Isegoria

You might describe a shark as a toothy torpedo covered in sandpaper:

A shark’s skin is covered in millions of microscopic denticles, rigid tooth-like scales that jut out from the soft skin beneath. By disrupting the flow of water over the fish’s skin, it is thought, the denticles reduce drag, making for a more efficient swimmer. But to really empirically understand how the denticles do their job, you need to see how different sorts of skin coverings affect the fluid dynamics as water washes over the skin of swimming fish. You can’t take a real shark and give it new skin, so Harvard University researchers Li Wen, James C. Weaver, and George V. Lauder created artificial shark skin instead. They manufactured it using a 3D printer.

Lauder’s group then subjected their 3D-printed faux skin to a series of tests in water. They found that it managed to reduce drag by 8.7% when the water flowing over it moved slowly, which is consistent with the thought that denticles reduce drag. But in faster currents, the denticles actually increased drag by 15% compared to a smooth sheet. That might seem surprising at first, but sharks don’t swim in a straight line, they wriggle their bodies. As soon as the researchers started wriggling their artificial skin, the swimming again become more efficient: swimming speed increased by 6.6% and the energy expended was reduced by 5.9%.

Researchers performed isotope analysis of hair and bone samples to study Yosemite bears’ changing diets over the past century:

Yosemite National Park was established in 1890, and Hopkins obtained samples from bears killed between 1915 and 1919 to represent the earliest time period. In those early years, bears were attracted to garbage dumps in the park and were often killed when they became a nuisance. Visitors liked to see bears, however, and in 1923 the park began intentionally feeding bears where visitors could watch them. The last artificial feeding area closed in 1971. There was also a fish hatchery in Yosemite Valley, from 1927 to 1956, where bears once helped themselves to fresh trout from the holding tanks. But closing the hatchery and the feeding areas didn’t stop bears from eating human food.

“The bears just went back to the campgrounds and hotels and continued to find human food,” Hopkins said.

The average figures for the proportion of human food in bear diets during the four time periods in the study were 13 percent for the period from 1915 to 1919; 27 percent for 1928 to 1939; 35 percent for 1975 to 1985; and 13 percent again for 2001 to 2007.

These results are based on a kind of chemical forensics in which Koch’s lab specializes. Isotopic analysis of an animal’s tissues can yield clues to its diet because of natural variability in the abundance of rare isotopes of elements such as carbon and nitrogen. Isotope ratios (the ratio of carbon-13 to carbon-12, for example) are different in human foods than in the wild plants and animals that black bears naturally eat in Yosemite, partly due to the large amounts of meat and corn-based foods in our diets.

In order to analyze the data from Yosemite bears that ate a mixture of human and natural foods, Hopkins had to get samples from bears that did not eat any human food, and he had to track down samples of the non-native trout that had been raised in the hatchery. He also needed data representing a 100 percent human food diet, for which he turned to the Smithsonian Institution for samples of human hair from different periods over the past century.

“He searched far and wide to get the collection of samples we analyzed, and that collection made the study powerful enough to answer the question of how management practices affect bear diets,” Koch said.

According to Hopkins, the key to managing bear problems is to prevent bears from becoming conditioned to eat human food in the first place.

Lynne Isbell was running through a glade in Kenya back in 1992, when she spotted a cobra. She froze in her tracks before she realized what she’d seen. Isbell is an anthropologist and behavioral ecologist at the University of California, Davis, and she believes that her life-saving reaction was the result of millions of years of evolution:

You can see large carnivores from afar, but the same is not always true for snakes: To pick out camouflaged snakes, you need great close-range vision. So to spot snakes better, primates evolved to have color vision and forward-facing eyes, which improves depth perception and allows 3D vision. They also evolved to have the best visual acuity among mammals, Isbell said. These visual features, which required the enlargement of some parts of the brain, were co-opted for other purposes, such as social interactions and reaching and grasping for objects.

Interestingly, the evolutionary interaction between snakes and mammals was not a one-way street, according to the snake detection theory. As mammals became better able to evade snakes — which till this point relied on squeezing their prey to death — the reptiles needed a new, easier way to kill. So they evolved venom. In response, primates evolved even better vision. Indeed, primates that live in areas without venomous snakes, such as on Madagascar, have poorer vision than other primates.

For the snake detection theory to be true, primates would have to be amazing snake detectors. And there is some research that appears to support this idea.

For example, a 1993 study by Arne Öhman, a psychologist at the Karolinska Institute in Sweden, showed that people are able to visually detect snakes before they’re even consciously aware of the reptile, just as Isbell learned firsthand.

More recently, Öhman and his colleagues compared how quickly people detected snakes and spiders. Based on Isbell’s snake detection theory, they predicted that participants would detect snakes more rapidly than spiders because the arachnids were historically less of a threat to primates — and this is exactly what they found. They also discovered that snakes are more distracting than spiders, and concluded that “attending to snakes might constitute an evolutionary adaptation.”