As we moved through the forest collecting data for our directed research in Carara National Park, we heard a high pitch crying sound in the canopy. Suddenly, there were spider monkeys (Ateles geoffroyi) darting through the branches above us. Several individuals were vocalizing close to the noisy trafficked road as they were looking for leaves and fruits.
The spider monkey is an endangered species that needs relatively large tracks of mature forest to survive. Carara National Park is 5,242 ha in size and protects mature as well as secondary forest along an altitudinal gradient. The downside of this park is that it is limited by a segment of a paved road with a traffic volume ranging from 426 to 1199 vehicles per hour in the last 10 years. This high number of vehicles generates loud noise levels that penetrate the mature forest and meet with the natural sounds produced by wildlife.
We have been studying bird bioacoustics in this park over the years and have also heard many other animal species that also produce sounds, including spider monkeys. The most notorious of all is that produced by a howler monkey (Alouatta palliata). Different from the spider monkeys, howlers emit a loud low-frequency call that can be heard several kilometers away. This roaring sound is produced in response to neighboring groups to locate each other to minimize confrontation. However, the loud sound produced by the howler monkeys is challenged by similar anthropogenic sounds such as those produced by car engines. Thus, the interaction between sounds could interfere with the effective communication among monkey groups. Nonetheless, monkeys vocalize from the canopy which confers advantages in the long range propagation of their sounds. In any case, anthropogenic noise notably alters the quality of the natural soundscape and the appreciation of natural protected areas.