Q. Dear Twig: Why did those great big mammals in North America — sloths, mammoths, those guys — go extinct?
A.: Scientists have four main ideas. Each one has to do with change and the creatures' inability to cope with it.
The overkill hypothesis ("hi-PAHTH-uh-siss"; an educated guess based on study) says this: That humans came to North America 14,000 years ago (maybe more). Spread across the continent. And in the process hunted, killed and unwittingly wiped out many great big mammals for food. Humans — smart, hungry, armed — were new to the neighborhood. The mammals didn't know what to do — Take them a Bundt cake? Run away screaming? — and/or lacked the ability to do it.
The climate change hypothesis blames, yes, the climate, which started to warm up again some 20,000 years ago. Grasslands shrank. Forests grew. And certain grassland mammals ended up fewer and fewer then gone.
Next: Hypotheses 3 and 4. A killer disease? All — or none — of the above?
P.S. Q.: How do you know when a mammoth is in your cave? A.: Its bike is outside! Hyuck!
Notes: Sources included the American Museum of Natural History, http://www.amnh.org/science/biodiversity/extinction/Intro/GiantBeasts.html (including and especially the link "What happened to all these mammals?"); Ice Age Mammals of North America by Ian Lange (2002, Mountain Press Publishing Company); Twilight of the Mammoths by Paul S. Martin (2005, University of California Press); Mammal Evolution by R.J.G. Savage and M.R. Long (1986, Facts on File); and the BBC's "Walking with Prehistoric Beasts."
"Smart Stuff with Twig Walkingstick," published by The Ohio State University College of Food, Agricultural, and Environmental Sciences - specifically, by the Ohio Agricultural Research and Development Center (OARDC) and Ohio State University Extension, the research and outreach arms, respectively, of the College - is a weekly column for children about science, nature, farming and the environment. It's written at a 4th-grade reading level. For details, to ask Twig a question, and/or to receive the column free by mail or e-mail, contact Kurt Knebusch, CommTech, OSU/OARDC,1680 Madison Ave., Wooster, OH 44691, firstname.lastname@example.org, (330) 263-3776. Online at http://extension.osu.edu/~news/archive.php?series=science.