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Overly close neighbors

Living close to human beings changes the diet of the brown howler monkey, exposing it to yellow fever

DAVID SANTOS DE FREITAS“On my grandmother’s farm, the brown howler monkeys invade the chicken coop to steal eggs.” Primatologists have heard such stories for many years and have ignored them as if they were legends. They knew that these monkeys only eat plants. Now this belief has changed. The team led by primatologist Júlio César Bicca-Marques, from the Pontifical Catholic University of Rio Grande do Sul (PUCRS), showed that the black howler monkeys (Alouatta caraya) actually do steal eggs from nests or chicken coops to complement their diet whenever the amount of available leaves and fruit cannot take care of their hunger. The scarcity of their favorite food, however, is not the only problem these monkeys face in the South of Brazil: yellow fever has infected these primates, who are also being unjustly accused of spreading this disease.

Suspicions about these monkeys’ unorthodox feeding habits first appeared about 20 years ago. Primatologists Bicca-Marques and Cláudia Calegaro-Marques were involved in fieldwork in the region of Alegrete, in the western part of the State of Rio Grande do Sul, as part of their master’s degree program. The owner of the property where they were working told them about these strange feeding habits. The primatologists were suspicious of the interest with which the brown howler monkeys examined nests in trees. The brown howler monkeys, of the Alouatta genus, are among the most heavily studied primates in the Americas. In the last decades, research teams from a number of countries spent over 50 thousand hours studying them, from Mexico down to Argentina. Despite all this observation, no researcher had recorded any diet other than radical vegetarianism. “These monkeys prefer to eat ripe fruit and green leaves,” the primatologist explains.

However, the rural areas of the State of Rio Grande do Sul have long been populated by farmers and ranchers. As a result, fruit groves and eucalyptus trees have replaced native forests. In the absence of the forests along riverbanks that yield most of the monkeys’ menu of fruits and leaves, the animals are driven to improvise. This is how Bicca-Marques justifies the alternative diet of the Rio Grande do Sul monkeys. To corroborate this hypothesis, he has focused his students’ work on areas where the vegetation has changed. At a fruit farm consisting mainly of orange groves next to an area where Bicca-Marques and Cláudia had worked 20 years ago, Helissandra Prates discovered that the monkeys feast on oranges at harvest time and even eat orange tree leaves, which are full of an oil not greatly appreciated by herbivores. Helissandra also saw two monkeys licking the inside of a nest.

On a eucalyptus-dominated property located in the municipal region of Tupanciretã in the center of the state, Carina Muhle saw monkeys eating the flowers and even the bark of these trees, which are not part of this region’s native flora. On this farm, she also saw monkeys invading the chicken coop to steal eggs, even though they had to squeeze through a narrow opening on top of the chicken coop’s closed door.

DAVID SANTOS DE FREITASBlond from birth: only the adult males have dark fur DAVID SANTOS DE FREITAS

The findings, published in the October issue of the International Journal of Primatology, were well received by experts. “It took 20 years to publish our findings, but the results have made up for this lead time,” says the primatologist, who has been contacted by colleagues from Brazil and abroad who are interested in the findings. After reading the scientific article, many of the primatologists are now taking more seriously the reports they heard over the years on monkeys invading chicken coops and being blamed for the lack of birds, such as the monk parakeets, in small town squares. Thus, the work of the group from Rio Grande do Sul may spur new findings on the feeding habits of several species of howler monkeys. The next step is to conduct experiments to evaluate the monkeys’ interest in eggs and observe their eating habits in more detail in several regions.

Guardian angels
The need to turn to an alternative diet is not the only problem, nor the most serious one, that howler monkeys face as a result of living too close to human beings. Several hundred howler monkeys died in 2008 and 2009 during an outbreak of wild yellow fever. “Yellow fever appeared in Africa and reached Brazil on slave ships,” says Bicca-Marques. “This is why the New World primates such as the howler monkeys are sensitive to the disease.” The fact that humans have been exposed to this disease for a longer time gives them greater resistance. This is why, says the primatologist, it is human beings that are responsible for spreading the disease – not only in the 16th century, but today as well. Of the infected humans, 40% to 60% have no apparent symptoms, yet spread the virus. The howler monkeys react differently; in the period of three to seven days during which the disease damages the body, the monkeys remain prostrate and inactive and most of them die. As a result, they do not have the opportunity to spread the virus.

“The howler monkeys are actually sentries that can warn public health authorities about the need to vaccinate the human population,” says Bicca-Marques. This is why, according to him, protecting these monkeys is also a public health issue and why he launched the “Protect your guardian angel” campaign to keep the inhabitants of rural areas from killing the monkeys out of fear that they may spread yellow fever. This initiative highlights a two-way relationship: just like the monkeys can protect people, acting as harbingers of the virus, the vaccination of people is also a biodiversity protection issue.

Scientific articles
BICCA-MARQUES, J. C. et al. Habitat impoverishment and egg predation by Alouatta caraya. International Journal of Primatology. v. 30, n. 5, p. 743-48. Oct. 2009.
BICCA-MARQUES, J. C. Outbreak of yellow fever affects howler monkeys in southern Brazil. Oryx. V. 43, n. 2, p. 169-175. Apr. 2009.