Timing of epiphyseal development in the flipper skeleton of the harbour porpoise (Phocoena phocoena) as an indicator of paedomorphosis

Abstract

Galatius, A., Andersen, M.E.R., Haugan, B., Langhoff, H.E and Jespersen, Å. Timing of epiphyseal development in the flipper skeleton of the harbour porpoise ( Phocoena phocoena ) as an indicator of paedomorphosis. -Acta Zoologica (Stockholm) 87 : 77-82Epiphyseal development was investigated on X-rays of flippers from 158 harbour porpoises from Danish waters. Development followed a proximodistal pattern similar to what is known in other cetacean species. Ossification of epiphyses was rare in the phalanges of the first and fifth digits and in the more distal phalanges of the second, third and fourth digits. Along with the morphology of the first metacarpal and the more distal phalanges this suggested paedomorphosis relative to delphinids. Male and female porpoises showed similar progression of epiphyseal development until approximately the sixth year. From then on, female porpoises showed more progressed development than males. This suggests a higher level of paedomorphosis in the male porpoise. The mechanism behind phocoenid paedomorphosis seems to be progenesis, probably as an adaptation towards a high reproductive rate relative to the delphinids.

Paedomorphic Ossification in Porpoises with an Emphasis on the Vaquita (<I>Phocoena sinus</I>)

“…However, no studies have addressed paedomorphosis in vaquita appendicular components, nor has it been studied across a broad range of phocoenids (Dawson, 2003;Gol'din, 2004;Galatius et al, 2006).…”

“…Each diaphysis was assigned a stage of ossification and ankylosis following the staging criteria used in harbor porpoises (Galatius et al, 2006): Stage 0 unossified epiphysis; Stage 1, epiphysis ossified but not ankylosed to the diaphysis; Stage 2, epiphysis initially ankylosed; Stage 3, progressing ankylosis; and Stage 4, complete ankylosis (Galatius et al, 2006).…”

“…Male harbor porpoises retain a juvenile limb proportiontheir limb skeletal development arrests around 6 y. Male harbor porpoises, therefore, reach sexual maturity sooner than females, but their skeletal morphologies are more paedomorphic (Galatius et al, 2006).…”

“…The striped dolphin (Stenella coeruleoalba) ossifies the epiphyses of metacarpals I through IV as well as all proximal and mid-digit phalangeal epiphyses (Calzada & Aguilar, 1996;DiGiancamillo et al, 1998). Unlike the striped dolphin, harbor porpoises fail to ossify the distal epiphysis of metacarpal I, and a greater number of phalangeal epiphyses lack ossification (Galatius et al, 2006).…”

“…Most phocoenids are also sexually dimorphic in their soft tissue and skeletal development ( McLellan et al, 2002;Galatius et al, 2006). Female harbor porpoises grow more relative to males, achieving larger body sizes.…”