At the end of the afternoon I was coming back from the forest with my field assistant, when we heard a noise some 500 metres away from us. We hurried towards it and followed the loud alarm calls. When I was some 20 metres away from it I saw a group of white-bellied spider monkeys (Ateles belzebuth) vocalising agitatedly and close by was another group of red howler monkeys (Alouatta seniculus), also vocalising loudly. I thought that they must be engaged in an agonistic meeting, possibly competing for some food source or other, but, to my utter surprise, I saw, just above them in an emergent tree, a majestic harpy eagle (Harpia Harpyja) holding an old female howler monkey in its claws that it had just attacked.
The howler was still debating occasionally, but was mostly hanging from the tree trunk. The harpy eagle had one foot on its chest and the other supporting itself in the tree. As soon as the harpy eagle saw us, it tried to fly away with the big howler in its claws, but instead, it released the monkey from its grip.
I then collected the howler, which was still alive, but she died on the way back to the field base. As soon as it died, hundreds of nematode worms came out of its body through the natural cavities (mouth, ears, rectum and vagina) and also through the wounds the harpy eagle had inflicted on it. It was also infested by hundreds of fly eggs and larvae (dipterans, or flies), which had spread all over its fur. Interestingly though, the dipterans eggs were carefully laid halfway along each strand of fur, almost in a calculated synchrony.
This apparent health condition, together with the fact that her teeth were heavily worn out and decayed, only one third intact and blackened with age, with three missing and one broken, makes me assume that this older female may well have been sick, weak and consequently more susceptible to attack.
Considering that assemblages of up to three primate species are fairly common in the study area, as are polyspecific choruses, including for instance, spider monkeys, howler monkeys, and squirrel monkeys (Saimiri sciureus), we think that it makes a lot of sense that harpy eagle predation may be more directed towards senile debilitated individuals. All the others would be well aware of predators even before they could see them. More individuals, and more individuals of different species, with different abilities to evade predators, may jeopardize the efforts of the predator to trap them. Adult howlers, specifically, however, may pose a transport problem to harpy eagles because of their large body size.
To know more about the subject, about animal protection and respect, and also about the life of a scientist in the jungle, his discoveries, challenges and even life threats, read: AMAZONIA AND OTHER FORESTS OF BRAZIL, by Janus Publishing, London, found from www.amazon.com.