How leaf cutter ants domesticated crops

Over tens of millions of years, fungus-farming ants have learned how to cultivate their crops to ensure a stable food supply, seemingly navigating challenges that human farmers still grapple with.

“Ants have managed to retain a farming lifestyle across 60 million years of climate change and leafcutter ants appear able to grow a single cultivar species across diverse habitats, from grasslands to tropical rainforest,” says Jonathan Shik from Denmark’s University of Copenhagen, lead author of a study published in the journal Nature Ecology and Evolution,

Left to their own devices, wild cultivars grow and adapt to optimise fitness. Domesticating plants to produce desired fruit with higher yields creates trade-offs, as they come to rely on a narrower set of conditions, including moisture, temperature and nutrient requirements to survive and thrive.

As farmers have increasingly pushed the boundaries of these trade-offs over the past 10,000 years, crops have become more vulnerable to environmental threats such as pathogens and varying weather conditions, leading to increased use of pesticides, irrigation and fertilisers.

Ants from the Attini tribe, comprising vast colonies with millions of workers, surmounted comparable challenges with their fungal cultivars (Agaricales).

“The ants appear to have faced a similar yield vulnerability trade-off as their crops became more specialised but have also evolved plenty of clever ways to persist over millions of years,” says Shik.

“For example, they became impressive architects, often excavating sophisticated and climate controlled subterranean growth chambers where they can protect their fungus from the elements.”

In a series of experiments, Shik showed that the fungal cultivar’s nutritional requirements narrowed over millions of years of domestication, and that the ant farmers collected highly specific substrates – fertilisers, effectively – from the environment to produce the crops.

201028 leaf clutter ant nest
Leaf cutter ant nest. Credit: Smithsonian Tropical Research Institute

He and his research assistants spent more than 100 hours lying next to ant nest entrances on the floor of Panamanian rainforests, nabbing the ants’ bounty from their jaws with tweezers as the critters returned from foraging expeditions to identify and analyse its nutritional content.

“This was very tedious work,” Shik says, “as the ants and their little substrate bits were very small.”

They discovered that the ants foraged substrates from a diverse range of leaves, fruit and flowers from hundreds of different trees, with a rich nutrient profile including protein, carbohydrates, vitamins and minerals.

Incredibly, the ants would bypass some resources to choose fodder to meet the selective needs of their fungal crops.

To understand this, Shik drew from the protein-leverage hypothesis, based on the observation that humans are driven to meet their protein requirements, even if it means eating loads of carbohydrates to get there.

In captive colonies, he offered ants – which have taste receptors in their mouth parts – a mixture of diets with various nutrient profiles and found that ants refused to eat mixtures with too much protein, which is toxic to fungi.

“If you feed the fungi outside their nutritional requirements, they die,” Shik explains.

“Human farmers know exactly what the fundamental niche of corn is and can target this using specific fertilisers. The ants appear to know the same thing, surviving by satisfying the nutritional needs of their fungus crops.”

This feat of organic farming, honed over the millennia, could explain the organisational complexity of attine ants, spanning 250 species that have evolved a remarkable diversity of agricultural domestication practices within the one ecosystem.

The researchers contrast this with sweeping human systems that “have tended to competitively displace subsistence farms in many habitats”.

“Our results underscore that naturally selected farming systems have the potential to shed light on nutritional trade-offs that shaped the course of culturally evolved human farming.”

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