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Embryonic brain growth in two shorebirds: Charadrius vociferus and Gallinago gallinago
Abstract:
The sizes of six brain regions and of the whole brain were measured for a series of embryonic killdeer, Charadrius vociferus, and common snipe, Gallinago gallinago, to examine (1) the allometric relationship between whole brain and body mass through ontogeny, (2) whether the longer incubation period of the killdeer corresponds to a larger brain at hatch, (3) whether different brain regions grow independently in size through ontogeny, and (4) whether relative size of particular brain regions relates to relative importance of hatching behavior or to the relative importance of behaviors in the adult. Although snipe are generally less precocial at hatch than killdeer, and hence are predicted to have lower allometric coefficients, the allometric relationships between brain and body mass for the two species were not significantly different and were comparable to those for other birds and mammals. The onset of the rapid growth phase of the whole brain, and each region, was very early in the snipe; as a consequence, brain sizes in both species are similar at hatch, despite the shorter incubation period of snipe. In hatchlings of both species, the brain comprises about 7% of body mass. The telencephalon grows most rapidly, the diencephalon and myelencephalon grow more slowly, and the optic tectum grows steadily throughout the embryonic period. The telencephalon of the hatchling snipe is relatively larger than that of hatchling killdeer and exhibits a large nucleus basalis, typical of tactile foragers, although snipe do not forage tactily until adulthood. The relatively large optic tectum of hatchling killdeer corresponds to the highly visual method of foraging of hatchlings. However, the degree to which brain regions grow in the embryonic period, with the exception of the optic tectum and cerebellum in killdeer, appears to relate very closely to their eventual size in adults, with large brain regions growing less in the embryonic period than small brain regions.