Heather N. Koopman

Ontogenetic allometry and body composition of harbour porpoises (Phocoena phocoena, L.) from the western North Atlantic

North Atlantic harbour porpoises Phocoena phocoena (L.) face considerable energetic challenges, as they are relatively small marine mammals with an intense reproductive schedule and a cold-water habitat. Postnatal growth of these porpoises was described using ontogenetic allometry and body composition techniques. The cross-sectional sample contained robust calves, immature, and mature porpoises (n = 122) incidentally killed in commercial fishing operations between 1992 and 1998. Total mass and the mass of 26 body components were measured using a standard dissection protocol. Most body components grew similarly in female and male porpoises. Blubber, brain and skull were negatively allometric, while muscle and reproductive tissues exhibited positive allometry. Female heart, liver, intestine and mesenteric lymph node grew at significantly higher rates than in males. Male locomotor muscle and pelvic bones grew significantly faster than in females. High growth rates for visceral and reproductive organs in porpoises, relative to other mammals, may underlie their early maturation and support their intensive, annual reproductive schedule. Relative to other cetaceans, porpoises seem to allocate a larger percentage of their total body mass to blubber. This allocation to blubber, which is greatest in calves (37% of body mass), may provide harbour porpoises with the thermal insulation required to live in cold water. The factors influencing growth rates and differential investments in body composition seem to change at various stages of a porpoise’s life. Energy allocation in porpoises seems to shift from an emphasis on developing an insulative blubber layer in young animals to preparing the body for annual reproduction at sexual maturity.

Topographical distribution of the blubber of harbor porpoises (Phocoena phocoena)

Thickness of blubber was measured at 48 sites on the bodies of 68 male and 46 female harbor porpoises ( Phocoena phocoena ) from the Bay of Fundy, Canada, and Gulf of Maine and mid-Atlantic coast of the United States. Blubber was thinner over the head than in the thoracic-abdominal region, where thickness varied little around the girth of the animal. Thickness of blubber anterior to the anus ranged from 15 to 25 mm. Posterior to the anus, blubber formed thick dorsal and ventral ridges but was extremely thin laterally. In the thoracic-abdominal region, blubber of calves ( X = 22.9 mm ± 3.6 SD; n = 11) was significantly thicker than that of all other animals, but lactating females had the thinnest blubber (14.5 ± 3.1 mm; n = 7). Immature porpoises of both sexes, mature males, and non-lactating mature females had thicknesses of blubber intermediate between those of calves and lactating females. Thickness of blubber anterior to the anus was correlated negatively with body size, an unusual relationship for a marine mammal. Blubber posterior to the anus was similar among reproductive classes. Blubber of the harbor porpoise appears to be divided into two functional components. The thoracic-abdominal blubber functions primarily as an energy store and insulation, and posterior blubber acts to maintain hydrodynamic shape on the peduncle. Calves and adult porpoises may have different thermoregulatory and energetic strategies, resulting in different adaptations of blubber structure.

High concentrations of isovaleric acid in the fats of odontocetes: variation and patterns of accumulation in blubber vs. stability in the melon

Isovaleric acid (iso5:0) is an unusual fatty acid that is important for echolocation and hearing in acoustic tissues of some odontocetes, but its functional significance in blubber is unknown. We examined patterns of accumulation of this compound in blubber in 30 species of odontocetes (n=299). Iso5:0 concentrations in blubber varied with phylogeny, ontogeny and body topography. Iso5:0 accumulated in greater quantities in superficial/outer blubber than in deep/inner blubber. In the outer blubber of northern right whale and Hectors dolphins, iso5:0 accounted for one-third to one-half of all fatty acids. Total blubber burden of iso5:0 in harbour porpoises represented up to 15 times the amount deposited in the melon. The composition of the melon does not change during starvation in harbour porpoises, supporting the hypothesis that lipids in melon are conserved for a specific function. Some odontocetes continually deposit iso5:0 in blubber after levels in melon have reached asymptotic levels, suggesting independent control of iso5:0 synthesis and storage in these compartments. Dolphins and porpoises inhabiting cold waters possess higher concentrations of iso5:0 in their outer blubber layers than species from warmer regions. We propose that this relationship represents an adaptive secondary role for iso5:0 in maintaining blubber flexibility in cold environments.