Biology Reference
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franciscana males from the northern coast of Rio de Janeiro State reach sexual maturity at two
years, and a total length of 115 cm, and the females at three years and a total length of 130
cm. Franciscanas from the southern coast of São Paulo and coast of Paraná states are sexually
mature between 112 and 116 cm for males, and between 122 and 126 cm for females, with
age at sexual maturity for both between four and five years (Rosas & Monteiro-Filho, 2002).
These results suggest that the populations from Paraná and southern São Paulo, particularly
females, attain sexual maturity at smaller sizes, but at an older age, than those inhabiting areas
to the north and to the south of these two neighboring states. The small relative size of the
testes (testes/body weight ratio of 0.12%) suggests a single-male breeding system without
sperm competition (Danilewicz et al., 2004; Rosas & Monteiro-Filho, 2002). Most of the
females from Rio Grande do Sul State and Uruguay have a larger and heavier left ovary, a
weight difference correlated to a higher number of corpora (Harrison et al., 1981 Brownell,
1984; Danilewicz, 2003). However, no ovulation polarity is observed in franciscanas from
São Paulo and Paraná states, with both ovaries being functional (Rosas & Monteiro-Filho,
2002).
The gestation period does not appear to vary much according to geographic location, with
estimates of between 10.2 and 11.2 months (Kasuya & Brownell, 1979; Harrison et al., 1981;
Di Beneditto & Ramos, 2001; Rosas & Monteiro-Filho, 2002; Danilewicz, 2003).
Reproduction is markedly seasonal in the southern range, with births occurring ―in a pulse‖
from October to February (Brownell, 1984; Danilewicz, 2003). In the north, births ―flow‖
year round (Di Beneditto & Ramos, 2001). Therefore, they can be classified as birth-pulse
and birth-flow populations (definition of the terms, in either biological or mathematical
language, can be found in ecology text books -
e.g.
Caughley, 1977; Krebs, 1994; Caswell,
2001). Lactation periods last approximately. 8.4 months in northern Rio de Janeiro State (Di
Beneditto & Ramos, 2001), around 7.4 months in São Paulo and Paraná states (Rosas &
Monteiro-Filho, 2002), 6 to 7 months in northern Argentina (Rodríguez et al., 2002), and 8-9
months for individuals in Uruguay (Harrison et al., 1981; Kasuya & Brownell, 1979).
Weaning is gradual with early predation on shrimps and small fish (Pinedo et al., 1989;
Rodriguez et al., 2002). Litter size is limited to one in utero and at birth. The length and
weight at birth of
P. blainvillei
are 70 to 80 cm and about 5 to 6 kg in the southern range
(Danilewicz, 2003; Kasuya & Brownell, 1979; Harrison et al., 1981). In the northern range
the newborn calves are smaller (Ramos et al., 2000; Rosas & Monteiro-Filho, 2002). In
Argentina, weaned calves exceed 97 cm in length and weigh 13 to 17 kg (Rodríguez et al.,
2002). Females do not show any evidence of reproductive senescence (Kasuya & Brownell,
1979; Danilewicz, 2003). Taking the age of first reproduction, life span and calving interval
into account, it is suggested that a female franciscana might produce four to eight offspring in
her lifetime (Danilewicz, 2003), though investment in reproduction seems to vary
geographically.
Survival probably varies accordingly as there seem to be a competing relationship
between reproduction and survivorship. There is empirical evidence for some bird and
mammal species that lower reproductive effort is compensated by higher survival. For
example Stearns (1976) and Millar and Zammuto (1983) stated that there is positive
correlation between age at maturity and life expectancy. It is therefore expected that survival
rates are lower in areas where franciscanas attain sexual maturity at younger ages and have
shorter calving intervals. Unfortunately very little is known about survivorship for franciscana
due to the lack of unbiased age-at-death data to construct its life table and due to unsuitability
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