Why Is a Pelican Like a Whale?

Pelicans and whales are not especially close relatives. I’m about as closely related to Tyrannosaurus rex as they are to each other. The specialized, flying dinosaurs and the highly modified, aquatic artiodactyls (who long ago lost their hooves) last shared a common ancestor over 306 million years ago in the form of a visually unremarkable, lizard-like creature. But there is something that unites both pelicans and baleen whales together in an unnatural grouping of predators — a large expandable pouch suitable for scooping up writhing masses of little fish.

The way pelicans and many baleen whales feed is called “engulfment.” It’s exactly what it sounds like. These predators open their jaws wide to surround masses of small prey, and both lineages share a similar framework of flesh and bone adapted to this method. Pelicans and baleen whales have huge, toothless maws — their U-shaped lower jaws typically measure about a quarter of their body length — and possess an expandable pouch of soft tissue slung from their lower jaws. (The terminology for this fleshy trap differs. Whales have a pleated pouch called the “ventral groove blubber,” while the corresponding structure in pelicans is called the gular sac.) These structural similarities are immediately obvious and are an example of convergent evolution — two distantly-related lineages becoming adapted in the same way due to a shared aspect of their natural history. But how deep do these resemblances go?

According to an Anatomical Record paper published by anatomist Daniel Field and colleagues, the correspondence between the pouch-mouthed birds and mammals extends to the very structure of their bones. It comes down to a matter of bending. Imagine a baleen whale headed towards a school of small fish at high speed. When that whale opens its ludicrously large mouth, the expandable flesh of the lower jaw is going to catch a huge volume of water and will therefore pull down and backwards on the mandible. (Think of trying to pull an empty plastic bag through the water.) No surprise, then, that a 2010 study by some of the same authors found that the density and shape of baleen whale mandibles are specialized to resist the stresses exerted by this kind of feeding. Since pelicans try to gulp down fish in a similar way, Field and co-authors expected the same principle to hold true for the birds.

The researchers took CT scans of mandibles from humpback and minke whales, as well as those of a white pelican, and brown pelican, and a double-crested cormorant (another fish-eating bird with a different method of fishing). These jaws were studied both in terms of their internal structure and their mechanical properties — specifically how rigid the jaws are and if the different jaws are similarly resistant to bending during the sorts of stress created by engulfment feeding.

On the basis of their tests, Field and collaborators suggest that the pelican mandibles have been evolutionarily modified in the same general fashion as those of baleen whales. Engulfment feeding requires a sturdy jaw. But such mechanical adaptations were not exactly the same in the two pelican species studied. The jaws of white pelican were not as stress-resistant as those of the brown pelican, and the reason why may have to do with differences in the way these birds go after fish. Whereas white pelicans often feed from the surface, brown pelicans dive bomb after fish and open their mouths underwater when they are still moving fast. The stiffer mandibles of brown pelicans may be a result of this disparity in approach. Even so, the anatomy and mechanics of the pelican jaws corresponded more closely to whale jaws than the mandible of the double-crested cormorant. This means that the specialized mechanical features in the pelicans truly do seem to be convergent adaptations for engulfing prey and are not common traits inherited by all fish-eating birds.

The new study focused entirely on modern species, but I wonder if the same approach may help pin down the timing of when engulfment feeding evolved. A recently-discovered fossil pelican has shown that the general pelican jaw shape evolved by about 30 million years ago and has remained in place ever since. Large-jawed whales have been around for quite a while, too. While early “baleen whales” — technically called mysticetes — still had teeth, forms such as the roughly 30 million year old Aetiocetus had long, low heads bearing a combination of prominent teeth and tufts of baleen. No one really knows how these whales fed with a tooth-baleen combination, nor does anyone know whether fossil pelicans scooped up prey just like their modern counterparts, but maybe the methods employed by Field and co-authors could be applied to prehistoric forms to detect when both these lineages began enveloping prey in expandable parachutes of flesh sewn to bow-shaped jaws. If we want to understand how this spectacular and specialized technique evolved, we should have no qualms about looking ancient cetaceans and prehistoric pelicans in the mouth.

References:

Field, D., Campbell-Malone, R., Goldbogen, J., & Shadwick, R. (2010). Quantitative Computed Tomography of Humpback Whale (Megaptera novaeangliae) Mandibles: Mechanical Implications for Rorqual Lunge-Feeding The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology, 293 (7), 1240-1247 DOI: 10.1002/ar.21165

Field, D., Lin, S., Ben-Zvi, M., Goldbogen, J., & Shadwick, R. (2011). Convergent Evolution Driven by Similar Feeding Mechanics in Balaenopterid Whales and Pelicans The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology, 294 (8), 1273-1282 DOI: 10.1002/ar.21406