Biology Reference
In-Depth Information
Sul State, Brazil, it is from 129.8 - 130.6 cm for males (
n
= 59;
n
= 96) and 146.3 - 152.6 cm
for females (
n
= 48;
n
= 69 - Barreto & Rosas, 2006; Botta et al., 2007). For specimens from
Uruguay, the asymptotic length is 133.3 cm for males (
n
= 137) and 153.0 cm for females (
n
= 123 - Kasuya & Brownell, 1979) and for franciscanas from Argentina it is 135.8 cm for
males (
n
=14) and 150.5 cm for females (
n
= 12 - Botta et al. 2007). Body weight also follows
the same geographic pattern (
e.g.
Rosas, 2000; Botta et al., 2006). It is worthwhile to note
that franciscana variation in size is not clinal as one would expect. Franciscanas are larger in
their southern range (Rio Grande do Sul State, Brazil; Uruguay and Argentina) followed by
animals from the northern range (Rio de Janeiro and Espírito Santo States, Brazil) and are
much smaller in the middle of its range (São Paulo and Paraná States, Brazil). Larger sizes in
the southern range might be explained as an evolved characteristic to cope with colder waters
as well as increased abundance of potential predators. Both sharks and killer whales are
predators of franciscanas (see below) and are more abundant in cold water (Heyning &
Dahlheim, 1988; Compagno, 1984). The larger size of franciscana in the north than in mid-
range latitudes could be due to some influence of the upwelling cold waters near Cabo Frio
(Rio de Janeiro State) northwards. The possibility, however, that franciscanas in the northern
range originate from larger individuals from the south (founder effect) should not be
overlooked.
Reversed sexual size dimorphism (RSSD) is observed in franciscana (Higa et al., 2002;
Kasuya & Brownell, 1979; Pinedo, 1995; Ramos et al., 2002 - see also references on growth
above). Females are larger than males in both the total body length as well as weight
(Brownell, 1989; Rosas, 2000; Botta et al., 2006). Larger females might represent an
evolutionary advantage for small species by allowing gestation and birth of larger, even
slightly, calves. Larger calves potentially have a higher likelihood of survival due to several
factors including a relatively lower metabolic rate and a lower surface/volume ratio which
reduces heat loss. Predation of larger calves might also be less likely. In fact, most of the
odontocetes presenting RSSD are the smallest species in the group. In general these species
produce relatively larger calves at birth than the other species within the taxonomic group
(
e.g.
Ralls, 1976).
R
EPRODUCTION AND
S
URVIVAL
Franciscana has one of the fastest reproductive cycles among all cetaceans (Danilewicz et
al., 2000). Sexual maturity of franciscanas from the coast of Uruguay is attained at about 131
cm in males and at 140 cm in females, with an estimated age at sexual maturity between two
and three years for both sexes (Kasuya & Brownell, 1979). The mean age at sexual maturity
of specimens from the coast of Rio Grande do Sul, southern Brazil, was estimated to be
around 3.5 years for both sexes (Danilewicz et al., 2000; 2004; Danilewicz, 2003). Mean
length and weight at sexual maturity in the same area was estimated at 138.9 cm and 32.8 kg
for females and 127.4 cm and 26.6 kg for males (Danilewicz et al., 2000; 2004; Danilewicz,
2003). In Rio Grande do Sul, the annual pregnancy rate was estimated to be 0.66, which is
equivalent to an average birth interval of 1.5 years. This suggests that within the population,
half of the females reproduce annually and the other half biannually (Danilewicz et al., 2000;
Danilewicz, 2003). Ramos et al., (2000) and Di Beneditto and Ramos (2001), estimated that
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