In a behaviour that finds a broad comparison with human tradition, a genus known as turtle ants, or cephalotes, pass on possessions from older individuals directly to younger ones.
In a mechanism that does not, however, find a human analogue, they do so through their anuses.
They have been doing this, researchers estimate in a paper published in the journal Nature Communications, for around 46 million years, during which time they have evolved in ways that mark them out from most other ant species in the world.
And these changes – which include the loss of the large mandibles and stinging abilities that give most ants their fearsome reputation – are all because of their long tradition of inheritance.
The content of a turtle ant’s faecal transplant comprises a community of bacteria, without which the recipient insect would very likely starve. And the microbes themselves, denied safe haven in a cephalote gut, would also perish.
The arrangement, thus, comprises one of the most robust symbiotic relationships ever discovered.
The true nature and purpose of the ants’ intergenerational anus exchange was discovered by a team led by Jacob Russell from Drexel University, in Philadelphia, US.
The clue was to be found in the ants’ diet, and, consequently, in their manner of acquiring food.
Many species of ant hunt aggressively for nutrient sources, swarming over territory and, if necessary, engaging in pitched battles against rival colonies, rival species, and even completely different and much larger animals.
Turtle ants, in contrast, are peaceful foragers, chowing down on nectar, pollen and fungi in forest canopies, as well as eating bird urine and mammal poo. All these sources, the scientists realised, shared one characteristic. They were poor sources of nitrogen, either because they contained very little of the chemical, or, as in the case of the excreta, contained a lot of it, but not in a form the ants could use.
This is where the gut bacteria come in. The ants’ microbiome contains species that are finely evolved to access and digest nitrogen in a way that makes it available to its hosts.
“The fact that they can subsist on such diets and have moved away from aggressively competing for more optimal food resources with other ants is almost certainly a function of their investment in symbioses with gut bacteria,” says Russell.
And while the ants have evolved in ways that make them much less predatory than other members of their family, there is little role for karma in the insect world. Without the active defences of strong jaws and powerful acid, cephalotes risk becoming juicy low-risk prey for more aggressive species.
In response to such threats, over many millions of years, the ants have developed a very thick defensive exoskeleton – a structure that requires a considerable amount of nitrogen in order to be produced and maintained.
“That armour may be possible due to the large contributions gut microbes make to their nitrogen budgets,” Russell said.
And, just as the actions of the bacteria protect the ants, so too do the ants return the favour. The researchers discovered that they have evolved a kind of fine mesh screen across their lower digestive tract, thought to prevent foreign invaders contained in food from reaching the symbionts.