The quest for elusive bugs spurred primate tool use and problem solving skills.
Learning how to survive on a lean-season diet of insects such as ants and slugs, which are tricky to catch, may have progressed the development of larger brains and higher-level cognitive functions in our ancestors and other primates, suggests research from Washington University in St. Louis.
“Challenges associated with finding food have long been recognized as important in shaping evolution of the brain and cognition in primates, including humans,” said Amanda D. Melin, PhD, assistant professor of anthropology in Arts & Sciences and lead author of the study.
“Our work suggests that digging for insects when food was scarce may have contributed to hominid cognitive evolution and set the stage for advanced tool use.”
Based on a study of capuchin monkeys that took place over a five year period in Costa Rica, the research provides support for an evolutionary theory that connects the development of sensorimotor (SMI) skills, such as increased manual dexterity, tool use and problem solving, to the more creative challenges of foraging for insects and other foods that are buried, or otherwise difficult to acquire.
Published in the June 2014 Journal of Human Evolution, the study is the first to offer detailed evidence on how seasonal changes in food supplies have influenced the foraging patterns of wild capuchin monkeys.
The study is co-authored by biologist Hilary C. Young, along with anthropologists Krisztina N. Mosdossy and Linda M. Fedigan, all of the University of Calgary, Canada.
The study refers to the fact that human populations also eat buried insects on a seasonal basis and suggests that this practice was a key aspect in human evolution.
“We find that capuchin monkeys eat embedded insects year-round but intensify their feeding seasonally, during the time that their preferred food – ripe fruit – is less abundant,” Melin said. “These results suggest embedded insects are an important fallback food.”
Research that has been conducted previously has shown that fallback foods help shape the evolution of primate bodies, including the formation of strong jaws, thick teeth and specialized digestive systems in primates whose fallback diets are reliant largely on vegetation.
The study also shows that fallback foods can play a crucial role in brain evolution among primates that fall back on insect-based diets. This influence is most prominent among primates that evolve in habitats with wide seasonal variations, such as the wet-dry cycles that can be found in South American forests.
“Capuchin monkeys are excellent models for examining evolution of brain size and intelligence for their small body size, they have impressively large brains,” Melin said. “Accessing hidden and well-protected insects living in tree branches and under bark is a cognitively demanding task, but provides a high-quality reward: fat and protein, which is needed to fuel big brains.”
However, when it comes to using tools not all capuchin monkey strains and lineages are identical, and Melin’s theories may help distinguish the reason for this.
Perhaps the most significant difference between the robust (tufted, genus Sapajus) and gracile (untufted, genus Cebus) capuchin lineages is their variation in tool use. Cebus monkeys are well known for their clever food-foraging skills, such as banging snails or fruits against branches, however they are nothing compared to their Sapajus cousins when it comes to the innovative use and modification of sophisticated tools.
Melin said that one explanation is that Cebus capuchins have historically and consistently resided in tropical rainforests, whereas the Sapajus capuchins spread from their origins in the Atlantic rainforest into areas with drier, more temperate and seasonal habitats.
“Primates who extract foods in the most seasonal environments are expected to experience the strongest selection in the ‘sensorimotor intelligence’ domain, which includes cognition related to object handling,” Melin said. “This may explain the occurrence of tool use in some capuchin lineages, but not in others.”
Genetic analysis of mitochondrial chromosomes has shown that the Sapajus-Cebus diversification occurred millions of years ago in the late Miocene epoch.
“We predict that the last common ancestor of Cebus and Sapajus had a level of SMI more closely resembling extant Cebus monkeys, and that further expansion of SMI evolved in the robust lineage to facilitate increased access to varied embedded fallback foods, necessitated by more intense periods of fruit shortage,” she said.
One of the more interesting modern examples of this behavior, said Melin, is the seasonal consumption of termites by chimpanzees, whose use of tools in order to extract the protein-rich food source is a crucial survival technique in their harsh environments.
So what does this mean for hominids?
It is difficult to decipher the extent of seasonal dietary variations in the fossil record; stable isotope analyses show seasonal variation in diet for at least one South African hominin, Paranthropus robustus. Other isotopic research suggests that early human diets might have included a range of extractable foods including termites, plant roots and tubers.
Modern humans are known to frequently consume insects, which are important when other animal foods are more limited.
This study suggests that the resourcefulness needed to survive on a diet of elusive insects has been a key factor in the development of uniquely human skills:
It may well have been bugs that helped shape our brains.
Contributing Source: Washington University in St. Louis
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