Danilewicz et al.: Reproductive biology of male Pontoporia blamvillei from Rio Grande do Sul, southern Brazil 



589 



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 matu- 

 rity 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 popula- 

 tions 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 (Lagenorhyn- 

 chus obscurus) and the harbor porpoise (Phocoena pho- 

 coena), 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 fran- 

 ciscana (Kasuya and Brownell, 1979, Harrison et al., 

 1981, Danilewicz, 2003), it would be expected that the 

 males would accompany the female rhythm, decreas- 

 ing 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 predic- 

 tion 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 ma- 

 ture 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. 



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 spermatogen- 

 esis rather than by changes in the testes weight. Nev- 

 ertheless, the preliminary results about these charac- 

 teristics (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 com- 

 bination of results presented here indicates that testicu- 

 lar 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 dur- 

 ing 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-sea- 

 son reproductive females. 



Franciscana reproductive strategy 



Although important advances in the knowledge of fran- 

 ciscana behavior in the wild have been made (e.g., Bor- 

 dino 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 charac- 

 teristics may provide indirect clues regarding mating 

 strategy in franciscana and are discussed below. 



Relative testis weight In mammals, there is a func- 

 tional 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 present- 

 ing 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). 



Using the data on 133 mammal species, Kenagy and 

 Trombulak (1986) presented a function describing the 

 relationship between body weight and combined-testes 

 weight without epididymis. Applying their equation 

 for the adult male franciscanas, we discovered that 

 mature franciscanas have testes 3 to 12 times lighter 

 than expected (mean = 6 times) for a mammal of its 

 body weight. Indeed, among the 133 species analyzed, 



