If they can’t hear you, make them smell you…

At the heart of the Amazon rainforest, near the confluence of the Rio Negro and Amazone, lies the metropole Manaus. Nicknamed the Paris of the Tropics, Manaus is home to over two million people and functions as the economical hub of the northern part of Brazil. Incidentally, it is also the last refuge of the critically endangered pied tamarin (Saguinis bicolor). This small species of New World monkey, measuring less than 30 cm, is characterized by a black, hairless head and a fluffy white and brown body. Originally found in the primary and secondary rainforest regions around Manaus, the city is now comprising the bulk of its habitat. This tamarin has moved from the green jungle to the urban jungle.

The pied tamarin is now considered Critically Endangered by the IUCN as life in the city poses several threats to its survival. With its habitat becoming more and more fragmented, the monkey faces its dire consequences. Road-kills, electrocutions and attacks by domesticated animals all contribute to its small numbers (Gordo et al., 2019). On top of that, urban noise makes it increasingly difficult for the monkeys to communicate to each other. And communication is important to the pied tamarin, for they are social creatures living in small family groups of around 10 individuals. Pied tamarins employ multiple ways of communicating: both scent and sound is used for communications. Especially their long calls are considered crucial in their fragmented home, because they allow different groups to communicate to each other. But the urban noise caused by humans makes it more difficult to be heard and that is why a group of scientists have tried to find out if pied tamarins deploy a change of communication strategy in the urban jungle of Manaus.

In a new study published this year, a team of scientists observed, followed and tracked nine different groups of pied tamarins in five different green areas in Manaus. Their aim was to find out how an increase in background noise caused by humans is battled by the monkeys. They found that the background noise varied among the green patches: from 40 to 80 dB, but averaging around 59 dB. This background noise was mainly comprised of road traffic, but also sounds from park visitors or gunshots from military activities were heard. It was predicted that an increase in background noise would increase the amount of scent marking behavior exhibited. This would then lead to a decrease in long calls. Armed with radio-collars, portable antennas and multiple receivers, the researchers set out to observe each of the nine groups for ten complete days. And after analysis their results turned out to be slightly different than expected (Sobroza et al., 2024).

As it turned out, the pied tamarins do increase their scent marking behavior when there is substantial background noise. When noise levels reach 65 dB, this results in an increase in the probability of scent marking of around 60%. So far, so good. Surprisingly however, is the observation that an increase in background noise does not necessarily mean a decrease in the long calls made. This means that instead of a shift from communicating by sound towards scent when facing urban noise, the monkeys just combine the two (Sobroza et al., 2024). And why not? In an interview with ScienceNews, the study’s leading author Tainara Sobroza compared it like this: “It is just like being at a noisy party, and you want to call a friend that can’t hear you from the other side of the room. Added to calling him or her, you also usually wave and do arm gestures to call the attention of that person”. (Rodrigues, 2023).

These findings are important in two different ways. First of all, it shows that the pied tamarins are able to cope with the effects of urban noise by combining two strategies of communicating. This helps in the conservation of this critically endangered species as communication is key in the survival of pied tamarins. It is therefore a small token of hope and hopefully more adaptations by pied tamarins to the harsh urban conditions will be found. Secondly, these findings are important in a more academic context. Already multiple strategies deployed by animals to overcome urban noise have been extensively studied. But these have mainly focused on changes in only one modus of communicating, for example only sound. Changes in multimodality, as done by the pied tamarins, have not been given the same attention. This study opens doors to more ways of viewing shifts in animal communication. In a world where human sounds are becoming more dominant, it is a small reassurance that at least the pied tamarins still make themselves heard. Albeit by smelling.

References

Gordo, M., Röhe, F., Vidal, M.D., Subirá, R., Boubli, J.P., Mittermeier, R.A. & Jerusalinsky, L. 2021. Saguinus bicolor (amended version of 2019 assessment). The IUCN Red List of Threatened Species 2021: e.T40644A192551696. https://dx.doi.org/10.2305/IUCN.UK.2021-1.RLTS.T40644A192551696.en. Accessed on 18 April 2024.

Rodrigues, M. (2023, October 3). In noisy environs, pied tamarins are using smell more often to communicate. ScienceNews. https://www.sciencenews.org/article/noisy-environs-pied-tamarins-smell-communicate

Sobroza, T. V., Dunn, J. C., Gordo, M., & Barnett, A. A. (2024). Do pied tamarins increase scent-marking in response to urban noise?. Ethology Ecology & Evolution, 36(2), 136-149.