeding them mouth-to-mouth. There are examples, as you said, of butterfly caterpillars - there's a whole family of butterfly caterpillars. But by far the largest number of examples of myrmecophiles are found within the beetles. And what this fossil beetle is an example of is one of the largest groups of these myrmecophile beetles that we know about today. What we found is that, actually, this association between these beetles and ants is evolutionarily very ancient; it really dates very close to the dawn of ant social behaviour. So you can imagine that these beetles have kind of been a constant presence, like a sort of constant part of ant ecology, from the very earliest beginnings of ant colony formation.

Chris - Tell us a bit about the fossil itself, and also how it came to light.

Joe - Burmese amber is an amber deposit from Myanmar. This amber is ninety-nine million years old, so it's kind of slap bang in the middle of the Cretaceous, you know, dinosaurs are walking around. But encapsulated in this amber is this ancient ecosystem with all these different amazing insect species. And I've had the kind of privilege of working with some of the beetles that have come out of this amber deposit. This particular specimen is absolutely incredible because it's immaculately preserved, and is the earliest known definitive example of one of these socially parasitic beetles. And so it has kind of anatomical hallmarks that show that it was specialized for life inside ant colonies. So it's got, for example, defensive modifications of its head and legs so, you know, if ants pick it up and detect it, you know, and try and kill it, it's physically very well defended, it can withdraw its appendages under itself. It's also got what we think are kind of chemical glandular openings on its body and in modern species that we've studied, these chemicals have the effect of manipulating ant behaviour. They produce secretions the ants find very attractive, and have the behaviour manipulating effect of pacifying the ant, preventing the beetle from being attacked. In modern species that we've studied, we also have found that these beetles mimic the ant nest mate pheromones. So ants have chemicals on their bodies that enable them to recognise members of their own colony or intruders, and these beetles mimic those chemicals.

Chris - Well, given that this beetle shows these quite considerable adaptations, it's clearly been evolving for a little while to have become that specialized. So two questions spring to mind. One of them is: when do you think that this symbiotic relationship between this beetle and ants began? And by 99 million years ago, how long had ants actually been around for?

Joe - Well this is the remarkable thing. We think the ants hadn't evolved complex social behaviour much before their first appearance in the fossil record in this Burmese amber; so that 99 million years ago, ants had probably only been social for a few million years. It's kind of just at the dawn of of complex social behaviour in ants. And so it's jaw-dropping that already, by this point, you find these beetles that were clearly living inside colonies of these early ants, and targeting them with the same adaptations for colony infiltration that you see in the modern species today that have this lifestyle. What this tells you is: the evolution of complex ant colonies that are resource-rich environments selected for the rapid evolution of organisms, like these beetles, which had adaptations to infiltrate them. Clearly there's intense selection for organisms to adapt to ant colonies, and these beetles were one of the first groups that were able to kind of turn up at the party. So it evolved very rapidly, you know, in a space of presumably just a few million years; and it's persisted ever since - so, you know, the best part of 100 million years. And what's happened is, as ants have split into modern species, these beetles have kind of hitchhiked along with them. So today there's hundreds of different species of them living with different species of ants in different parts of the world.