Aspects of the biology of the amazonian dolphins Inia and Sotalia fluviatilis

Abstract

This thesis presents a combination of analysis, re-analysis and review of data related to the two contrasting dolphins that occur in the Amazon river of Equatorial South America. The first, a Platanistoid dolphin restricted to freshwater habitat, is generally known by the binomial Inia geoffrensis and the common names Amazon river dolphin or boto. The second, Sotalia fluviatilis or tucuxi, is the smallest representative of the essentially marine family Delphinidae. Chapter 1 is an introduction to the study and discusses the current and historical taxonomic status of these animals. Chapters 2-6 are devoted to aspects of the biology and morphological evolution of the genus Inia. Chapter 2 discusses the geographical distribution of Inia in relation to the geological history of the region where it occurs. Currently, this dolphin exists almost everwhere it can physically reach, although one population is geographically isolated and those of the Amazon and Orinoco rivers are linked by only one, hazardous waterway. Chapter 3 looks at the external morphology of these dolphins, establishing a slight but significant degree of sexual dimorphism and a change in body colour with age and water type. Chapter 4 reports on the first successful attempt to determine the age of individual Inia using dental laminae, applying a range of laboratory techniques of which the staining of thin, decalcified sections proved most effective. Chapter 5 updates our knowledge of reproduction in this dolphin, using a combination of recently-acquired biological samples and a review of all published information. Chapter 6 is a statistical evaluation of skull morphology across the distributional range of the genus. It demonstrates long-established and clear genetical differences between the reproductively isolated Bolivian population and all others, justifying its recognition as a separate species: 1. boliviensis. Chapter 7 is a review of all existing information on Sotalia, including new data analyzed during the course of this study. Chapter 8, the Conclusion, attempts to put the major results of this study into context, and to consider the way forward, both in terms of what gaps in our knowledge of these animals remain to be filled, and how newly developing research tools might be employed to assess and monitor the effect of increasing anthropogenic activity in this rapidly developing region.