Severe Bone Loss as Part of the Life History Strategy of Bowhead Whales

Abstract

The evolution of baleen constituted a major evolutionary change that made it possible for baleen whales to reach enormous body sizes while filter feeding on tiny organisms and migrating over tremendous distances. Bowhead whales (Balaena mysticetus) live in the Arctic where the annual cycle of increasing and decreasing ice cover affects their habitat, prey, and migration. During the nursing period, bowheads grow rapidly; but between weaning and approximately year 5, bowhead whales display sustained baleen and head growth while limiting growth in the rest of their bodies. During this period, they withdraw resources from the skeleton, in particular the ribs, which may lose 40% of bone mass. Such dramatic changes in bones of immature mammals are rare, although fossil cetaceans between 40 and 50 million years ago show an array of rib specializations that include bone loss and are usually interpreted as related to buoyancy control.

Beyond thermoregulation: metabolic function of cetacean blubber in migrating bowhead and beluga whales

“…Development of fully functional baleen plate takes years to complete, during which time young whales are unable to feed efficiently, subjecting them to unique life history constraints (Lowry 1993; Lambertsen et al 1989; Lambertsen et al 2005; Reeves and Leatherwood 1985; George, 2009; George et al, 2016). The young feed heavily on lipid-rich milk, and as a result, possess high total fat content (George et al 1999; George et al 2007; Rosa 2006 and COSEWIC 2005).…”

“…When born, the baleen of bowhead calves is less than 20cm in length (Schell and Saupe 1993; Schell et al 1989; Lowry 1993 and Lambertsen et al 1989; George, 2009). After weaning, bowhead youths experience a unique 4–6 year maturation period, not observed in other mysticetes (Nerini, et al, 1984; Koshi, et al, 1993; Schell et al, 1993; George et al 2016). During this time, individual growth is almost entirely limited to the head region and the baleen lengthens and frays increasing feeding efficiency (Schell and Saupe 1993; Schell et al 1989; Lowry 1993 and Lambertsen et al 1989; Lubetkin et al 2008; George, 2009; Fortune et al, 2012; George et al 2016).…”

“…After weaning, bowhead youths experience a unique 4–6 year maturation period, not observed in other mysticetes (Nerini, et al, 1984; Koshi, et al, 1993; Schell et al, 1993; George et al 2016). During this time, individual growth is almost entirely limited to the head region and the baleen lengthens and frays increasing feeding efficiency (Schell and Saupe 1993; Schell et al 1989; Lowry 1993 and Lambertsen et al 1989; Lubetkin et al 2008; George, 2009; Fortune et al, 2012; George et al 2016). As a result, immature youth whales are less efficient feeders and thought to rely primarily on lipid reserves for energetic demands (Lowry and Burns, 1980; Koski et al 1993; Moore and DeMaster 1998; George et al 1999; Dehn et al 2006 and Rosa 2006).…”

“…The evolution of blubber contributed to cetaceans’ ability to colonize a wide range of aquatic habitats, ranging from tropical to polar, and its anatomy is highly variable across species (Aguilar and Borrell 1990; Crandall et al 1997; Koopman 1998; Webb et al 1998; Koopman 2007; Montie et al 2008; Bagge et al 2012; McClelland et al 2012; Ball et al 2015). Exclusively Arctic species, such as bowhead and beluga whales, require and utilize extensive blubber reserves that cannot be substantially altered without negatively affecting homeostasis, locomotion, and buoyancy (Iverson 2002; George et al 2007; George et al 2016). Our work is the first to examine seasonal and developmental transcript patterns of lipolytic genes in cetaceans.…”

Whale tear glands in the bowhead and the beluga whales: Source and function

“…These observations are based on seven bowhead whales, Balaena mysticetus Linnaeus, 1758, captured 2016–2018. In the fall (September/October), the bowhead whales migrate from the Beaufort and Chukchi Sea to the Bering Sea as the ice closes off the Artic (George et al, ). A small number of migrating bowhead whales are harvested by native Alaskans (Inupiat) during this seasonal migration in Utqiagvik (Barrow, Alaska).…”