Allometric scaling of morphology and engulfment capacity in rorqual whales

Abstract

Body length is one of the most important factors that influence organismal function and ecological niche. Although larger animals tend to have a suite of physiological advantages, such as lower mass-specific metabolic rates and lower costs of transport, they may also experience significant limitations to unsteady locomotor performance or maneuverability because of the relative scaling of control surface areas and body mass. Rorqual whales are the largest of all animals and thus represent a unique study system for understanding how animals function at the extreme of body mass. Rorquals are characterized by an engulfment-filtration foraging strategy facilitated by a complex set of morphological adaptations. We studied the scaling of key morphological structures related to locomotion and feeding in six rorqual species in a comparative framework. Our analyses show that most rorqual species exhibit positive allometry of both the control surfaces and body length, but the large scaling differences between these parameters suggest that larger rorquals will predictably suffer from decreased maneuverability and unsteady locomotor performance. However, we found that the dimensions of the engulfment apparatus also exhibit positive allometry, and thus engulfment capacity was relatively greater in larger rorquals. We posit that the allometric growth in the engulfment apparatus may be an adaptation that ameliorates the detrimental effects of large size on maneuverability. Our analyses also reveal significant differences in the scaling of mass-specific engulfment capacity among rorqual species that may reflect the evolution of unique foraging behaviors and the exploitation of divergent ecological niches.

Predator‐scale spatial analysis of intra‐patch prey distribution reveals the energetic drivers of rorqual whale super‐group formation

“…To examine the distribution of krill within dive‐sized cells (Figure 3c and 4h), we used Echoview to calculate S v within analytical cells the size of an average whale engulfment volume ( S v _gulp , symbol definitions in Table 3) as calculated from the morphology of an intermediately sized representative of each species of interest (blue whale total length = 22.5 m, humpback whale = 10.5 m). Jaw length was used for the vertical size of the cell (blue whale = 4.3 m, humpback whale = 2.3 m), and the ventral groove blubber length (blue whale = 12.8 m, humpback whale = 6.0 m) was used for the horizontal cell size (lengths calculated from ordinary least squares regression relationships in Kahane‐Rapport & Goldbogen, 2018). At the observed prey patch depths, all return echoes had y ‐axis values larger than the head width, so the extracted cells represented a 2D projection of the gulp size.…”

“…Baleen whales are the largest predators of all time, and rorqual whales (in the clade Balaenopteroidea) including blue whales Balaenoptera musculus and humpback whales Megaptera novaeangliae can engulf volumes of water (means ~130 and 15 m 3 respectively) that approach or exceed their own body masses (Goldbogen et al., 2012; Kahane‐Rapport & Goldbogen, 2018). Most typically, lunge‐filter‐feeding whales forage singly or in small groups (≤3 animals), and large groups of up to 10–20 animals, often fish‐feeding humpback whales, have also been reported in some ecosystems (Jurasz & Jurasz, 1979; Kirchner et al., 2018; Whitehead, 1983).…”

Prenatal developmental sequence of the skull of minke whales and its implications for the evolution of mysticetes and the teeth‐to‐baleen transition

“…Their filter‐feeding strategy known as lunge‐feeding involves engulfment of a large amount of water and prey, and the employment of baleen to filter out the water before swallowing prey (Goldbogen et al. ; Kahane‐Rapport & Goldbogen, ). This feeding style required these mammals to evolve a characteristic skull shape, with a broad and flat rostrum and an elongated mandible (Bouetel, ; Pyenson et al.…”

“…This feeding style required these mammals to evolve a characteristic skull shape, with a broad and flat rostrum and an elongated mandible (Bouetel, ; Pyenson et al. ; Kahane‐Rapport & Goldbogen, ).…”

The range of atlanto‐occipital joint motion in cetaceans reflects their feeding behavior

“…This motion subjects the atlanto‐occipital joint to a large resistance force from the water. The joint inflexibility in C filt taxa may be an adaptation to resistance from inpouring water into the oral cavity during ram‐filtering (Kahane‐Rapport & Goldbogen, ). However, unlike other members of the mysticetes, Eschrichtius is known to select particular prey on the sea floor (Ray & Schevill, ).…”