They have shown that brains in mammals have grown over the course of evolution without the digestive organs having to become smaller. The researchers have further demonstrated that the potential to store fat often goes hand in hand with relatively small brains – except in humans, who owe their increased energy intake and correspondingly large brain to communal child care, better diet and their ability to walk upright.
Brain tissue is a major consumer of energy in the body. If an animal species evolves a larger brain than its ancestors, the increased need for energy can be met by either obtaining additional sources of food or by a trade-off with other functions in the body. In humans, the brain is three times larger and thus requires a lot more energy than that of our closest relatives, the great apes. Until now, the generally accepted theory for this condition was that early humans were able to redirect energy to their brains thanks to a reduced digestive tract. Zurich primatologists, however, have now disproved this theory, demonstrating that mammals with relatively large brains actually tend to have a somewhat bigger digestive tract.
Ana Navarrete, the first author on the study published today in Nature, has studied hundreds of carcasses from zoos and museums. “The data set contains a hundred species, from the stag to the shrew,” explains the PhD student. The scientists involved in the study then compared the size of the brain with the fat-free body mass. Senior author Karin Isler stresses that, “it is extremely important to take an animal’s adipose deposits into consideration as, in some species, these constitute up to half of the body mass in autumn.” But even compared with fat-free body mass, the size of the brain does not correlate negatively with the mass of other organs.
Nathalie Huber | Universität Zürich
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