Rosas et al.: Age and growth of Sotalia gutanensis 



381 



of the GLG of the first year, frequently 

 appear in a very conspicuous manner, 

 especially in the tip of the tooth, and can 

 be easily confused with annual layers. 

 The assumption that accessory layers 

 are annual could result in a duplication 

 of the real age of young animals up to two 

 years old, with significant consequences 

 in the interpretations of populational bio- 

 logical parameters ( Hohn, 1990 ). The ideal 

 situation would be that a GLG deposition 

 model already existed for the species being 

 studied, thereby avoiding counting acces- 

 sory layers as being annual (Hohn et al., 

 1989). In most odontocete species, includ- 

 ing S. guianensis, accessory layers do not 

 continue up to the end of the root of the 

 tooth, in contrast to annual layers, which 

 can be seen from the tip to the base of the 

 root of the tooth. However, to identify ac- 

 cessory layers it is necessary that the sec- 

 tions selected for age determination are 

 central, or close to the center of the pulp 

 cavity (Pinedo and Hohn, 2000). Off-center 

 sections can be used for age estimation, 

 but reading errors increase markedly and 

 consequently induce unreliable age esti- 

 mates (Pinedo and Hohn, 2000). 



The reasons for GLG deposition in teeth 

 are unknown (Hohn et al., 1989). Howev- 

 er, several reasons have been suggested, 

 including seasonal variations in growth 

 rate, genetic physiological cycles, dietary 

 changes, hormonal influences, and intrin- 

 sic factors on the metabolism in general 

 (Boyde, 1980; Klevezal, 1980; Scheflfer and 

 Myrick, 1980). Although all these factors 

 could be influential, variations in the diet 

 certainly play a significant role. According 

 to Klevezal ( 1996), a descriptive record of 

 the dietary changes of an animal during the year should 

 initially be looked for in structures that have a large degree 

 of sensitivity, such as teeth. It is known that dentine reacts 

 to the introduction of fluoride, calciferol and a series of other 

 components in the organism, forming layers with different 

 degrees of mineralization (Klevezal, 1996), which is known 

 as a calcium-traumatic reaction of dentine. Therefore, it is 

 possible to find a record of dietary changes in the dentine, 

 starting from weaning (Klevezal, 1996). 



We believe that the accessory layer in the dentine found 

 at approximately 419.6 pm from the neonatal line, could be 

 a record of the end of weaning in the estuarine dolphin. It 

 was observed in all the teeth of individuals older than 6.7 

 months and could be a hypomineralized layer caused by a 

 reduction of calcium in the body due to the absence of milk 

 in the diet (Klevezal, 1996). The other accessory layer found 

 closer to the neonatal line (mean of 248.9 pm) was not 

 observed in all animals and the interpretation of this layer 

 remains uncertain. It may be related to the beginning of 

 weaning, as has been suggested for the bottlenose dolphin 



Table 3 



Von BertalanfTy growth model parameters for male 

 Sotalia guianensis on the Parana coast, southern Brazil. 

 "TL,"= asymptotic length (cm), "*"= growth constant and 

 '7,j"= theoretical age at which the length of the animal is 

 zero. "SQ" represents the residual sum of squares, and 

 "Expl. var" represents the variance of the data explained 

 by the model. 



Parameters 



Up to 5 years More than 5 years 



k 



SQ 



Expl.var (%) 

 n 



159.64 

 2.02 



-0.38 

 510.99 



94.20 



15 



186.41 



0.53 



-1.40 



1013.98 



50.90 



19 



