Growth, proportions and variation of the skull of harbour porpoises (Phocoena phocoena) from the Sea of Azov

Abstract

Skulls of 153 porpoises from the Sea of Azov and the Black Sea were studied. Thirty-seven skull measurements were examined. Skull growth in porpoises can be divided into five periods. The greatest changes in growth patterns occur after the first weeks of life. A ‘relay effect’ in growth of some structures (e.g. rostrum) was observed. Individual variation usually increases when a given structure grows intensively; however, some structures are highly variable over the entire lifespan. Sexual dimorphism is demonstrated in most of the measurements in adult animals. Temporal variation is minor. Skull structures are characterized by six types of allometry; three of them demonstrate drastic changes of allometry during lifespan, thus requiring two allometric equations to describe them. Skull proportions change critically during the first weeks of life. Six groups of correlated measurements differing in the pattern of correlation links during ontogeny were identified. Three factors play the most important roles in forming the definitive skull proportions: correlation between adjacent structures, impact of total skull size and inclusion in a certain functional complex.