Macroscopic and histologic characteristics of sexual maturation in the Burmeister’s porpoise Phocoena spinipinnis from Peru

Abstract

The morphology and maturation process of gonads of 70 Burmeister’s porpoises Phocoena spinipinnis, with body lengths ranging 135 - 183 cm (n = 34 females) and 64.5 - 182 cm (n = 36 males) are described. Samples were collected in six ports of central and northern Peru from 1987 to 1999. In the field, sexual maturity was determined through macroscopic examination of gonads. Ovarian corpora indicated mature females (resting, lactating, pregnant). Semen in epididymides was considered evidence for mature males. The ovaries of Burmeister’s porpoises are ovoid or bean-shaped and flattened, with corpora modifying surface appearance. In the laboratory, ovaries were examined macro- and microscopically, measured, weighed and sliced. The number of corpora ovarica (lutea, albicantia, atretica) and their morphology were documented. The follicles, oocytes, and nuclei were measured. Follicular development was predominantly left-sided, but occurred in both ovaries in 16.3% of females, mainly in those with numerous corpora. Macroscopically, several corpora atretica with luteinization had characteristics similar to those of corpora albicantia, making microscopic determination essential. Inconclusive evidence of recent ovulation was reported for January. Two of three immature females showed good follicular development in March and April.

The testes of Burmeister’s porpoise are elongated and cylindrical. Of 36 males examined macroscopically, seven were immature, five pubescent and 24 mature. The histological analysis allowed to confidently determine sexual maturity status. Field evaluation of maturity based on the presence of sperm in the epididymides is a useful but in 8.3% of cases not an exact method. No histological evidence of male reproductive seasonality was found. Spermatogenesis was perceptible year-round and tubule diameters showed stochastic monthly variations. The body length at 50% sexual maturity was estimated at 157 cm and 152.7 cm, for males and females, respectively. There was no evidence of reproductive senescence.

This study is the first evaluation of the sexual maturation process in the Burmeister’s porpoise, using both macroscopic observations and elaborate gonads histology. It provides important new information on the natural history of the species, including parameters (e.g., mean body length at maturation, reproductive seasonality) highly relevant to its conservation.