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space, which can complicate the reading of the layers. Additionally, the growth of the skull of
these dolphins could be more or less linear from two until 27 years but not from zero to two
years and after 27 years. However, this is not a grave problem, because the animals younger
than two years of age or very old are those easiest to determine in relationship of their ages.
The multiple regression equation with the absolute value distance also offered comparable
results with the linear multiple regression procedure.
Therefore, the four aforementioned skull measurements and the linear and distance
regression methods seem to be adequate and precise tools to determine the age of pink river
dolphins using their skulls. Nevertheless, some more sophisticated procedures could be
applied to obtain more accurate estimations especially in the youngest and oldest animals. For
example, Stevick (1999) applied the von Bertalanffy equation showing that is a good
descriptor of the growth in marine species and it has been applied to describe the relationships
between longitude and age in several mysticete species, such as hunchback whales
(Chittleborouh 1965,) and some odontoces (Kasuya, 1972; Martin, 1980; Bloch et al., 1993).
The equation
,
L
t
=L
∞
(1-
e
-
k
(
t
+
t
o)
),
(
where L
t
is the longitude of the body at certain
t
moment, L
∞
is the corporal longitude at the sexual maturity or the asymptotic longitude of the individuals
in one given population,
k
is a growth constant,
t
is the age at that time and
t
o
is a time
constant), could be adapted to the growth of several skull morphometric traits and through it,
the age could be obtained. However, in this case, it should be necessary a constantly monitor
the growth of the implied individuals.
In the current chapter, we showed the usefulness of several skull measures to determine
accurate age estimations in pink river dolphins.
A
CKNOWLEDGMENTS
The authors want to thank different peoples and institutions for their collaboration in
carrying out the present research. Thanks to Dr. Vera da Silva and Dr. Fernando Rosas for
sharing their research experiences on this topic to the first author during the tooth analysis in
the INPA (Manaus, Brazil). Thanks to Sandra Beltrán-Pedreros for her hospitality in Manaus.
Also, a thank-you goes to the von Humboldt Institute (at Villa de Leyva, Colombia) for
providing access to its
Inia
skull collection. Finally, thanks go to diverse Indian communities
which helped to obtain pink river dolphin material in Colombia (Ticunas and Cocamas in
diverse communities such as Puerto Nariño, Patrulleros or in the Tarapoto lake), in Perú
(Ticuna, Ocaina, Capanahua, Angoteros, Orejones, Cocamas and Alamas) and in Brazil
(Marubos, Kanaimari and Kulina in diverse points of the Yavarí river).
R
EFERENCES
[1]
Anli, G. & Zhou, K. (1992). Sexual dimorphism in the baiji,
Lipotes vexillifer
.
Canadian Journal of Zoology,
70, 484-493.
[2]
Archer, F. & Chivers, S. (2002).
Age Structure of the Northeastern Spotted Dolphin
Incidental Kill By Year, for 1971 to 1990 and 1996 to 2000. Administrative Report LJ-
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