Body density and diving gas volume of the northern bottlenose whale (Hyperoodon ampullatus)

Abstract

Diving lung volume and tissue density, reflecting lipid store volume, are important physiological parameters that have only been estimated for a few breath-hold diving species. We fitted 12 northern bottlenose whales with data loggers that recorded depth, 3-axis acceleration and speed either with a fly-wheel or from change of depth corrected by pitch angle. We fitted measured values of the change in speed during 5 s descent and ascent glides to a hydrodynamic model of drag and buoyancy forces using a Bayesian estimation framework. The resulting estimate of diving gas volume was 27.4±4.2 (95% credible interval, CI) ml kg−1, closely matching the measured lung capacity of the species. Dive-by-dive variation in gas volume did not correlate with dive depth or duration. Estimated body densities of individuals ranged from 1028.4 to 1033.9 kg m−3 at the sea surface, indicating overall negative tissue buoyancy of this species in seawater. Body density estimates were highly precise with ±95% CI ranging from 0.1 to 0.4 kg m−3, which would equate to a precision of <0.5% of lipid content based upon extrapolation from the elephant seal. Six whales tagged near Jan Mayen (Norway, 71°N) had lower body density and were closer to neutral buoyancy than six whales tagged in the Gully (Nova Scotia, Canada, 44°N), a difference that was consistent with the amount of gliding observed during ascent versus descent phases in these animals. Implementation of this approach using longer-duration tags could be used to track longitudinal changes in body density and lipid store body condition of free-ranging cetaceans.

Assessing cetacean body condition: Is total lipid content in blubber biopsies a useful monitoring tool?

“…Wax esters also have lower densities (specific gravities) than triacylglycerols (Sargent, 1978), and they are less sensitive to phase change with changing temperature (Nevenzel, 1970), both of which affect overall tissue buoyancy. As such, small changes in the blubber lipid content may considerably affect their overall buoyancy and diving capabilities, so they exist within a small range (Miller et al, 2016). So, stores of wax esters may provide various elements of physiological or mechanical advantage in terms of buoyancy and insulation that contribute to their extreme deep-diving abilities.…”

“…These species may therefore prioritize maintaining consistent blubber hydrodynamic, thermal, and buoyancy parameters, which comes at the expense of having variable fat deposits for energy storage. As such, small changes in the blubber lipid content may considerably affect their overall buoyancy and diving capabilities, so they exist within a small range (Miller et al, 2016).…”

High diving metabolic rate indicated by high-speed transit to depth in negatively buoyant long-finned pilot whales

“…During glides, acceleration along the swimming path was determined as the difference between drag (F D ) and buoyancy (F B ) parallel to the swimming path: ma=F B sin( p)−F D (Miller et al, 2004a). Therefore, acceleration is given as (Miller et al, 2016):…”

“…The value 101,325 converts pressure in atmospheres to pressure in Pascals, so that the units of body tissue compressibility are proportion ×10 −9 Pa −1 . The model consists of three terms that estimate external forces acting on gliding whales: the first term is drag, the second term is buoyancy derived from the density of the non-gas component of the whale body, and the third term is buoyancy derived from the residual air carried by the animal (see Miller et al, 2016, for more detailed explanation of the equation).…”

“…Deep-diving animals are predicted to benefit from transiting to and from depth at the optimal speed and having neutral buoyancy, because this strategy maximizes the amount of oxygen available for efficient aerobic metabolism at depth. Indeed, tissue body density of sperm whales and northern bottlenose whales that employ steady swimming during dives is close to neutral buoyancy (Miller et al, 2004a(Miller et al, , 2016. In contrast, short-finned pilot whales make relatively short dives and perform sprints reaching 4-9 m s −1 , possibly to chase prey, at the deepest portion of the dive (Aguilar Soto et al, 2008).…”