Abstract
There are different indirect methods for age determination in mammalian species; few of them have been evaluated in marine mammals. Hard tissues (especially teeth with growth layer groups in dentine and cementum) and ear plugs of baleen whales are commonly studied for age determination in cetaceans. Also, more modern laboratory techniques are right now under research in order to increase accuracy of the estimations and decrease invasiveness of the standard methods. New techniques still pending for verification include the examination of aspartic acid racemisation in teeth and eye lens, the detection of stable isotopes in tissues, the examination of blubber fatty acid profiles and genetic age determination.
Determining the age of an animal with an unknown/unreliable origin is always relevant. In cetaceans, this is particularly tricky as aging signs are less evident compared with other mammal groups. This project makes use of radiographs to estimate age through bone maturation. This technique could pose many advantages in the daily work with this species compared with other techniques, as radiographs are non-invasive, fast, readily available for most veterinarians and relatively easy to interpret. Additionally, working with digital images instead of tissues samples is much simpler regarding handling and movement or transport restrictions.
Age determination through bone radiological morphology has proven to be very useful in other mammalian species, including humans. According to the present study this is also valid in most cases for the bottlenose dolphins. The aim of the present study was to create a bank of radiographic images from animals of known age and evaluate consistent changes throughout the life of the individuals correlating them with chronological age. This information will allow clinicians and researchers to classify dolphins in different groups of age.
For this purpose, images from pectoral fins, mandible and peduncle were collected from multiple animals. Preliminary results point out that although age changes are evident in all three areas, pectoral fins seem to be more reliable and precise in order to estimate age. Contrary to other mammals, dolphins seem to experience relevant changes in the carpal region even after puberty. The most informative areas to evaluate morphologically were the metaphyseal regions (epiphyseal plates) of the radius and ulna. According to observations, long bones complete epiphyseal closure correlates well with average sexual maturity in males and females. The presence, absence or fusion of certain bones seems to be genetically determined and not related with the age of the animals. This also could offer relevant info about paternities. Additionally, one animal with an abnormal physical growth also showed an atypical pattern of ossification revealing that, as in human medicine, this technique could also be used as a diagnostic tool for abnormal growth development.
More cases are needed to satisfy age class distinctions for the complete development of a dolphin specific data base. Future research goals include correlation of radiographic aging estimation with other validated methods for age determination in cetaceans.
Acknowledgements
The authors would like to thank all the institutions and veterinarians that participated submitting images for this study (Zoomarine Portugal, Mundomar, Zoomarine Italy, Nuremberg Zoo, Zoo-Aquarium Madrid, Dolphinarium of Brugge, SeaWorld South Africa and Ocean Park Hong Kong) with special thanks to Dr. Geraldine Lacave, and all the staff involved in the project from the Oceanográfic, SeaWorld San Diego, and the U.S. Navy Marine Mammal Program for their great effort and dedication.