Hippos can stand in the hot sun all day without getting a sunburn, and now researchers know why: a red-colored glandular secretion known as "hippo sweat" contains microscopic structures that scatter light, protecting the hefty mammals from burns, according to a new study.
In the future, scientists hope to create a product inspired by hippo sweat that we may be slathering on our bodies before long. The stuff could be an advertiser's dream.
"It would be nice to also try and replicate the antiseptic and insect-repellent characteristics of the sweat, to obtain a four-in-one product: sunscreen, sunblock, antiseptic, insect repellent," co-author Christopher Viney told Discovery News.
"Just so long as the stuff doesn't smell like hippo," added Viney, a professor in the School of Engineering at the University of California, Merced.
He and colleagues Emily Reed, Lisa Klumb and Maxwell Koobatian had staff at Fresno's Chaffee Zoo retrieve the oily secretion from an indoor enclosure where the zoo's hippos had rested. The sweat was transferred to sealed plastic containers. Even after several months of storage, the red sweat showed no signs of yeast, bacteria or fungal contamination.
Microscopic analysis of the sweat revealed that it contained two types of liquid crystalline structures: banded and non-banded. Viney explained that the banded structures are "characterized by concentric dark rings" when viewed under certain magnification.
"The rings are the result of a structural periodicity that occurs on a scale comparable to the wavelengths of visible light," he said. "This means that the sweat is an effective scatterer of light, so that it combines both sun-blocking and sun-screening properties."
The non-banded structures, in turn, "enhance the ability of the sweat to spread over the surface of the animal, by reducing the viscosity of the sweat."
The sweat's unusual red coloring is due to red and orange pigments present in the secretion. Prior research demonstrated that molecules in these pigments are capable of absorbing ultraviolet light. While the substance may be one of nature's most perfect skin creations, it has caused many an onlooker to think that hippos were bleeding when they were really just "sweating" out the oily secretion.
The findings are published in the latest issue of Philosophical Transactions of the Royal Society A.
The researchers specialize in biomimicry, or learning lessons from nature that can be applied to man-made products and materials. They have also studied worm and spider silk along with filamentous phage, a type of virus that can infect bacteria.
David Kaplan, chair of the Department of Biomedical Engineering at Tufts University School of Engineering, told Discovery News that the study's authors "have a terrific handle on novel functional materials that come out of nature." He added that Viney is one of the world's leading experts in this field.
Kaplan's own department often works with natural materials, such as spider silk, because "nature has provided us with substances that have emerged after eons of evolution. There is so much invaluable data that has yet to be mined."
"As a student said to me several years ago, nature is full of solutions to our technical problems," he said. "We just have to figure out how to look in the right place, and ask the right questions."