Ants: ancient and complex

Ants might be small in stature, but they’re big on complexity. Ant societies divide tasks between three different social groups that each perform distinct roles. The queen lays the eggs, which are then fertilised by the males, while workers look after the offspring and are responsible for finding food and building nests.

This behaviour is known as eusociality and is particularly complex in ants because it effects not only their behaviour but their physical traits. Females with wings become queens, whereas wingless, infertile females are relegated to lowly workers.

Recent research has found that ants developed this unique teamwork and cooperative way of life as far back as the early Cretaceous period, over 100 million years ago.

The earliest proof of cooperative behaviour in ants is four fossils enclosed in amber – three adult wingless females and one incompletely developed pupa. Because an ant pupa isn’t mobile, researchers determined that an adult insect had carried it. So even this far back in their evolution ants used brood transport – a feature of their behaviour that’s based on their division of labour – and had different queen and worker castes.

The eusociality of ants inspired the 2019 SCINEMA International Science Film Festival short animated film Ant Sisters, which used the evolution of social insects to weave a tale about the complex dynamics of human and ant sisterhood. In it, a woman takes inspiration from the social lives of ants to pen a letter to her estranged sister and discovers what it takes to survive as a collective.

You can watch Ant Sisters here and read on to learn more about ant behaviour research.

At a time when humans are expressing increased outrage, we might have something to learn from ants.

Researchers have discovered they use a clever, precise mechanism to switch on aggression towards intruders from other colonies to defend their own, which could add more insight into their evolutionary success.

Ants have been around for tens of millions of years and have complex, highly organised social structures that help their colonies thrive; telling nestmates apart from non-nestmates is a key aspect of this.

“Accepting friends and rejecting foes is one of the most important decisions an ant worker must make,” says Stephen Ferguson from Vanderbilt University, US, lead author of a study published in the Journal of Experimental Biology.

Scientists have a long-standing interest in how insects use their chemical senses – smell and taste – to respond to their environment, identify each other and facilitate their interactions.

This research found that they need to smell and interpret fragrant compounds on intruder ants to “unlock” aggressive behaviour and protect their nest, which senior author Laurence Zwiebel refers to as a “coat of many odours”.

He and Ferguson put a chemical that his lab had previously discovered to a series of tests, establishing that it could block or overexcite ant odourant receptors.

They then collected wild eusocial Camponotus floridanus ants from nine different colonies across the Florida Keys.

Setting up mini-duelling arenas for ants from the same or different colonies, they filmed them to score aggressive behaviours, most commonly lunging, biting or dragging, which the insects typically display towards non-nestmates – usually fighting to the death.

Ants with normal receptors recognised and fought with those from other colonies, but ants with blocked or overactivated receptors showed dramatically reduced aggressive behaviour.

The aggression is thus triggered by what the researchers term a “lock and key” mechanism.

“The tumblers of the lock are the odourant receptors, and the teeth of the key represent the mixture of odourants that an ant might encounter on the cuticle of a non-nestmate,” Ferguson explains.

“Neither a toothless key (antagonist) nor a rake (agonist) are sufficient to unlock aggression. Rather, the presence of a particular set of teeth on the key are required to elicit aggression between non-nestmates.”

The tolerant stance appears to be the norm, and the insects will only become aggressive if this very precise signal is correctly “decoded” by their smell receptors, proving a long-held theory.

“Basically, this means that hundreds of millions of years of social evolution has provided a system in which the default pathway is not to fight; only when ants are absolutely sure do they trigger aggression,” Zwiebel says.

“Put another way, ants, which have some of the most sophisticated and successful social structures in biology, do not practice a ‘shoot first and ask questions later’ social policy.”

“This process may have contributed to the evolutionary success of these insects,” says Ferguson, “and there may be important lessons about tempering aggression for other social beings such as humans.”

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