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While mammals collectively exhibit the highest ratio of brain size to body size among vertebrates, a team led by paleontologist Ornella Bertrand demonstrates that for ten million years following the extinction of dinosaurs, that ratio in fact shrank for mammals as they evolved to fill newly vacated ecological niches and their bodies increased in size more rapidly than their brains. Competition for resources tends to favor intelligence and thus large, complex brains, but during this period, the abundance of resources relative to mammalian population numbers likely moderated competition and facilitated an increase in body size. Bertrand and her team reason that as population numbers swelled, competition intensified, creating conditions that