Drag, but not buoyancy, affects swim speed in captive Steller sea lions.

Ippei Suzuki,Andrew W Trites,Nobuyuki Miyazaki,Yasuhiko Naito,Katsufumi Sato,Andreas Fahlman

doi:10.1242/bio.20146130

Abstract

ABSTRACTSwimming at an optimal speed is critical for breath-hold divers seeking to maximize the time they can spend foraging underwater. Theoretical studies have predicted that the optimal swim speed for an animal while transiting to and from depth is independent of buoyancy, but is dependent on drag and metabolic rate. However, this prediction has never been experimentally tested. Our study assessed the effects of buoyancy and drag on the swim speed of three captive Steller sea lions (Eumetopias jubatus) that made 186 dives. Our study animals were trained to dive to feed at fixed depths (10–50 m) under artificially controlled buoyancy and drag conditions. Buoyancy and drag were manipulated using a pair of polyvinyl chloride (PVC) tubes attached to harnesses worn by the sea lions, and buoyancy conditions were designed to fall within the natural range of wild animals (∼12–26% subcutaneous fat). Drag conditions were changed with and without the PVC tubes, and swim speeds were recorded and compared during descent and ascent phases using an accelerometer attached to the harnesses. Generalized linear mixed-effect models with the animal as the random variable and five explanatory variables (body mass, buoyancy, dive depth, dive phase, and drag) showed that swim speed was best predicted by two variables, drag and dive phase (AIC = −139). Consistent with a previous theoretical prediction, the results of our study suggest that the optimal swim speed of Steller sea lions is a function of drag, and is independent of dive depth and buoyancy.

Highlights

Breath-hold divers have morphologically and physiologically evolved to live in aquatic environments
Animals Experiments were conducted in July and September 2007 using three female Steller sea lions Eumetopias jubatus housed in a specially designed floating pen located in the Indian Arm
Buoyancy and drag adjustment Both buoyancy and drag conditions were manipulated using a pair of polyvinyl chloride (PVC) tubes attached to a webbing body harness that was worn by each animal (Fig. 1)

Summary

Introduction

Breath-hold divers have morphologically and physiologically evolved to live in aquatic environments. In addition to reducing drag, breath-hold divers have morphological traits that allow efficient propulsive forces with paddle- or hydrofoil-like appendages (Berta et al, 2006). These morphological adaptations allow the animals to efficiently swim and dive, but do not address the limiting effects of a restricted oxygen supply on time spent underwater. The physiological adaptations for economical usage of oxygen are generally known as the dive response, which includes a decrease in heart rate and a restricted peripheral blood flow to conserve the available O2 for the heart and brain (Hill et al, 1987; Kooyman, 1989; Butler and Jones, 1997; Kooyman and Ponganis, 1998). Various marine vertebrates augment these physiological traits using a range of behavioral responses that are believed to extend time spent underwater (Kramer, 1988)

Methods

Results

Conclusion