Extreme climate dictates how animals live in groups, 33-year-long study reveals

A study tracked 12 neighbouring groups of white-faced capuchin monkeys across three decades, combining on-the-ground observations of 335 individual animals with satellite data measuring forest conditions.

In Short

• Animals live in groups for safety and resource control
• Larger groups lose foraging edge under severe droughts, heavy rains
• Climate change may cause group splits, altering animal social structures

There is a reason animals live in groups. Living collectively translates to safety in numbers, more eyes for predators, and the collective muscle to push rivals off a good feeding spot.

But a 33-year-long study of wild monkeys in Costa Rica has found that this logic, which is arguably one of the most fundamental in animal behaviour, begins to unravel when the climate turns extreme.

This finding comes with obviously sobering implications for wildlife across the world as Earth faces rising extremes and warming. These implications are not limited to national territories either, carrying risks for wildlife across the planet, including for India's richly social primate populations.

The study tracked 12 neighbouring groups of white-faced capuchin monkeys across three decades, combining on-the-ground observations of 335 individual animals with satellite data measuring forest conditions.

It is one of the longest-running primate field studies in the world.

"It takes decades to see how climatic cycles interact with the complex social dynamics of animal societies," said Professor Meg Crofoot, a director at the Max Planck Institute of Animal Behaviour.

BIGGER THE GROUP BIGGER THE PROBLEMS

Under normal conditions, the study found, larger groups of capuchins managed a predictable trade-off.

More members meant more competition for food, but it also meant more power to push smaller groups out of the best foraging areas, effectively compensating for the internal pressure.

"The textbook view is that bigger groups need more space and need to travel further every day to find enough food for everyone," said lead author Dr Odd Jacobson. "What we found is more nuanced. How groups use space depends not just on their size, but also on the size of their neighbours and the climate conditions they all experience."

During the dry season, resources like water, food, and shade are concentrated along rivers, forcing groups into closer contact and intensifying competition.

Larger groups still prevailed, but only just, and only under typical seasonal conditions.

CLIMATE CHANGING WILDLIFE RULES

That balance is now being broken down by the climate extremes that are growing in frequency as well as intensity.

El Nino events brought severe drought and then La Nina events brought unusually heavy rainfall in the region.

Both weather phenomena disrupted the foraging advantages that larger groups normally enjoyed, intensifying internal competition and undermining the benefits of living in a large group.

While this study focused on Costa Rican capuchins, the underlying dynamics of group living, resource competition, and climate stress are universal across social primates.

The relevance of this study for India is undeniable, especially when the country's range of primate species is considered, along with the fact that India is on the frontlines of climate extremes.

India is home to some of the world's most socially complex primates, including rhesus macaques, bonnet macaques, langurs, many of which live in large, tightly structured groups in forests already under pressure from habitat loss and increasingly erratic monsoons.

"If we are to truly forecast whether increasingly erratic climates could permanently destabilise the costs and benefits of group living," said Jacobson, "we will need to connect birth and death rates to what we now understand about competition and climate."

- Ends