Diving behavior in a free-living, semi-aquatic herbivore, the Eurasian beaver Castor fiber.

Klaus Hacklӓnder,Lea Cohen Sanchez,Rory Paul Wilson,Frank Rosell,Patricia Maria Graf

doi:10.1002/ece3.3726

Abstract

Semi‐aquatic mammals have secondarily returned to the aquatic environment, although they spend a major part of their life operating in air. Moving both on land, as well as in, and under water is challenging because such species are considered to be imperfectly adapted to both environments. We deployed accelerometers combined with a depth sensor to study the diving behavior of 12 free‐living Eurasian beavers Castor fiber in southeast Norway between 2009 and 2011 to examine the extent to which beavers conformed with mass‐dependent dive capacities, expecting them to be poorer than wholly aquatic species. Dives were generally shallow (<1 m) and of short duration (<30 s), suggesting that the majority of dives were aerobic. Dive parameters such as maximum diving depth, dive duration, and bottom phase duration were related to the effort during different dive phases and the maximum depth reached. During the descent, mean vectorial dynamic body acceleration (VeDBA—a proxy for movement power) was highest near the surface, probably due to increased upthrust linked to fur‐ and lung‐associated air. Inconsistently though, mean VeDBA underwater was highest during the ascent when this air would be expected to help drive the animals back to the surface. Higher movement costs during ascents may arise from transporting materials up, the air bubbling out of the fur, and/or the animals’ exhaling during the bottom phase of the dive. In a manner similar to other homeotherms, beavers extended both dive and bottom phase durations with diving depth. Deeper dives tended to have a longer bottom phase, although its duration was shortened with increased VeDBA during the bottom phase. Water temperature did not affect diving behavior. Overall, the beavers’ dive profile (depth, duration) was similar to other semi‐aquatic freshwater divers. However, beavers dived for only 2.8% of their active time, presumably because they do not rely on diving for food acquisition.

Highlights

Mammals evolved on land (Kemp, 1980, 2005) and were under selection pressure to operate efficiently in an air-based medium
These extreme specializations are possible because fully, or almost fully, aquatic mammals need only to operate in water
We studied the diving behavior of a free-living, amphibious herbivore, the Eurasian beaver Castor fiber

Summary

| INTRODUCTION

Mammals evolved on land (Kemp, 1980, 2005) and were under selection pressure to operate efficiently in an air-based medium. The thick, air-filled fur or plumage of semi-aquatic species provides valuable insulation but makes these animals positively buoyant and, increases upthrust and energy use during submergence (Butler & Jones, 1997) Their drag-based propulsion with paddles is less efficient and uses more energy than the lift-based propulsion with hydrofoils employed by fully aquatic mammals (Fish, 1994). A common finding of such studies is that semi- aquatic species face increased energetic costs during diving, mainly due to higher thermoregulatory needs (MacArthur & Krause, 1989), in cold water (MacArthur, 1984), as well as increased physical effort needed to overcome buoyancy and drag (Fish, Smelstoys, Baudinette, & Reynolds, 2002; Williams, 1983). This work examines the diving behavior of beavers in light of these hypotheses and attempts to interpret them with respect to behavioral mechanisms for increasing diving efficiency before putting the mass-specific diving capacities of beavers into a broader diving endotherm perspective

| MATERIALS AND METHODS

| DISCUSSION

Findings

| CONCLUSION