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

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 odontocete 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 Grampus, Physeter, Berardius, Hyperoodon, and Mesoplodon used in male-male combats (MacLeod, 1998). In species 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 franciscanas 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, absence 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 odontocete species where males combat each other for copulation. 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) provided molecular evidence suggesting that franciscanas may travel in kin groups that include mothers with their calves and the father of the youngest offspring. The authors 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 franciscahas; 2) quantification of the intraspecific teeth scars in franciscanas of different sexes and reproductive status in order to confirm the absence of intrassexual aggressions among males; 3) investigation of the relationship of relative testis weight, SSD, and reproductive strategies in cetaceans, by phylogenetic methods (see Harvey and Pagel, 1991) to understand the evolution of these characters in this group.

Table 1
Summarized information on age, length, mass, and testicular
characteristics for male franciscanas (Pontoporia blainuillei)
in the Rio Grande do Sul at different sexual maturity stages.

Characteristics and        n      Mean      Standard       Range
maturity state                              deviation

Age (years)
  Immature                 31        1.29     1.01          0-5
  Pubertal                  4        2.0      0.82          1-3
  Mature                   12        3.8      1.14          2-6
Standard length (cm)
  Immature                 62      111.2     13.62      70.0-137.5
  Pubertal                  7      118.5      9.75      107.8-132.5
  Mature                   37      133.7      7.71      120.5-155.0
Total mass (kg)
  Immature                 53       19.0      5.6        4.95-29.7
  Pubertal                  6       21.4      4.62       17.1-28.0
  Mature                   30       29.9      5.22      20.25-41.5
Mean diameter of
seminiferous tubules
([micro]m)
  Immature                 54       69.6     12.2       50.0-105.0
  Pubertal                  6       95.0     19.2       74.5-121.2
  Mature                   33      154.1     21.7       113.0-197.0
Combined testes mass (g)
  Immature                 63        1.59     0.84       0.33-4.78
  Pubertal                  7        2.73     1.28       1.30-4.8
  Mature                   37       10.24     3.94      4.27-20.08
Testes length (mm)
  Immature                 62       27.2      4.9        15.7-35.5
  Pubertal                  7       32.6      6.2        25.0-41.0
  Mature                   35       45.4      5.6        31.6-59.7
Index of testicular
maturity
  Immature                 61        0.03     0.01       0.01-0.06
  Pubertal                  7        0.04     0.01       0.02-0.06
  Mature                   36        0.11     0.03       0.05-0.18

Table 2
Comparison between average age, weight, and length at
sexual maturity between male and female franciscanas
from Rio Grande do Sul and Uruguay. The means of the
animals from Rio Grande do Sul were estimated by using
the DeMaster method (modified) and those from Uruguay
were estimated by using a linear regression to determine
the moment when 50% of the animals are mature.

               Rio Grande do Sul        Uruguay (1)

              Males   Females (2)   Males       Females

Age             3.6      3.7          2-4         2.7
Weight (kg)    26.6     32.6       25.0-29.0   33.0-34.0
Length (cm)   127.4    135.9         131.4       140.3

(1) Data from Uruguay were compiled from Kasuya and Brownell
(1979).

(2) Data from Damlewicz (2003).