Wednesday, July 10, 2013

An Evolutionary Tour of the Galapagos (5): Sibilicide Among the Boobies

June 20
Overnight, the Cormorant navigated to Marchena, which is one of the most isolated islands and one rarely visited by tour boats.

We hiked over Black Beach.  The beach is black because it's composed of volcanic ash.  The large pieces of driftwood on the beach that have floated in from hundreds of miles away suggest how the islands could have been originally settled by animals floating on wood from the South American mainland.

Reptiles could have survived such a long trip without water better than mammals, which could explain why there are no large land mammals on the Galapagos, and why the reptiles are dominant.

There are marine iguanas here, as there are on all the main islands.  They are the only marine lizards in the world.  They live largely on land, but they feed almost entirely on red and green algae.  They can remain submerged for over 10 minutes.

Here's a video of marine iguanas feeding.


On Marchena, there are no land iguanas or tortoises, which feed on cacti.  Consequently, as Carlos pointed out to us, the species of cactus on this island grows low to the ground, because it does not need a high stalk to protect itself from iguanas and tortoises eating it.  Opuntia helleri is a low-growing species that grows only in Marchena and other northern islands that have never had a tortoise or land iguana population.

                                                         Opuntia helleri

                                       Opuntia echios, Found on Santa Cruz

June 21
Having navigated overnight to the island of Genovesa, the Cormorant anchored in Darwin Bay, which is an almost perfectly circular bay created when a crater collapsed into the sea.


In the morning, we hiked a trail where we were surrounded by thousands of breeding sites for nazca boobies, who make their nests on the ground, and red-footed boobies, who make their nests in trees.  The third species--the blue-footed boobies--is the most famous of the three.

                                                            Nazca Boobies

                                                     Red-Footed Booby

                                                 Blue-Footed Booby

                                    Mating Dance of the Blue-Footed Boobies

The Nazca Booby is a species endemic to Galapagos--that is, it is found nowhere else in the world.  The Blue-footed Booby and the Red-footed Booby are endemic subspecies that are found only in Galapagos, but other closely related members of the species are found elsewhere in the world. 

They all feed by plunge diving for fish.  Although they breed close to one another, they avoid competition in fishing by not fishing close to one another.  Blue-footed boobies fish close to shore.  Nazca boobies fish farther out to sea within the islands.  Red-footed boobies fish even farther out in the open ocean outside the islands.

Although he did not use the terms, Carlos often appealed to what Darwinian biologists call "competitive exclusion" and "niche differentiation."  Where similar species compete for the same food resources, either one will drive out the other, or they will coexist when the species differentiate their niches so that resources are divided between them.  The boobies can coexist and avoid competing for fish by fishing in different spots and thus carving out distinct niches.  So while natural selection is driven by competition between species, natural selection can also favor species finding ways to avoid directly competing with one another for scarce resources.

People are amused by the clownish look of the mating dance of the blue-footed boobies.  Indeed, the name "booby" is derived from the Spanish word bobo--a fool or a clown.  Here's a video of the mating dance.

Despite their clownish appearance, the reproductive and parenting strategies of the boobies are deadly serious.  As Carlos explained to us, the supply of food determines how many offspring they can rear.  Blue-footed boobies lay up to three eggs that are incubated by both parents for up to 42 days.  If the food supply runs short, the young can eat their siblings.  Nazca boobies can lay up to two eggs, but they cannot rear more than one offspring.  Laying two eggs seems to be insurance if one of the eggs is bad.  If both eggs hatch, one sibling will kill the other.  Biological studies have shown that the killers tend to be innately more aggressive, with higher levels of testosterone.  Oddly, those young that are born without siblings have been observed attacking the young of other birds, as though their instinct for sibilicide needs to be satisfied.  Some pictures of Nazca booby sibilicide can be found here.

Some of the people in my tour group had spoken previously about how "cute" the boobies were, but then they were disturbed by these Darwinian explanations about their need for siblicide and cannibalism as reproductive strategies.  This illustrates the point that Darwin made in his debate with Frances Cobbe--that evolutionary science reveals a world that is without moral design, and thus there is no cosmic grounding for right and wrong.  Those with Platonic longings for a moral cosmology conclude from this that Darwinism is nihilism, because they cannot accept the moral anthropology of Darwinian science--that human morality arises from evolved human nature without any cosmic normativity to support it.  Here is where Darwinian science continues the tradition of Lucretian liberalism as revived by David Hume and Adam Smith, which forces us to consider the possibility that Friedrich Nietzsche was right in claiming that we could properly "describe the entire phenomenon of morality as animal."

The Darwinian evolution of family conflict, including siblicide, in birds and other animals has been explained by behavioral ecologist Douglas Mock.  See Mock, More than Kin and Less than Kind: The Evolution of Family Strife (Harvard University Press, 2006), and Mock and Geoffrey A. Parker, The Evolution of Sibling Rivalry (Oxford University Press, 1997).  See also Douglas Mock, Hugh Drummond, and Christopher H. Stinson, "Avian Siblicide," American Scientist, 78 (1990): 438-49, which can be found online.  For some evidence that hormonal changes in Nazca boobies promote the aggression of dominant, first-hatched chicks against subordinate, second-hatched chicks, see Elisa Tarlow, Martin Wikelski, and David Anderson, "Hormonal Correlates of Siblicide in Galapagos Nazca Boobies," Hormones & Behavior 40 (2001): 14-20, available online.

Some posts on related themes can be found here and here.

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