The fast and the furious: more prey and more energy for big, hungry rorquals

Lunge filter feeding is the unique foraging method used by rorqual whales. During this dynamic and intermittent feeding strategy, a rorqual accelerates towards a dense swarm of prey. At high speed, a huge volume of prey-laden water in engulfed, enabled by the rapid, parachute-like expansion of the ventral groove blubber (VBG) that comprises the throat. This ‘parachute’ full of water significantly increases the drag on the animal, slowing it down while simultaneously making it possible to engulf a massive volume of prey and water. The water is then filtered out through baleen and the retained prey is swallowed. Despite the drag costs, previous studies have demonstrated that when the targeted prey is in dense, large, patches, rorquals will gain energy from this energetically expensive maneuver.

Gough et al. (2022) (me, the blog author, included!) used data collected by CATS tags, deployed on four species of krill feeding rorquals, historical records, drones, and hydrodynamic modeling. Combining these data sources allowed the authors to produce a highly detailed exploration of rorqual feeding and energetics across scale, focusing on how variations in morphology and kinematics affect the balance between energetic cost and gain.

Figure 1, adapted from Gough et al. (2022): (A) Illustration of a foraging rorqual, swimming and lunging, active swimming and acceleration are highlighted in the orange box, while gliding and deceleration periods are highlighted in blue. (B) Kinematic data and camera views for paired blue whales lunge feeding on krill. The images correspond with specific times during the lunge and are represented in the kinematic data as dotted lines; the point of mouth opening at the beginning of the lunge (MO), the maximum gape during the lunge (MG), and the mouth closing at the end of the lunge (MC).

The authors aimed to expand upon three areas regarding this extreme foraging method:

Aim 1: Do rorquals ‘fluke-through’ or ‘coast’ after engulfment?
Gough et al. discovered that rorquals use both methods! Rorquals use small amounts of thrust to minimize the effects of post-engulfment drag and control the precise timing needed when lunging. By minimizing drag, rorquals can increase their energy gain from a lunge and can better prepare for the next lunge.
Aim 2: What can incorporating swimming speed tell us about rorqual kinematics?
The authors found that rorquals can reach speeds greater than krill escape speeds (a common prey item) and also speeds greater than the mechanical VBG inflation speeds. This means whales must balance reaching a minimum speed for prey capture and VGB inflation with ensuring that energy is conserved. Gough et al. found that whales average lunge speeds of about 4 meters per second! However, these speeds do not scale predictably with body size.
Aim 3: Can we use this new data to improve scale-dependent energetic models?
The authors concluded yes! The new estimates created in this study show that as body size increases, the energetic cost of a lunge increases slower than the energetic gain- a very good trade-off for large rorquals. Larger rorquals may spend more time looking for optimal prey patches, slightly lowering the energetic efficiency at long time scales (such as over the entire day), but their overall energetic balance remains higher than that of smaller rorquals.

Gough et al. (2022) combined data in a novel way to determine that larger whales capture more prey and more energy at a lower cost than smaller whales. This study shows that reexamining data with new questions and analyses can yield fresh insights into the fine-scale variations that dictate the long-term successes of a species.

Shirel Kahane-Rapport is a postdoctoral fellow at CSU Fullerton. Google Scholar

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