Difference in Age, Gender, Body Weight, Body Length and Sexual Maturity in Association with Variations in Thyroid Size and Morphology of Indo-Pacific Bottlenose Dolphin, Tursiops aduncus
Abstract
Assessment of thyroid gland morphology and function is one of the diagnostic challenges in cetacean clinical endocrinology.7,8 Many factors affect thyroid function and morphology, such as demographic parameters, physiological cycles and both thyroidal and non-thyroidal illness.1,2,3,4,6,7,8 It is important to recognize the potential influence of these factors when interpreting physical thyroid changes and serum thyroid hormone levels. To the best of our knowledge, the formal literature is devoid of any reference to the relationship between thyroid size/morphology and age, gender, body weight, body length or sexual maturity in bottlenose dolphins. Recognizing any changes on the thyroid by these demographic factors in bottlenose dolphins will help in the diagnosis of pathological states and monitoring of the thyroid gland during the course of treatment. Therefore, this study was undertaken to investigate the possible variation in thyroid size and morphology of captive Indo-Pacific bottlenose dolphins with different demographic factors using ultrasonography.
All ultrasound examinations were performed with a Philips HD-11 ultrasound unit in conjunction with a 2-6 MHz curvilinear 3-D transducer. Sixteen Tursiops aduncus, at Ocean Park, Hong Kong (6 males and 10 females) were included in the study. Of these, 11 subjects were sexually mature and five were immature. The mean age of the subjects was 15.1 years (range, 2-35 years); the mean body weight was 126.1 kg (range, 101-193.5 kg); and the mean body length was 215.8 cm (range, 191-241 cm). None of the subjects was receiving medication at the time of the study. A 3-D ultrasound examination of the thyroid gland was performed in each subject, and the thyroid volume was measured. Pearson Product Moment Correlation (SPSS for windows 14.0, SPSS Inc., Chicago, Illinois) was used to investigate any statistically significant association between thyroid volume and age, body weight or body length. Mann-Whitney U test (SPSS for windows 14.0, SPSS Inc., Chicago, Illinois) was used to investigate any significant differences in gender and sexual maturity.
Results showed that there was no significant association between the thyroid volume and age (P=0.69), body weight (P=0.10) or body length (P=0.10). There was no significant difference in the thyroid volume with different gender (P=0.64) or sexual maturity (P=0.91). However, the thyroid tended to be larger in females (n=10, mean 14.32 cm3 ± 5.19 cm3) than males (n=6, mean 12.20 cm3 ± 6.50 cm3). The borders of the thyroid were usually well-defined (81%), however ill-defined borders were observed in two mature subjects and one immature subject. The echo-pattern of the thyroid glands was generally homogeneous (81%) but the relative echogenicity mostly varied between animals from hypoechoic to isoechoic when compared with adjacent musculature.
Cowan & Tajima2 found that the ratio of thyroid weight/body length was most useful in reflecting the relationship between animal size and thyroid size, however, this study found no significant association between thyroid volume and animal size. Variability of follicle size and colloid density of the thyroid glands might account for the varied relative echogenicity in the animals.5
Results of the present study suggest that thyroid size does not correlate with body size. Demographic parameters such as age, gender, body size and sexual maturity may not be the significant determining factors on thyroid size alternation. A longitudinal study is on-going to investigate any difference in reproductive status and health in association with variations in thyroid size, morphology and function of same Indo-Pacific bottlenose dolphin populations.
Acknowledgements
The authors are grateful to Ocean Park's Marine Mammal Department for dolphin training, husbandry and guidance. We would also like to thank Ocean Park Corporation for the continuous support in this project. This project was funded by The Hong Kong Polytechnic University Research Studentship (G5556 RGGH).
References
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