Thyroid hormone concentrations associated with age, sex, reproductive status and apparent reproductive failure in the Amazon river dolphin (Inia geoffrensis).

T R Robeck,R S Amaral,V M F Da Silva,A R Martin,G A Montano,J L Brown,Steven Cooke

doi:10.1093/conphys/coz041

Abstract

This study was conducted to characterize immunoreactive thyroid hormone concentrations in wild Amazon river dolphins, also called boto (Inia geoffrensis) by age group, sex, pregnancy and lactation status, and to determine if thyroid hormone concentration differences could be detected between pregnant females with and without successful parturition outcomes. Radioimmunoassays were used to analyse total T 3 and total T 4 in 182 serum samples collected from 172 botos living in the Mamirauá Sustainable Development Reserve, in the Brazilian Amazon from 2003 through 2015. Age significantly affected tT 3 and tT 4 concentrations in males, with values in immature males and females being significantly lower than those in adult males, whereas no age effects were noted between immature females and adult non-pregnant, non-lactating females. Significant sex differences were noted in tT 3 concentrations between immature males and females and in tT 4 concentrations between adult males and females. These resulted in significant differences in the tT 3:tT 4 ratio between males and females within the immature and adult groups. Lactating and non-pregnant adult females had significantly higher tT 3 concentrations than pregnant females, and this difference was primarily driven by a 12% drop in tT 3 concentrations during the last two-thirds of pregnancy. No differences in thyroid hormone concentrations were detected between females diagnosed as pregnant and later found to have or not have a live calf. These results are the first to define thyroid hormone reference intervals and normal physiological variations in a wild population of river dolphins.

Highlights

Global freshwater habitats are often adjacent to or within dense and expanding human populations and are increasingly being degraded due to anthropogenic activity
This study is the first report of thyroid hormone concentration reference ranges in relation to sex, age and reproductive status for any species of river dolphin and provides novel information regarding river dolphin physiology, species that are some of the least understood and most endangered of the cetaceans (He et al, 2017)
It has been postulated that relatively high concentrations of T4 in marine cetaceans compared with nonhuman terrestrial mammals may be an adaptation to the relatively cold climate associated with an aquatic environment (Fair et al, 2011; Flower et al, 2015)

Summary

Introduction

Global freshwater habitats are often adjacent to or within dense and expanding human populations and are increasingly being degraded due to anthropogenic activity. As apex predators, these species often directly compete with man for food, and many of the environmental factors that may be detrimental towards their overall health and well-being could affect human populations. These species, and cetaceans in general, may represent one of the best environmental sentinels for human populations living nearby (Bossart, 2006; Chen et al, 2017; Lailson-Brito et al, 2008)

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