A colony of feral rhesus macaques calls the banks of the Silver River in Silver Springs State Park in central Florida its home. The monkeys are part of a larger feral population living throughout the Cross Florida Greenway. Many locals enjoy having the monkeys in the park, but wildlife officials are concerned about overpopulation caused by human feeding, the nonnative animals’ ecological impact and the potential for interspecies disease transmission. A study released this week in the journal Primates by anthropologists at San Diego State University has found that the park’s macaque population is smaller than many previous estimates and that the vast majority of the monkeys’ diets come from environmental–not human-given–food. No one knows exactly how or when the monkeys, which are native to southern and southeast Asia, were introduced to the central Florida wetlands, but they have lived in the region’s wetland parks for decades and have adapted to the environment. Over the years, population numbers have waxed and waned due largely to intermittent trapping efforts. Their presence has been contentious.
“The local authorities, like the Fish and Wildlife Service, have been less thrilled with the monkeys,” said SDSU anthropologist Erin Riley, one of the paper’s authors. “Their purview is to maintain a natural environment, and these animals are not natural to this area. They have concerns about the local ecological impact of these animals, and then there are also health issues if people interface and get close to them.”
With funding from the National Geographic Society/Waitt Grants Program, Riley and SDSU graduate student Tiffany Wade spent several months in 2013 taking a census of the monkey population along the Silver River and documenting human-monkey interactions. Over the years, some alarming estimates have suggested upwards of a thousand or more individuals, but the researchers’ census came to a much more sober figure: 118 individuals living in four separate social groups.
How many monkeys live in other parts of the Florida greenway remains unclear, Riley said. There are likely a number of groups living beyond the Silver River that would raise the overall Florida population into the hundreds, if not more.
Another prong of the study looked at how frequently visitors were feeding–“provisioning” is the scientific term–the monkeys, and conversely, how much of their diet came from eating natural foods in the environment. Riley and Wade observed that the Silver River monkeys, like their relatives who live in other temperate climates like Pakistan, ate mostly leaves, buds, flowers, shoots and a type of dry fruit called a samara. The ash tree formed the cornerstone of their diet. The researchers did note one novel, local adaptation: The macaques also ate grass-like sprouts called sedges that grow in wetlands.
When boaters came along, Riley and Wade inconspicuously recorded how many visitors interacted with the monkeys and how many gave them food.
“People would sometimes throw them whole oranges and you’re like, ‘Watch out, don’t nail them in the head!'” Riley said. “They love peanuts. Grapes also seemed to elicit what are called ‘food calls.’ Really, they’re excited about pretty much anything you give them.”
The researchers noticed the monkeys seemed to have a rule of thumb for deciding whether it was worth their time to come to the tree line in search of human handouts.
“They tend to ignore canoes and kayaks because people on those boats generally aren’t the ones feeding them,” Riley said. “It’s the big boats, the pontoons and the motorboats, that are feeding them. As soon as the monkeys hear the sounds of those boats, they come running up to the river’s edge.”
After five months of observation, Riley and Wade totaled up the numbers for the monkeys’ feeding behavior and found that 87.5 percent of their diet came from the natural environment and a relatively small 12.5 percent came from provisioning. The researchers do note, however, that even though provisioned food represented only a small percentage of their overall diet, the fleshy fruits provided to the monkeys may be an important fallback food.
“From the park’s perspective, they know that provisioning occurs, and their sense is that it’s because of this provisioning that this population persists,” Riley said. “What our data show is that provisioning actually doesn’t occur that often anymore, and as a result the monkeys have learned to rely primarily on local food.”
When it came to interactions between macaques and humans, the researchers found that most were benign. Out of 611 recorded interactions, only two involved people directly handing food to a monkey. That suggests worries about the monkeys’ health hazards are out of proportion to the actual risk, Riley said. The river itself acts as a hurdle to disease transmission, as the dense cypress roots all along its banks make it difficult to land a boat and explore the vegetation.
The findings underscore the need for official education and outreach programs, the researchers said. They would like to see park officials introduce some kind of signage or brochure educating visitors about where the monkeys come from and how to safely interact with them, and they believe increasing patrols along the river would go a long way toward discouraging people from feeding the monkeys.