by Greg Mayer
While Jerry’s photos of the fabulous wildlife in the Galapagos show us the marvels of life there, one thing that stood out to me when I visited the Galapagos was how frequently we came across dead animals, much more frequently than anyplace else I’ve been. This observation inspired two thoughts. First, could this abundant evidence of the struggle for existence have influenced Darwin? And, second, might the prevalence of carcasses be due to a dearth of scavengers in the depauperate biota of these islands?
On the mainlands of the Americas, there is no dearth of scavengers, two familiar ones being the Turkey Vulture (Cathartes aura) and the Black Vulture (Coragyps atratus), both found throughout the Neotropics and, for varying distances, into the United States (both) and Canada (Turkey only). Here are both of them, more or less in action.
This Turkey Vulture is standing next to a deceased Virginia Opossum (Didelphis virginiana) on a sidewalk in Oviedo, Florida.
These two Black Vultures are feeding on the carcass of a Nine-banded Armadillo (Dasypus novemcinctus), also in Oviedo, Florida. (Both of the scavenged species, opossum and armadillo, are more or less recent recent invaders from the south; the Black Vulture, too, is moving north.)
And here they are in action:
The Galapagos have one large raptorial bird, the Galapagos Hawk (Buteo galapagoensis), an insular derivative of Swainson’s Hawk (Buteo swainsoni). Although it hunts and eats a variety of prey, both small vertebrates and largish invertebrates, it also eats quite a bit of carrion. Darwin, in fact, thought it rather resembled caracaras, a group of hawk-like falcons that are largely scavengers, especially in its habits:
When a tortoise is killed even in the midst of the woods, these birds immediately congregate in great numbers, and remain either seated on the ground, or on the branches of the stunted trees, patiently waiting to devour the intestines, and to pick the carapace clean, after the meat has been cut away.
Jackson (1985:177) concurs:
… on every island they are also major scavengers. They will feed on virtually any dead animal. I have seen them at the carcasses of sea lions, marine iguanas, seabirds and even fish … They seem to be very fond of goat meat. … On one occasion, within five minutes of a goat being killed, thirteen birds arrived and sat within 5 m of myself and the carcass. [Invasive goats were/are being eradicated in the Galapagos by hunters employed for the purpose.]
The Galapagos Hawk, apparently, fills part of the scavenging niche filled by vultures on the mainland. It has disappeared from some islands since settlement, so its decline may account for the frequency of unscavenged carcasses. The generally dry conditions at sea level, which lead to rapid mummification, may also lead to a proliferation of unscavenged carcasses, so that Darwin, even before the hawk’s decline, may have come across them as well. I’ll have to query Jerry as to his observations in this regard.
Jackson, M.H. 1985. Galapagos: A Natural History Guide. University of Calgary Press, Calgary.