How Does the Largest Animal Eat the Smallest? Hair in the Mouth
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The biggest animals that have ever lived catch their food with fringed plates made of the same material as fingernails. Losing teeth and growing a sieve is what let whales get that large.
What the plates are
Hanging from the upper jaw in two rows are hundreds of stiff plates, set close together like the pages of a book turned on edge, each running down from the gum and fraying along its inner edge into coarse bristles. The plates are keratin, the same protein as hair, horn and fingernail, and they grow continuously from the gum and wear away at the tip, exactly as a nail does. The fraying is not damage but structure, since the interlocking bristles from adjacent plates form a dense mat across the whole inside of the mouth, and that mat is the filter.
How a mouthful is processed
The sequence is the same across the group with variations in method:
- •Open the mouth and take in water containing prey
- •Close the mouth, trapping the water inside
- •Force the water out sideways and forwards through the plates
- •Prey too large to pass is held on the bristle mat
- •Use the tongue to scrape the mat clear and swallow
- •Repeat, with the largest species taking tonnes of water at a time
The two ways of doing it
The group divides into two feeding styles that have driven very different anatomy. Lunge feeders accelerate at a patch of prey with the mouth open, engulfing a volume of water that can exceed their own body volume, which is possible because the throat is pleated and expands enormously and the lower jaw unhinges. That manoeuvre costs a great deal of energy and only pays when the prey is densely packed, which is why these animals feed in short bursts on concentrated swarms. Skimmers instead swim slowly with the mouth held open, filtering continuously for long periods, which suits thinly spread prey and requires very long fine plates rather than a huge throat.
Where it came from
The group evolved from toothed ancestors, and the transition is documented well enough to be described. Fossils from around thirty million years ago show animals with teeth still present and with features of the skull associated with filter feeding beginning to appear, and a series of intermediate forms follows, with teeth reducing and eventually being lost while the plates develop. Modern embryos confirm the sequence independently, since a foetus develops tooth buds in the jaw that are reabsorbed before birth and never erupt, leaving an animal that grows a filter instead. The change is generally linked to a reorganisation of ocean circulation that made dense swarms of small prey abundant.
How it was used by people
The material was a major commercial product for three centuries and its properties explain why. Strong, light, and able to be split, shaped with heat and hold that shape, it did the job that spring steel and later plastic would do, and no other available material matched it. It stiffened corsets and collars, framed hooped skirts, made umbrella ribs, buggy whips, brushes and fishing rods. Demand was high enough that some fisheries pursued whales primarily for it rather than for oil. The trade collapsed when spring steel and then celluloid became cheap in the early twentieth century, and the change in fashion after the First World War removed most of the remaining market.
The takeaway
Hundreds of keratin plates hanging from the upper jaw fray into bristles that interlock into a dense mat, and water forced out through that mat leaves the prey behind. Lunge feeders engulf more than their own volume in one gulp and need dense swarms to make it pay, while skimmers filter continuously on thin prey. The material replaced spring steel before spring steel existed, and the trade collapsed once it did.