Abstract:
This study investigates possible regional variations
in size composition of adult female sperm whales
(Physeter macrocephalus) using data from 3302 pregnant
individuals taken on Soviet whaling expeditions to the
Southern Hemisphere 1961/62–1974/75. A general linear
model (GLM) was used to take the covariates of expedition,
latitude and ocean basin into account. The average body
size decreased from south to north in each ocean basin,
with the biggest decrease (about 200 cm) in the Indian
Ocean; followed by the Pacific Ocean (about 110 cm),
and the Atlantic Ocean (about 80 cm). Independent data
confirm the small size of female/immature sperm whales
in some tropical areas of the Indian and Pacific Oceans.
The mechanism responsible for this geographic variation
in sperm whale growth could reflect culturally transmitted
differences in foraging behaviour between clans of
female/immature sperm whales in response to differing
availabilities of prey resources by geographical region –
McNab’s resource rule. However there is little available
information for such a mechanism to be readily identifiable.
Although data for oceanic squids (sperm whale’s
main source of food) are lacking, there is evidence that the individual sizes of neritic species are positively correlated
with latitude. Hence feeding in equatorial regions may be
energetically more demanding due to smaller individual
prey size, with consequent effects on growth rate.
