590 



Fishery Bulletin 102(4) 



the relative testes weight of the franciscana is heavier 

 than that of gorilla {Gorilla gorilla), humpback whales 

 (Megaptera novaeangliae), and fin whales iBalaenoptera 

 physalus), indicating that sperm competition does not 

 occur in franciscanas. 



Sexual size dimorphism Males are larger than females 

 in most mammal species. Nevertheless, the reversed 

 sexual size dimorphism (RSSD) (i.e., females are larger 

 than males) is more common than previously thought 

 and has been documented for 12 out of the 20 orders of 

 mammals (Ralls, 1976, 1977). Among the odontocetes, 

 four (Ziphidae, Pontoporiidae, Phocoenidae, and Delph- 

 inidae) out of the eight families present RSSD. 



Although sexual selection may be the main reason 

 why males are the larger sex in most mammal species, 

 it has been systematically refused as an explanation in 

 the cases where females are the larger sex (Ralls, 1976, 

 1977; Andersson, 1994). In species with RSSD, females 

 do not mate with many males, they are not dominant, 

 and are not more aggressive than males of the same 

 species. Moreover, they do not show secondary sexual 

 characteristics associated with intrasexual selection 

 (e.g., horns in Artiodactyla and large canine teeth in 

 Primates). Therefore, the occurrence of RSSD in mam- 

 mals may be explained more satisfactorily by natural 

 selection (Andersson, 1994). 



Slooten (1991) proposed an interesting hypothesis 

 for the occurrence of RSSD in cetaceans, suggesting 

 that a minimum size may be necessary for a newborn 

 cetacean to survive. In odontocetes, the smallest mean 

 sizes at birth are about 70-80 cm. Because the size of 

 the newborn is directly related to the size of the moth- 

 er, in species of small dimensions the females would 

 suffer a selective pressure to be a larger size, so that 

 they could produce offspring with the minimum viable 

 size. This hypothesis is reinforced by the fact that most 

 of the odontocete species with RSSD (e.g., Pontoporia 

 blainvillei, Cephalorhynchus hectori, Cephalorhynchus 

 commerssoni, Phocoena phocoena, Phocoena sinus) are 

 the smallest species within the group. Moreover, spe- 

 cies presenting RSSD also have larger relative size at 

 birth than the other species within the taxonomic group 

 (Ralls, 1976). 



The degree and direction of SSD (sexual size dimor- 

 phism) in mammals is the result of the difference of 

 the sum of all selective pressures affecting the female's 

 size and the sum of all selective pressures affecting the 

 male's size (Ralls, 1976). Thus, it is very probable that 

 more than one factor may act selectively on animals 

 of both sexes in Pontoporia, molding the degree and 

 direction of SSD. We propose that the requirement of 

 a neonate minimum viable size (70-80 cm in length) 

 is one of the main selective pressures acting on female 

 franciscanas. It is important to emphasize that other 

 factors may also be influencing SSD in franciscana, 

 and in some species it was evident that different selec- 

 tive pressures could affect body size in opposing direc- 

 tions in males and females and in different age classes 

 (Grant, 1986; Andersson, 1994). Among the factors that 



may be simultaneously acting on franciscana body size 

 are intrinsic genetic and physiological limitations, and 

 the requirement of maintaining an optimum size for the 

 species' ecological niche. 



Secondary sexual characteristics The presence and 

 intensity of secondary sexual characteristics in males 

 is a more precise indication of the degree of intrasexual 

 selection than is body size (Andersson, 1994). In odonto- 

 cete males, these characteristics are present in the form 

 of "weapons," such as the tusk of the narwhal (Monodon 

 monoceros) and the teeth in species of the genus Gram- 

 pus, Physeter, Berardius, Hyperoodon, and Mesoplodon 

 used in male-male combats (MacLeod. 1998). In spe- 

 cies of these genus, the teeth were reduced in number, 

 enlarged in size, and their form was modified (specially 

 in males of Ziphiidae). The teeth of these species also 

 lost their function in feeding because of a diet comprising 

 almost exclusively cephalopods and were used uniquely 

 in intrasexual combats. There is no evidence that the 

 same evolutionary process occurred in male francisca- 

 nas because their teeth are very small and numerous 

 (around 200), their diet is primarily fish, and the number 

 of combat scars is apparently low. These characteristics 

 support the hypothesis that male-male combat must be 

 very rare or even nonexistent in franciscanas. 



The sexual features presented in this study (extremely 

 low testis weight, reversed sexual size dimorphism, ab- 

 sence of secondary sexual characteristics in males, and 

 a low number of scars in males) indicate the absence of 

 sperm competition in the franciscana, and these features 

 differ drastically from those characteristics of odonto- 

 cete species where males combat each other for copula- 

 tion. This finding may indicate that franciscanas form 

 temporary reproductive pairs during the reproductive 

 period, where a male pairs and copulate with only one 

 female. Recently, Valsecchi and Zanelatto (2003) pro- 

 vided molecular evidence suggesting that franciscanas 

 may travel in kin groups that include mothers with their 

 calves and the father of the youngest offspring. The au- 

 thors also suggested that male franciscanas may prolong 

 their bond with their reproductive partner, providing 

 some form of paternal care. For a better understanding 

 of franciscana social structure and mating system, the 

 following suggestions are proposed: 1) an increase in the 

 efforts of behavioral studies of free-ranging francisca- 

 nas; 2) quantification of the intraspecific teeth scars in 

 franciscanas of different sexes and reproductive status 

 in order to confirm the absence of intrassexual aggres- 

 sions among males; 3) investigation of the relationship 

 of relative testis weight, SSD. and reproductive strate- 

 gies in cetaceans, by phylogenetic methods (see Harvey 

 and Pagel. 1991) to understand the evolution of these 

 characters in this group. 



Acknowledgments 



This study could not be made without the cooperation 

 and friendship of the fishermen from Tramandai/Imbe 



