Brain, behavior and evolution
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Evolution has generated mammalian brains that vary by a factor of over 100,000 in mass. Despite such tremendous diversity, brain scaling in mammalian evolution has tacitly been considered a homogeneous phenomenon in terms of numbers of neurons, neuronal density, and the ratio between glial and neuronal cells, with brains of different sizes viewed as similarly scaled-up or scaled-down versions of a shared basic plan. According to this traditional view, larger brains would have more neurons, smaller neuronal densities (and, hence, larger neurons), and larger glia/neuron ratios than smaller brains. ⋯ Strikingly, all brains appear to gain nonneuronal cells in a similar fashion, with relatively constant nonneuronal cell densities. As a result, while brain size can no longer be considered a proxy for the number of brain neurons across mammalian brains in general, it is actually a very good proxy for the number of nonneuronal cells in the brain. Together, these data point to developmental mechanisms that underlie evolutionary changes in brain size in mammals: while the rules that determine how neurons are added to the brain during development have been largely free to vary in mammalian evolution across clades, the rules that determine how other cells are added in development have been mostly constrained and to this day remain largely similar both across brain structures and across mammalian groups.
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We previously reported that in a eusocial rodent, the naked mole-rat (Heterocephalus glaber), traditional neural sex differences were absent; instead, neural dimorphisms were associated with breeding status. Here we examined the same neural regions previously studied in naked mole-rats in a second eusocial species, the Damaraland mole-rat (Fukomys damarensis). Damaraland mole-rats live in social groups with breeding restricted to a small number of animals. ⋯ Thus, both sex and breeding status influence neural morphology in Damaraland mole-rats. These findings are in accord with the observed sex differences in body weight and genitalia in Damaraland but not naked mole-rats. We hypothesize that the increased sexual dimorphism in Damaraland mole-rats relative to naked mole-rats is related to reduced reproductive skew.