Exuberant Life by William H. Durham

Author:William H. Durham
Language: eng
Format: epub
Publisher: Oxford University Press
Published: 2021-11-15T00:00:00+00:00

Adaptations of the Galápagos Cormorant

Leading researchers of the Galápagos cormorant, especially Carlos Valle (1994, 2013), Rory Wilson (Wilson et al. 2008), and the late Brad Livezey (1992), have filled in many of the details of this bird’s unique lifestyle. It evolved away from the flying capabilities of its ancestors and became a fully dedicated seafloor hunter in the heart of the Cromwell upwelling. Indeed, Valle (1995, 612–13) argued that the cormorants’ geographic range is small because they can only meet daily food requirements in the very center of the upwelling. This restricted range contributes to their being “the smallest naturally occurring marine bird population in the world,” with only about 1,000 individuals in the best of times.

Living in this special setting, the cormorant evolved a list of unusual features—extra-big beak, large body mass, stunted wings, strong legs and feet, and a mating pattern found in no other cormorant—each of which enhanced survival and reproduction, we infer, in its new niche. Consider, first, the cormorant’s enlarged beak. It’s a highly efficient tool for probing the nooks and crannies of the rocky seabed and for extracting marine prey (octopus, eel, blennies, and other small fish) from their hideouts (Figure 8.2). Under normal conditions, cormorants are able to feed themselves in a narrow band of water close to the coastlines, somewhat like the penguins, but always along the bottom and often at greater depths.14 The cormorant beak is also a potent weapon, allowing the birds to rest and nest in the open, protecting themselves and progeny from aerial predators, especially the Galápagos hawk. One would be hard-pressed to disentangle these two benefits, probe and protector, but it hardly matters because the equipment works so well for both. Livezey’s (1992, 176) careful measurements showed that there’s an impressive 40% increase in average adult beak length in the Galápagos cormorant compared to the closely related Neotropic cormorant.15

Second, consider the cormorant’s extra-large body mass. In contrast to the penguin, which, coming from Antarctica, evolved a smaller body in response to warmer conditions in Galápagos, the cormorant faced the opposite challenge. Cormorants came from tropical origins and had a hybrid flying–diving lifestyle, so selection pressures favored the foraging efficiency of a larger body for diving deep in the cold waters of the archipelago. Indeed, the advantages of “big” would only have grown as selection pressures for flight relaxed. Such advantages would have included reduced buoyancy, higher oxygen storage capacity, and lowered heat loss (owing to a lower surface-area-to-volume ratio) in the colder water. In response to these conditions, and with no terrestrial predators from which to flee, the cormorant eventually evolved the largest average body size in its zoological family. In addition, just as a larger body mass in marine iguanas improves their odds of surviving El Niño, so it may be with Galápagos cormorants. Extra mass gives them more stores to live off during El Niño and may also enable them to dive deeper than usual for fish when El Niño first arrives.

As expected, the cormorant’s large body mass enables it to carry out longer dives than its close relatives.

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