Spread of humans across planet led to other mammals getting smaller
Q. Why don’t deciduous trees — maples, oaks, birches, etc. — keep their leaves year round? A. They’re saving their lives by shedding their leaves as the season changes, says Dan Lewis on his “Now I Know” website. During the warmer months, the green leaves absorb sunlight and turn it into food for the trees. But as the air gets colder, water in the leaves starts to freeze and so the trees, unable to rely on photosynthesis, go into a sort of hibernation. According to the National Wildlife Foundation, if they’re not eating, “these trees need to conserve energy and reduce themselves to their toughest parts.”
So off come the leaves, since any water in their veins will freeze and kill them. Moreover, when a tree comes out of hibernation, too many dead leaves will doom it. But before that happens, a hormonal change takes place: “abscission” cells (the same root as “scissors”) appear where the leaf stem meets the branch and eventually a thin bumpy line of cells push the leaf away from the stem (NPR). Because the connection is weakened, an autumn breeze will blow the leaves off the tree, leaving their colorful remains on the ground below. And the trees are saved to await new leaves when the season changes again. Q. “Honey, I Shrank the Mammals,” the article title reads. What’s the story here? A. “Where humans migrate, mammals become smaller,” says Mark Fischetti in “Scientific American.” For 65 million years, mammals got bigger, and extinction rates among all sized mammals were similar. But in the past 100,000 years, larger species started dying off faster, as hominin species shifted across the continents — Eurasia, Australia, then North and South America 15,000 years ago.
According to paleoecologist and lead researcher Felisa Smith, “hefty animals suffered from being hunted, as well as from habitat change and fires caused by human activities . ... Two centuries from now, cows may top the size chart.”
“We have changed the entire Earth,” Smith says. “Now we have to be nature’s stewards.” Q. Whether it’s melodic birdsong, a chorus of insects, the howl of a coyote, or the barks and purrs of our domestic companions, the animal kingdom is a delightfully noisy place — on land, that is. But why not underwater? A. In fact, underwater animals are just as noisy! But we are not “well adapted to hearing when our ears are full of water,” says Helen Scales in “Discover” magazine. Also, “most sound waves don’t pierce the waterline, but instead bounce back into the depths.” Even if we do manage to hear underwater noises, since sound travels much faster in water than in air, we are not adept at pinpointing the source.
The intensity and diversity of underwater noise became clear with the development of sonar and hydrophones in World War II, and while some is attributable to waves, wind and tides, it was soon determined that animals were chiefly to blame. “Fish were so noisy they triggered underwater bombs which were supposed to detonate only at the sounds and vibrations of a nearby submarine.”
Following the war, one scientist at the forefront of underwater noise research was a woman aptly named Bobbie Fish, who for 20 years recorded and identified underwater sounds, eventually coauthoring the seminal “Sounds of Western North Atlantic Fishes.” Booms, rattles, pig-like grunts, rusty-hinge squeaks-the diversity is staggering, and the fish species involved number in the hundreds.
Yet little is known about the purpose of fish talk, or even how and how well fish can hear these sounds. After all, “they don’t have ears, at least not ones that stick out of their heads!”