Miniaturization in plethodontid salamanders (Caudata: Plethodontidae) and its consequences for the brain and visual system

Abstract

In seven species of plethodontid salamanders (Desmognathus ochrophaeus, Euycea bislineata, Plethodon cinereus, Batrachoseps attenuatus, Hydromantes italicus, 7horius narisoualis and Bolitoglossa subpalmata), absolute and relative volumes of the eye, the brain, major regions of the brain, and regions containing the major visual and visuomotor centres (ix. thalamus, praetectum, tectum and tegmentum mesencephali), and the density and number of neurons in these regions were determined. The seven species range from moderately large to extremely small in body size and from the smallest to the largest genome sizes found in terrestrial salamanders. The following processes were observed in miniaturized salamanders with intermediate to large genome and cell sizes (Batrachoseps, 7horius) as compared to small and medium-sized salamanders with small genome and cell sizes: (I) increase in the relative size of the brain, from 3.9 to 12.4% of head volume; (2) reduction in relative size of the ventricles from 10.9 to 5.8% of brain volume; (3) increase in relative volume of those brain regions containing the major visual and visuomotor centres from 29.2 to 37% of brain volume; (4) increase in volume of grey matter relative to white matter, from 33.2 to 44.4% of midbrain volume; (5) increase in volume of tectal relative to tegmental grey matter, from 54.8 to 76.8% of total midbrain volume; (6) increase in neuron packing density in the regions containing the visual centres, from 16 to 31.5%. Because of these compensatory processes, irhorius, the smallest species with a head 1/27 and a brain 119 the size of that of the largest one, Hydromantes, has 1/3 as many central visual neurons (58000 us. 187 000). Some of these processes found in miniaturized salamanders, such as increase in tectal cell density, also occur in large salamanders with very large genome and cell sizes, viz. in Bolitoglossa (25%) and Hydromantes (29%). Thus, increase in genome size and cell size seem to pose functional problems similar to miniaturization; both cases involve an increase in cell size relative to overall organismal structure.