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Why do anhingas have less-pneumatized bones than many other pelecaniform birds?

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Three eastern white pelicans with black-tipped wings take off from a muddy shoreline, wings outstretched, with blurred green vegetation in the background. — Pelecaniform European white pelicans (*Pelecanus onocrotalus*) in flight.

Anhingas have less-pneumatized bones than many other pelecaniform birds because they require a high specific gravity to remain submerged while motionless underwater.1

While many aerial pelecaniforms possess extensively pneumatized skeletons—where bones contain air sacs connected to the respiratory system to reduce weight—this adaptation is least developed in anhingas. By having denser, less-pneumatized bones, anhingas can maintain the necessary buoyancy levels to hunt and remain underwater without the need for an aerial dive.1