Monday, October 04, 2004

Update on post-Pleistocene extinctions Dog Extinctions Show Why Bigger Isn't Better

Fossils from extinct dogs show why bigger is not better -- giant meat-eating animals died out because they relied too heavily on hunting other big animals, scientists reported on Thursday.

Smaller, quicker carnivores could vary their diet more, hunting small rodents and mixing in berries, roots and other food sources, said Blaire van Valkenburgh and colleagues at the University of California Los Angeles.

But once a carnivore reached a certain size, it would spend more energy hunting than it would get from small prey, and had to rely on big game, they report in this week's issue of the journal Science. And that made them less adaptable.

"Among living meat eaters, almost all species larger than about 21 kg (45 pounds) prey on species as large or larger than themselves, whereas smaller carnivores can subsist on much smaller prey (such as invertebrates and rodents)," the researchers wrote.

Summary: Selection for individual traits and characteristics, or specialization, can be
immediately beneficial but detrimental to a clade in the long run. Van Valkenburgh et al. (p. 101) show that this likely happened to canids (the group that includes dogs) repeatedly during the past 50 million years, and perhaps to many large predators and mammals. Canid clades repeatedly show an increase in body size. Those clades that had the largest increase in body size and became specialized for attacking large prey, on the basis of teeth size and morphology, typically span only about 6 million years before becoming extinct, which is considerably less than for clades with smaller individuals and a more diverse diet.

Actual paper here. (subscription only)

Summary: They plotted an index score based on craniodental measurements which measure hypercarnivory against estimates of species duration and found that "none of the hypercarnivorous species (solid symbols) persisted for more than six million years, whereas some more omnivorous species (open symbols) endured for as much as 11 million years" concluding that large hypercarnivores tend to be more susceptible to extinction. The mechanism they suggest is that selection for large size and hypercarnivory locked them into a suite of specializations which make them more vulnerable to extinction than more generalized critters. This is a general characteristic of mammals known as Cope's Rule.

Update: Another paper on extinctions here. (Subscription only again) Summary:

One of the great debates about extinction is whether humans or climatic change caused the demise of the Pleistocene megafauna. Evidence from paleontology, climatology, archaeology, and ecology now supports the idea that humans contributed to extinction on some continents, but human hunting was not solely responsible for the pattern of extinction everywhere. Instead, evidence suggests that the intersection of human impacts with pronounced climatic change drove the precise timing and geography of extinction in the Northern Hemisphere. The story from the Southern Hemisphere is still unfolding. New evidence from Australia supports the view that humans helped cause extinctions there, but the correlation with climate is weak or contested. Firmer chronologies, more realistic ecological models, and regional paleoecological insights still are needed to understand details of the worldwide extinction pattern and the population dynamics of the species involved.

Abandonment archaeology The Jewish millionaire who surrendered to the Romans

Dozens of coins from the tenth Roman Legion, uncovered during the last excavation season at the Herodian palace in Ramat Hanadiv, offer some insight on the demise of the glamorous palace. Prof. Yizhar Hirschfeld, a Hebrew University archeologist who has been managing the excavations at the site since the 1980s, says that it is possible to learn from the presence of the coins that that the palace was abandoned during the Great Rebellion that started in 66 CE not far away from there, in Caesarea.

The findings at the site do not make it possible to determine whether the palace was captured by force or abandoned and then fell into Roman hands, says Hirschfeld. But they do say something about the haste of the residents as they left. Among other things found at the site were a gold earring and a gold clasp - jewels that even a person of means does not leave behind during a leisurely moving to another place.

This is a generalized interpretation of the mode of abandonment: valuable objects left behind are thought to indicate rapid abandonment, while areas left fairly clean or full of low-cost, low-value objects are presumed to indicate gradual or planned abandonment.