Abstract
The population biology of most phocoenid species is still rather poorly understood. It is important that we improve our knowledge in view of concerns which have been expressed in recent years about the status and long-term viability of some populations. A survey of the literature suggests that it would be unwise to assume that an unknown life-cycle parameter of a phocoenid species would necessarily be similar to that of another species, simply on the basis of taxonomic relationship. Each population and species has experienced a unique evolutionary history in a different ecological regime, and selective influences will have moulded life cycle parameters accordingly.
Most quantitative published data relate to Neophocaena phocaenoides (coastal waters of eastern Asia), Phocoenoides dalli (boreal – temperate North Pacific), and Phocoena phocoena (boreal – temperate N. Pacific and N. Atlantic, and Black Sea). Phocoena dioptrica (southern South America and the New Zealand subantarctic), P. sinus (Gulf of California), and P. spinipinnis (temperate – subtropical waters of South America) are not well known yet. Body-size variation within the family is relatively limited: 65-192 cm (N. phocaenoides); 100-225 cm (P. dalli); 67-190 cm (P. phocoena); and gestation periods are very similar in these three species: 11 months (P. phocoena and N. phocaenoides) and 11.4 months in P. dalli. Far greater differences are apparent in estimates of the age at sexual maturity, the life span, the duration of weaning, and the mean calving interval. It is important to determine if such differences result from misinterpretations or if they relate to radically different evolutionary adaptations (among animals of about the same body size) which have arisen in response to different environmental histories and ecological constraints.
The age at which sexual maturity is attained in N. phocaenoides is not yet known. The maturation process seems to be complete between 4-5 and 5-6 years of age in the western North Atlantic and North Sea populations of P. phocoena, respectively, and at about 7 years in P. dalli. The mean calving intervals appear to be 2 years in N. phocaenoides, 3 years in P. dalli, and variable in P. phocoena from 1-3 years, depending perhaps on nutritional levels. The duration of weaning is not well known in N. phocaenoides (estimated between 6-15 months). It could be as short as a few months (6-8) in P. phocoena and as long as 2 years in P. dalli. As with other odontocetes, controversies exist concerning the accuracy of age determination, but all direct evidence points to P. phocoena being relatively short-lived, to a maximum of about 12-13 years. On the basis of age determined from dentinal or cementum layers, P. dalli and N. phocaenoides have longer life spans than this; at least 16-17 years and 23 years respectively. Attempts to correlate numbers of corpora albicantia with body length have generally not been very successful.