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

In species that possess small testes, as in the case of the franciscana, the variation in the testicular activity may be better reflected by changes in the diameter of the seminiferous tubules and the rate of spermatogenesis rather than by changes in the testes weight. Nevertheless, the preliminary results about these characteristics (mature males with spermatids or spermatozoa [or both] in the seminiferous tubules in nonreproductive months and little monthly variation in the diameter of the seminiferous tubules) also do not support the hypothesis of a male reproductive seasonality. The combination of results presented here indicates that testicular activity is not completely interrupted in all males within the population, and that at least some of them may remain capable of fertilizing females during the year. This conclusion is supported by the observation of pregnancies outside the normal gestation season and that the births resulting from these pregnancies were estimated to take place in September and in late March (Danilewicz, 2003).

The hormone and sperm production by the testes during periods when the females are not able to reproduce may represent an unnecessary energetic expense by the male (Dewsbury, 1982) and may be an explanation for the period of reproductive inactivity for males of several mammal species. In species with large relative testes weight, the maintenance of high levels of sperm production in the testes is a considerable energetic cost for the individual. However, as discussed earlier, this is definitely not the case for the franciscana. For this reason, we suggest that the small energy investment in producing sperm all over the year, due to the small testicular mass, may be an evolutionary advantage for male franciscanas in case of the appearance of off-season reproductive females.

Franciscana reproductive strategy

Although important advances in the knowledge of franciscana behavior in the wild have been made (e.g., Bordino et al., 1999; Bordino, 2002), there is no information on the species' reproductive behavior and its mating strategy remains unknown. Relative testis weight, sexual size dimorphism, and secondary sexual characteristics may provide indirect clues regarding mating strategy in franciscana and are discussed below.

Relative testis weight In mammals, there is a functional relationship between relative testis weight and the species' mating system (Kenagy and Trombulak, 1986). Testes are relatively small in species presenting monogamy or extreme poliginy (several females few males), i.e., where a male copulates with all females of a group or harem. Comparative studies have demonstrated that males tend to be larger than females and show secondary sexual characteristics in species presenting extreme poliginy. On the other hand, the relative testis weight is high and the sexual size dimorphism is reduced or nonexistent in species where several males copulate with only one estrus female (polyandry). In this case, the evolution for a large testis is attributed to the sperm competition in a system where different males attempt to fertilize the same female and where a higher copulatory frequency and higher levels of sperm production are required (Harcourt et al., 1981; Kenagy and Trombulak, 1986).