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The large ape had just started ‘huituing’, or calling. The young researcher, whose childhood love of animals had led to her becoming a specialist in their acoustic communication, was gripped with excitement. Most attempts to get clear recordings had failed. This time everything was perfect … although one part of her was hoping the big ape’s call wasn’t going to be a long one: “I was being eaten alive by mosquitos.”

Dr Davila-Ross, from the 1024�˹���’s Centre for Comparative and Evolutionary Psychology, recalls those early field expeditions when she was studying for her Master’s degree and how hard it was to get good-quality recordings that she could acoustically dissect: “Most days I would just get lots of birds, or orangutans that were too far away for a clear analysis.”

This analysis of animal communication, which explores the structure of their acoustic signals, has become a key element in behavioural and evolutionary biology. It’s why many years after this fieldtrip, Dr Davila-Ross was part of an international team that made headlines with their discovery of a new orangutan species.

Discovering – and securing – a species

The team had been studying a bearded and frizzy-haired orangutan found in an isolated pocket of the South Tapanuli region in the province of North Sumatra, Indonesia, as part of a conservation programme under the auspices of the International Union for Conservation of Nature. Scientists had determined there were only about 800 of the apes still surviving and their habitat was under threat from a proposed hydro dam. The Tapanuli orangutans looked different to the two known species of Sumatran and Bornean orangutans, but they couldn’t be confirmed as a separate species until an exhaustive study of their genetics, morphology and behaviour had been conducted, and this included their particular acoustic signals.

The team’s momentous finding – a new species of large ape – was published in 2017, by which time the researchers were not only informing the world of a new species, but one that was already endangered. Tapanuli orangutans exist in a 1,000-square-kilometre pocket of jungle and are thought to have been isolated from other Sumatran orangutans about 3.5 million years ago, most likely during a period of volcanic activity.

“Hunting, small population size and geographic isolation had already raised a particularly high conservation concern, and now there was the hydro dam proposal that would have further reduced their domain,” Dr Davila-Ross explains.

However, as a result of the research showing the Tapanuli orangutans are a separate species, the dam has been put on hold while further research is undertaken on ways to protect and secure the population.

Dr Davila-Ross says had it not been for the research identifying the Tapanuli orangutans’ uniqueness, there would be a dam and power station being built right now, placing extreme pressure on the apes’ long-term survival.

Her role in the species’ discovery drew on her earlier recordings of the orangutans and matching the acoustic patterns of male ‘long calls’ to different population groups.

While her research since has spanned many animal groups, she finds orangutan acoustics particularly fascinating. The adult males are able to produce two sounds at the same time – a rare trait known as biphonation. Dr Davila-Ross measures these with a sonogram that shows audio frequency on a Y-axis and duration on the X-axis.

“You can see distinct shapes in these long calls and the different durations,” she says. “They consist of roars and ‘huitus’ in the middle of the call … I call them huitus because I don’t know how else to describe the sound. It’s a call with bubbling sounds and sighs at the beginning and the end of the call … that description is a rough structure of an orangutan long call.”

Tracing the origins of laughter

In her research into great ape communication, Dr Davila-Ross has also studied the evolution of laughter and smiles for apes and humans. “I developed a phylogenetic (evolutionary reconstruction) approach to test whether laughter is a pre-human trait by asking great ape keepers to tickle the apes. The recordings that I obtained of their ‘laugh vocalisations’, as well as human laughter, were phylogenetically measured and we were able to conclude they have the same evolutionary origin.

“In fact, we can say laughter is a pre-human form of communication going back at least 13 million years.”

However, Dr Davila-Ross says a major difference between ape ‘laughter’ and human laughter is that apes only use this form of expression in a positive context. They don’t have different forms, such as ironic or cruel laughter, that humans can express. For apes it is also only inwardly directed from something they are doing, not something they are seeing.

“Apes don’t ‘laugh’ when they see something funny. It has to be something they are doing.”

For Dr Davila-Ross this is a fascinating distinction requiring further research into the possible evolutionary reasons: “Plus you cannot study laughter without considering facial expression, in particular smiles. Where do they come from? Are they from primordial positive expressions, or fear grins as is more generally believed?”

For the moment, these are mysteries, but they still form part of her quest to better understand animal communication, which fits with her overarching involvement in conservation.

In particular, Dr Davila-Ross is concerned with improving the rehabilitation of orphaned or injured apes back into the wild and methods through which to better know when an animal is ready.

“We work with rehabilitation centres to develop approaches that help them more reliably identify candidates for release. Rehabilitated animals face a lot of challenges and knowing when an animal is ready isn’t straightforward; [it’s about] being able to recognise, for example, what skills an individual still might need to work on, if it is to survive in the wild.

“You are dealing with individuals from diverse backgrounds and experience. Some have been orphaned by really horrible events; some might have been pets. You have to understand these differences and work with them individually to prepare them for release.

“I am passionate about conservation and have always wanted to contribute in some way. Our research and the data we’ve been collecting for many years is helping this crucial work being done by rehabilitation centres to sustain the populations of these wonderful animals that are in our care.”

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