Reproductive biology of male franciscanas from Rio Grande do Sul, southern Brazil

Fishery Bulletin, Oct, 2004 by Daniel Danilewicz, Juan A. Claver, Alejo L. Perez Carrera, Eduardo R. Secchi, Nelson F. Fontoura

Index of testicular maturity

An index of testicular maturity may be very useful in studies where it is necessary to know the sexual maturity of a large sample of animals without the need of histological analysis, which is time consuming and requires expertise. Although Hohn et al. (1985) recommended the investigation of the applicability of this indirect index of sexual maturity for male cetaceans, the research on this subject has shown no progress. To date, the index of sexual maturity has been calculated only for the common dolphin, Delphinus delphis (Collet and Saint Girons, 1984), and from the pantropical spotted dolphin, Stenella attenuata (Hohn et al., 1985). For both species, this index distinguished satisfactorily the mature from the immature and pubertal dolphins. Given the results presented, we also recommend the use of the index of testicular maturity as an alternative, nonhistological method, to determine the sexual maturity of male franciscanas. Males with index values lower than 0.05 can be safely classified as immature, and males with index values above 0.08 can be classified as mature. It is recommended that for animals with intermediate values their testes be analyzed histologically so that their reproductive status may be determined definitively.

Besides making intra- and inter-populational comparisons possible, the index of testicular maturity also permits interspecific comparisons because size differences between species are eliminated. The mean index of testicular maturity of mature franciscanas (0.12) is considerably lower than mature pantropical spotted dolphins (1.9) (Hohn et al., 1985). This difference is a consequence of the relatively small increase of the testes weight of male franciscanas when sexual maturity is attained. Although male spotted dolphin show a marked increase of about 25-fold in testes weight at this moment, franciscanas show an increment in testes weight of about ninefold only.

Reproductive seasonality

The reproductive activities in male mammals are usually restricted to the periods when the females are in estrus (Lincoln, 1992). Reproductive seasonality in males has been reported for several cetacean species and populations through the identification of temporal variations in the testes weight and histological characteristics. In species where the reproductive period is restricted for a few months, as with the dusky dolphin (Lagenorhynchus obscurus) and the harbor porpoise (Phocoena phocoena), the testes weight presents marked fluctuations accompanying the reproductive period (Read, 1990; van Waerebeek and Read, 1994; Neimanis et al., 2000). Even in species with a diffuse reproductive period (i.e., with more than one peak for births per year) as in the case of dolphins of the genus Stenella in the tropical Pacific, it was possible to detect seasonal variation in the male reproductive rhythm (Perrin et al., 1976, Hohn et al., 1985).

Because of the known seasonality for births for franciscana (Kasuya and Brownell, 1979, Harrison et al., 1981, Danilewicz, 2003), it would be expected that the males would accompany the female rhythm, decreasing or even ceasing testicular activity in autumn and winter months. Kasuya and Brownell (1979) examined the seasonal change in testes weight in the months of January, June, and December. From our knowledge of the species' reproduction period, testes weight would be expected to be higher in December and January. However, the authors could not confirm this prediction and attributed the lack of seasonality to the small sample size of mature animals. Nevertheless, the lack of seasonality, even when the testes weight of the mature males from Rio Grande do Sul are included, may indicate what is occurring in the population, and not be a bias introduced by a small sample size.