Social Status Could Impact Lifespan and Reproduction Rates in Animals

Picnicking on the beach is a delightful experience for many, but it can quickly turn sour when a sandwich ends up with an unexpected crunch of sand. However, for primates like long-tailed macaques, the tolerance for sand in their food may depend on their social rank and the trade-off between immediate energy consumption and long-term health, as revealed in a recent study conducted by researchers at Dartmouth.

The study, published in the journal eLife, observed the eating habits of long-tailed macaques on Thailand’s Koram Island. The researchers found that both dominant and lowest-ranked animals would briefly rub sand-covered food on their fur or between their paws before consuming it, along with most of the sand, and moving on to the next morsel. In contrast, middle-ranked monkeys, having more time on their paws, would carry their food to the water’s edge and wash it in the sea to remove the sand. These monkeys would often spend extra time and energy scrubbing their snacks clean, sometimes even walking down the beach on their hind legs with their front paws full of food.

Nathaniel Dominy, the study’s corresponding author and the Charles Hansen Professor of Anthropology at Dartmouth, suggests that the findings provide insight into how animals, even those in hierarchical groups, make decisions based on individual needs. The study supports the disposable soma hypothesis, which proposes that animals may prioritize immediate survival and reproduction over longevity. Dominy notes, “Delayed gratification has its limits.”

The researchers observed that high-ranking males, constantly engaged in aggressive behaviors to maximize their siring of offspring, would quickly consume food, sand and all, without bothering to clean it. For these dominant males, the urgency of energy consumption outweighed concerns about tooth damage caused by sand. On the other hand, monkeys that washed their food, particularly middle-ranked individuals, may be playing a long game by investing time in cleaning their food to prevent tooth wear and potentially increase their longevity and reproductive success.

The study also raises questions about how wear and pitting observed in the fossilized teeth of early humans may relate to social structure and access to resources like water. Dominy suggests that tooth wear could potentially be linked to an individual’s rank and behavior, rather than just diet.

To observe the macaques’ eating habits, the researchers set up trays with cucumber slices on the beach, some placed on sand and others buried in sand. They recorded nearly 1,300 instances of food-handling by 42 individual macaques over six weeks. The monkeys that washed their food spent significantly more time cleaning each cucumber slice compared to those that simply brushed off the sand, indicating a trade-off between time spent washing and the effectiveness of sand removal.

The study highlights the impact of social rank on decision-making among animals and how different individuals within a group may prioritize energy consumption or long-term health based on their position in the hierarchy. The researchers emphasize the importance of being curious, asking questions, and testing assumptions, even when something seems intuitive. By studying the behavior of animals like long-tailed macaques, we can gain a better understanding of the complex trade-offs that individuals make within social structures and how these decisions shape their behavior.