Brain size variation along altitudinal gradients in the Asiatic Toad (Bufo gargarizans)

Size changes of brain and brain regions along altitudinal gradients
provide insight into the trade-off between energetic expenditure and
cognitive capacity. We investigated the brain size variations of the
Asiatic Toad (Bufo gargarizans) across altitudes from 700m to 3200m. A
total of 325 individuals from 11 sites and two transects were sampled. To
reduce confounding factors, all sampling sites within each transect were
within a maximum distance of 85km and minimum altitudinal difference close
to 2000 m. Brains were dissected and five regions were both
measured directly and with 3D CT scan. There is a significant negative
correlation between the relative whole brain volume (to snout-vent length)
and altitude. Furthermore, the relative volumes (to whole brain volume) of
optic tectum and cerebellum also decrease along the altitudinal gradients,
except the telencephalon, which increases along the altitudinal gradients.
Therefore, our results are mostly consistent with the expensive tissue
hypothesis and the functional constraint hypothesis. We suggest that most
current hypotheses are not mutually exclusive and data supporting one
hypothesis are often partially consistent with others. More researches on
mechanisms are needed to explain the brain size evolution in realistic
adaptation events.