The giant, air-breathing, aquatic mammals known as whales have been objects of fascination, legend, and study for millennia. Tales of sea serpents and the kraken likely arose from observations of seas roiling with feeding whales. Yet until this century, scientific studies were largely limited to anatomical studies and surface observations. Behavioral observations in situ, at hundreds of meters deep where these large mammals typical access their food, have only been possible since the advent of bio-logging tags over the past two decades. As the behavior of some species, like humpback whales and even endangered blue whales began to come to light, others, like the evasive minke whale that tended to forage alone and were difficult to approach in boats, remained enigmatic.
At 8 m long and 5 tons body mass, minke whales are some of the largest animals on earth – but they are tiny in comparison to their larger cousins like the blue whale (26-30 m and 100 tons or more). Interestingly for questions of how size affects foraging efficiency, both animals share a common prey (krill) and a common feeding type (engulfment filtration feeding, or lunge feeding). Lunge feeding is an energetically dynamic event, involving acceleration to 3-4 m/s, engulfment of prey laden water followed by filtration of that water through racks of keratinized baleen. While this whole process can take a minute or more in a blue whale, in minke whales it appeared to be much shorter, allowing more rapid feeding on smaller patches. But how to quantify these differences if the whales could not be tagged?
In 2013, my colleague Ari Friedlaender deployed two acousonde tags on Antarctic minke whales foraging in a group near sea ice. The high resolution data from those tags first gave us a hint of the extremely high foraging rates these animals are capable of. In 2018 and 2019 scientists from UC Santa Cruz, Stanford University and Duke Marine Labs teamed up to attempt to gather additional data. It turns out that in the fjords of the West Antarctic Peninsula, minke whales were more approachable, particularly when they were in groups of 2-5 animals. Our team deployed and recovered 29 tags with high resolution data, and used Unoccupied Aerial Vehicles (drones) to measure their lengths.
The data our team collected enabled us to study how feeding rates, and the overall water engulfed per unit time, scaled with body length and allowed us to get at a question had been nagging me. Lots of effort has gone in to analyzing what it is about baleen whales that allows them to be the biggest animals of all time. Less effort has gone into why they have to be big. An 8 m long, 5 ton aquatic animal is still pretty impressive when it swims up next to your boat. Additionally, there do not seem to be physiological restrictions to the general cetacean body plan that would limit body size below minke whales (think about dolphins), and baleen whale ancestors were all smaller than today’s extant species.
The answer seems to be that there is a maximum feeding rate allowed for whales of any given size, and this rate is constrained by the time it takes to perform the lunge feeding maneuver. While some constraints, like filter time, can get smaller with smaller body sizes, some constraints, like the time to search between patches, are more environmentally dependent than size dependent. The overall effect of these limitations is to reduce the amount of water filtered per time, an aspect of our study that can be applied to other filter-feeding systems. So, even though our study focused on a specific type of filter-feeding, the findings likely also help explain why mobile filter-feeders, like basking sharks, whale sharks and manta rays, are the largest animals of their respective clades.
All of today’s large filter-feeders are thought to have evolved from smaller ancestors, yet today seem to be subject to a minimum size constraint like that observed for rorqual whales. The evolution of large-bodied, obligate filtration, then, paints a picture of what seas must have been like when specific feeding modes evolved. If today’s smallest lunge-feeders, minke whales, appear to maximize intake by feeding on shallow swarms at night, it suggests that their smaller bodied ancestors might have existed in oceans where such conditions were even more prevalent. In the absence of direct observations of the past, sometimes the survivors of prehistoric arms races can tell us something about where they came from – and that knowledge can be critical when trying to understand the risks that wildlife face in today’s rapidly changing oceans.
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