A year in Rwanda

We conducted a field study on eastern chimpanzees in Nyungwe National Park, Rwanda, a cool and humid mountain forest. Over a 12-month period, we collected detailed data on the structure of nests, the characteristics of their chosen sites, and the kinds of trees the chimps chose.

We also measured how well different kinds of nests insulate against cold and heat. At the same time, we made detailed records of weather conditions when the nests were being built and throughout the night.

This let us test whether chimpanzees respond primarily to immediate environmental conditions, or whether their nesting decisions are better explained by the conditions they experience later during the night.

Chimpanzees are always adjusting their behaviour

Our results show chimpanzees consistently adjust their nesting behaviour in relation to environmental conditions. They preferred to build nests in places that were warmer, more humid and less exposed to wind than surrounding areas.

Nest structure and insulation varied systematically with environmental conditions. In cooler and wetter conditions, nests were thicker and deeper – indicating the chimpanzees put more effort into insulation when conditions are tougher.

We also found that factors such as the width and depth of the nest influenced its insulating ability.

The chimpanzees tended to build more insulating nests when weather was colder and when it was more humid, both during nest-building and overnight.

In cooler and wetter conditions, the chimps also built their nests higher, in taller trees with denser leaf cover. This makes sense: it would be a more stable microclimate with more shelter from rain.

Are chimps thinking ahead?

Importantly, nesting decisions aligned more closely with overnight environmental conditions than with those at the time of construction. When we took overnight weather into account, we found we could explain the variation in nesting behaviour much better than if we used only the current conditions.

One possible explanation is that chimpanzees use environmental cues, such as shifts in temperature, humidity or atmospheric pressure, that are linked to upcoming weather.

These cues may allow them to adjust nest-building behaviour in advance. Does this mean they predict or forecast future weather? Not quite.

But it does show their behaviour is consistent with reacting to environmental signals that are associated with later conditions. Either way, the chimps display a remarkable sensitivity to their environment – and a grasp of how to live in it.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Ettore Camerlenghi, Deakin University and Ari Martínez, University of California, Santa Cruz

You might go for a walk in the forest to disconnect from work and calm your nerves after a busy week. The chirping and calls of birds in the canopy above might be exactly what allows you to relax.

But what sounds soothing to humans may signal danger to other animals – and trigger fear across the forest.

In our research, published today in Current Biology, we show that when some animals spot a predator they issue a warning cry that is picked up by others and spread through the rainforest canopy. For a time, different species are linked into a shared information network, and parts of the forest briefly fall silent.

Birds and monkeys

During an expedition to a remote area of the Peruvian Amazon, working with a falconer, we used trained raptors to trigger warning calls from birds and primates. We recorded the calls then played them back into the forest and monitored how the community responded.

We already knew that birds sometimes repeat the warnings of others – occasionally even those of different species, or of primates. What we wanted to know was how widespread this behaviour is across the animal community.

Researchers released birds of prey in the Amazon rainforest to study how the alarm calls of other animals travel through the ‘internet of the forest’.

We discovered that alarm calls produced by small bird species – those weighing less than 100 grams – were most often passed on. Other small birds living in the canopy were the most likely to relay the call, but other animals joined in too.

Larger species, including capuchin and spider monkeys, sometimes responded as well. Two canopy species in particular – the black-fronted and the white-fronted nunbirds – stood out as especially likely to repeat and propagate the warnings of their neighbours throughout the forest.

Sounds and silence

Alarm calls from species living in the forest understorey were far less likely to spread and be propagated by other birds or primates.

However, even when these alarm calls were not repeated, they changed the forest’s soundscape. Small canopy birds almost completely stopped singing after hearing a predator alert. At the same time, animals in lower forest layers often continued to make sounds despite the perceived threat.

Together, these findings suggest that the Amazonian canopy is not only the rainforest’s most mysterious layer – largely unexplored and home to much of its biodiversity – but also functions as an information highway, like a fibre-optic network through which animals rapidly share signals of danger.

A new layer of the ‘internet of the forest’

In the past decade, the idea of an “internet of the forest” has become popular through the concept of the “wood wide web”, where plants exchange resources and information via root systems and fungal networks. Our work points to another communication system, one operating high above the ground.

Suspended above our heads is a vast ecosystem where animals constantly listen to one another, forming an eavesdropping network that spreads critical information within seconds.

The vocal activity of birds is usually associated with finding mates and defending territories. However, we now know that sometimes this activity, or lack of it, may represent pulses of a soundscape of fear.

Next time you walk through a rainforest, look up and listen to the birds. A sudden silence may mean a raptor is gliding somewhere above the canopy.

Ettore Camerlenghi, Associate Research Fellow, Avian Behaviour, Deakin University and Ari Martínez, Assistant Professor of Ecology and Evolutionary Biology, University of California, Santa Cruz