Researchers in Bolivia have found evidence of feeding behaviors in monkey very similar to those in humans.
In the year of Charles Darwin and his theory of evolution, another brick comes to strengthen the ties between humans and monkeys. Behavioural ecologists working in Bolivia have found that wild spider monkeys control their diets in a similar way to human: rather than trying to maximize their daily energy intake, these monkeys tightly regulate their daily protein intake, so that it stays at the same level regardless of seasonal variation in the availability of different foods.
Tight regulation of daily protein intake is known to play a role in the development of obesity in humans. Until now it was thought humans’ eating patterns originated in the Palaeolithic era (between 2.4 million and 10,000 years ago). Findings from this research suggest that the evolutionary origins of these eating patterns may be far older than suspected.
Published online in the journal Behavioral Ecology, the research also provides valuable information about which trees are important for the monkeys’ diet, which is relevant to conservation; in addition, it may help to improve the care of captive primates, which can be prone to obesity and related health problems due to their diet.
Dr Annika Felton, a Departmental Visitor at the Fenner School of Environment and Society, The Australian National University, Canberra, Australia, spent a year in the Bolivian rainforest familiarising the Peruvian spider monkeys (Ateles chamek) to her presence and then observing their feeding habits.
During her year long observations of 15 individual monkeys (7 adult males, 8 adult females) she recorded everything they did and ate and for how long, counting every fruit and collecting samples. These would be dried and sent to the laboratory in Australia to be analyzed for their nutritional content. Such an unusual way to conduct a research on primates has enabled Dr Felton and her colleagues to calculate how much an individual monkey had consumed and the nutrients involved.
“We found that the pattern of nutrient intake by wild spider monkeys, which are primarily fruit eaters, was almost identical to humans, which are omnivores. What spider monkeys and humans have in common is that they tightly regulate their daily protein intake. They appear to aim for a target amount of protein each day, regardless of whether they only ate ripe fruit or mixed in other vegetable matter as well. Finding this result in spider monkeys was unexpected because, previously, ripe fruit specialists were thought to be ‘energy maximizers’. In other words, they would aim to maximise their daily energy intake. Our findings show this is not the case.”
The consequence of tight protein regulation is the same for monkeys and humans: if the diet is poor in protein but rich in carbohydrates and fats (energy dense food) individuals will end up ingesting a great deal of energy in order to obtain their protein target, which can lead to weight gain.
Findings are also interesting from an evolutionary point of view. Similarity in the regulatory pattern of protein intake between distantly related species, such as humans and spider monkeys, possessing very different dietary habits, may indicate that the evolutionary origins of such regulatory patterns are quite old, potentially far older than the Palaeolithic era. If we are not dealing with convergent evolution here – in other words that spider monkeys and humans have evolved this trait independently – then this trait may have been shared by our common ancestor. Spider monkeys are New World primates that split from the Old World primates about 40 million years ago.
She concluded: “What is perhaps most fascinating about our paper is not the answers we provide, but the questions that our findings raise. For example, why do these frugivores have the same pattern of nutritional intake as human omnivores? Is this due to convergent evolution or is it a remaining trait from a common ancestor?”