THE APPLICATION OF EQUATION MODELS TO DETERMINE THE AGE OF PINK RIVER DOLPHIN SKULLS - Biology, Evolution and Conservation of River Dolphins Within South America and Asia - page 85

Pinedo (1991, 1994) and Pinedo & Hohn (2000) showed that most individuals of Pontoporia
blainvillei caught in nets in Southern Brazil and Uruguay were between 0 and 4 years old by
using GLGs, with females of one year old being the most common. Chivers (2002) found that
the age range most affected by nets for Stenella longirostris orientalis in the eastern tropical
Pacific Ocean was between 12 and 17 years old without significant differences between males
and females. Archer & Chivers (2002) analyzed a sample of 1,160 dead spinner dolphins
obtained from 1973 to 2000, with the highest frequency between 0 and 5 years old. On the
contrary, Rosas (2002) did not find any age trend in the Sotalia guianensis studied in the
Paraná coast of southern Brazil. A unique study of Inia geoffrensis conducted by Da Silva
(1994) determined the ages of several animals and reported that the oldest individual found
was a male of 36 years.
Studies that determine the relationships between age and skull´s measurements have not
been numerous in Cetacean species. One example is Mead and Potter (1990) who studied the
relationship of age with the pre-maxilla -maxilla coalition (distal fusion) in Tursiops
truncatus . They concluded that this coalition begins to occur at 7 years of age, before they
reach sexual maturity. Perrin (1993) did not find any relationship between skull maturity and
the age in the common dolphin, Delphinus delphis . Stevick (1999) employed the von
Bertalanffy equation to determine a relationship between the age of the animal Megaptera
novaeangliae and its length. Kemper (1999) determined, that for a whale species ( Caperea
marginata ), several post-cranial measures were highly correlated with the age of the animals.
Finally, Nummela et al. (2001) determined the existence of a significant relationship between
two measures of the lateral-mandibular grosor and the age in Tursiops truncatus .
In this chapter, we present the first evidence of a significant relationship among age and
several craniometric traits in a river dolphin species.
M ATERIAL AND M ETHODS
The ages of 71 dead river dolphins were determined by means of GLGs using the
laboratory procedures described by Da Silva (1995) and Castellanos-Mora (2007). Some Inia
teeth and GLGs are shown in Figure 1.
Descriptions of the geographical origin of these exemplars are as follow: Twelve (12)
samples were from the Ucayali, Marañón, Napo and Amazon rivers at the Peruvian Amazon;
nine (9) samples were from the Yavarí river, which follows the frontier among Peru and
Brazil; twenty-nine (29) samples proceeding from the Colombian Amazon (mainly from
Leticia to the Loreto-Yacu River); three (3) samples from the Negro river at the central
Amazon in Brazil; two (2) samples from the Mamoré River in Bolivia and, finally, sixteen
(16) samples from diverse rivers of the Colombian Orinoquia (10 from the Orinoco river, 2
from Meta river, 2 from Tomo River, one from Bita River and other from the Inirida River).
Of these 71 exemplars, 45 had complete skulls (28 from the Colombian Amazon, one
from the Bolivian Amazon and 16 from the diverse rivers of the Colombian Orinoquia). Fifty
(50) variables were recorded from each skull; 42 were skull measurements and eight were
teeth counts (Figure 2 and Table 1).