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0.5449; r
2
= 0.2969), V29 (fifth order: r = 0.5757; r
2
= 0.3314), V30 (fifth order: r = 0.6013;
r
2
= 0.3616), V35 (fifth order: r = 0.5542; r
2
= 0.3071) and V42 (fifth order: r = 0.5545; r
2
=
0.3074).
Furthermore, we obtained a multiple regression that included the four variables which
presented the highest correlations with the linear regressions (V15, V21, V30 and V42; Y = -
61.2597 + 2.251799 (V15) + 2.336477 (V21) + 4.37399 (V30) - 2.52504 (V42), r = 0.8059 ,
r
2
= 0.6493), the four variables with the highest correlations with the polynomial regressions
(V15, V21, V23 and V30; Y = -61.9367 + 1.783891 (V15) + 2.176527 (V21) + 3.460791
(V23) + 1.24999 (V30), r = 0.8080 , r
2
= 0.6529) and the four variables with the highest
correlations independent of the 24 regression models employed (V15, V1, V30 and V11; Y =
-64.3776 + 4.3223 (V15) - 0.2514 (V1) + 6.350545 (V30) - 1.75691 (V11), r = 0.8088 , r
2
=
0.6541). Therefore, for these multiple regression equations the age of a dead pink river
dolphin is determined by only four cranial measurements which explained 64-65% of age
variation.
D
ISCUSSION
Two morphometric traits seem to be extremely important in determining the skull age of
a dead animal. These variables are the maximum width between zygomatic processes of the
squamosal bones (V15) and the maximum width of the internal nares (V21). The maximum
width between the zygomatic squamosal processes offered the highest correlation values
practically in all the analyses undertaken. In relation to other dolphins,
Inia geoffrensis
is
characterized to have a zygomatic process quite conspicuous, and that it increases its size
notably with increase of years. Keeping in mind that the masseter muscle (in charge of the
mastication) has part of its insertion on the zygomatic process. Its conspicuous size can be
related to a particular diet and heterodonty (cusped posterior teeth) of this species, which
allows for crushing of even armored catfishes (for instance, Loricariidae such as
Hypostomus
plecostomus,
Pterygoplichthys multiradiatus
or Doradidae as
Oxydoras niger
), and to have
the capacity to manage a diverse diet. Da Silva (1983) identified 43 fish species in 22
Inia
stomachs including those from benthic, littoral and pelagic environments. Several analyses
indicated that two other morphometric traits; the internal longitude of the skull and the left
condyle width; were also important in the determination of age. Therefore, of all the variables
tested, these four were the most useful in determining the ages of pink river dolphin skulls. It
is interesting to note that no dental variables showed any relationship with age.
The obtained regression equations estimated the age with accurate precision. The biggest
errors between the observed and estimated ages were in the
individuals of less than one year
old as well as one year olds. This is probably due to the hindrance of counting dental layers in
calves and young individuals which have a thin cement width, and which have incompletely
differentiated layers. At about two years of age the layers in the cement begin to differ from
each other more and are more clearly defined and then stay this way more or less through age
27. In this age range (ages 2-27) we found the best fit between the observed and estimated
ages. Post 27 years, we observed some differences among the observed and expected ages
with the linear multiple regression equation. According to our experience in the laboratory,
this can be due to that teeth of mature individuals present numerous lines in a very limited
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