The enormous, air-breathing, aquatic mammals often called whales have been objects of fascination, legend, and research for millennia. Tales of sea serpents and the kraken doubtless arose from observations of seas roiling with feeding whales. But till this century, scientific research have been largely restricted to anatomical research and floor observations. Behavioral observations in situ, at a whole bunch of meters deep the place these giant mammals typical entry their meals, have solely been doable because the creation of bio-logging tags over the previous 20 years. Because the conduct of some species, like humpback whales and even endangered blue whales started to come back to gentle, others, just like the evasive minke whale that tended to forage alone and have been tough to method in boats, remained enigmatic.
At 8 m lengthy and 5 tons physique mass, minke whales are among the largest animals on earth – however they’re tiny compared to their bigger cousins just like the blue whale (26-30 m and 100 tons or extra). Apparently for questions of how dimension impacts foraging effectivity, each animals share a typical prey (krill) and a typical feeding kind (engulfment filtration feeding, or lunge feeding). Lunge feeding is an energetically dynamic occasion, involving acceleration to 3-4 m/s, engulfment of prey laden water adopted by filtration of that water via racks of keratinized baleen. Whereas this entire course of can take a minute or extra in a blue whale, in minke whales it seemed to be a lot shorter, permitting extra speedy feeding on smaller patches. However the right way to quantify these variations if the whales couldn’t be tagged?
In 2013, my colleague Ari Friedlaender deployed two acousonde tags on Antarctic minke whales foraging in a gaggle close to sea ice. The excessive decision information from these tags first gave us a touch of the extraordinarily excessive foraging charges these animals are able to. In 2018 and 2019 scientists from UC Santa Cruz, Stanford College and Duke Marine Labs teamed as much as try to collect further information. It seems that within the fjords of the West Antarctic Peninsula, minke whales have been extra approachable, notably once they have been in teams of 2-5 animals. Our crew deployed and recovered 29 tags with excessive decision information, and used Unoccupied Aerial Automobiles (drones) to measure their lengths.
The info our crew collected enabled us to review how feeding charges, and the general water engulfed per unit time, scaled with physique size and allowed us to get at a query had been nagging me. Plenty of effort has gone in to analyzing what it’s about baleen whales that enables them to be the largest animals of all time. Much less effort has gone into why they have to be massive. An 8 m lengthy, 5 ton aquatic animal remains to be fairly spectacular when it swims up subsequent to your boat. Moreover, there don’t appear to be physiological restrictions to the overall cetacean physique plan that may restrict physique dimension under minke whales (take into consideration dolphins), and baleen whale ancestors have been all smaller than as we speak’s extant species.
The reply appears to be that there’s a most feeding price allowed for whales of any given dimension, and this price is constrained by the point it takes to carry out the lunge feeding maneuver. Whereas some constraints, like filter time, can get smaller with smaller physique sizes, some constraints, just like the time to look between patches, are extra environmentally dependent than dimension dependent. The general impact of those limitations is to cut back the quantity of water filtered per time, a facet of our research that may be utilized to different filter-feeding methods. So, despite the fact that our research centered on a selected kind of filter-feeding, the findings doubtless additionally assist clarify why cellular filter-feeders, like basking sharks, whale sharks and manta rays, are the biggest animals of their respective clades.
All of as we speak’s giant filter-feeders are thought to have developed from smaller ancestors, but as we speak appear to be topic to a minimal dimension constraint like that noticed for rorqual whales. The evolution of large-bodied, obligate filtration, then, paints an image of what seas will need to have been like when particular feeding modes developed. If as we speak’s smallest lunge-feeders, minke whales, seem to maximise consumption by feeding on shallow swarms at night time, it means that their smaller bodied ancestors may need existed in oceans the place such circumstances have been much more prevalent. Within the absence of direct observations of the previous, generally the survivors of prehistoric arms races can inform us one thing about the place they got here from – and that information could be vital when making an attempt to know the dangers that wildlife face in as we speak’s quickly altering oceans.