| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Horizons Squirrel, Interrupted
When faced with the rigors of a subzero winter, many mammals will curl up in a den and siesta until spring. Animals from mice to bats to bears wait out the cold months by dropping their body temperatures down to a few degrees above freezing. The inactivity helps ensure that precious fat stores will last for long months of down time. But hibernation isn’t the multimonth snooze it’s commonly believed to be. Most hibernating mammals arouse on a fairly regular schedule throughout winter. Every few days to every few weeks, they stoke their body furnaces all the way back up to normal, stretch, eliminate their wastes, and then fall back into torpor 12 to 20 hours later. For most hibernating mammals, these periodic arousals are the most metabolically costly activity they will engage in all winter. Scientists have proposed a number of explanations, ranging from the need to void wastes to restoring the ability to burn stored fats. Yet no one has come up with a totally convincing explanation for why some hibernators burn up to 80 percent of their seasonal energy budgets getting up early. Now biologist Brian Prendergast of Ohio State University in Columbus and colleagues have thrown a new idea into the ring. They propose in the American Journal of Physiology: Regulatory, Integrative and Comparative Physiology that hibernating mammals arouse to fight disease. The scientists stumbled across the idea while studying fever responses in hibernating golden-mantled ground squirrels, Spermophilus lateralis. These California natives drowse for five to six months of the year, while their body temperatures drop to a just a few degrees above ambient temperatures. In their study, the scientists injected 31 hibernating squirrels with harmless bacterial agents known to spur active animals into spiking a fever. But to their surprise, the squirrels in torpor didn’t react at all. When the squirrels awakened on schedule a few days later, their body temperatures zoomed to a fevered 39 C—as though no time had elapsed since the injection. “There’s reason to believe that bacteria are quite capable of colonizing a mammal at such a low temperature,” Prendergast says. “If animals are in jeopardy of being colonized by a bacterial infection while hibernating, it would behoove them to do a system check to look around for any bacteria they may have acquired during the previous hibernation bout.” Without such a check, he says, animals on the verge of getting sick might not survive until spring. No one knows for certain why the animals’ immune systems shut down during hibernation. It may be that temperature-sensitive enzymes stop working when the mercury drops too low. Other bodily functions, such as neural activity in the brain, cease to occur below certain temperature thresholds. This is one reason why hibernation cannot be considered sleep. Other scientists aren’t yet convinced that animals arouse to ward off infection. “It doesn’t seem to me the way natural selection would have mounted such a response. If you get an infection, you should wake up now and deal with this as soon as possible, as opposed to waking up periodically and checking,” says neuroscientist Norman Ruby, who studies hibernation at Stanford University. “Hibernating animals are more sensitive to the outside world than people think. Given that squirrels don’t wake up after they get infected in the lab, you have to wonder what kind of infectious agents hibernating squirrels would be exposed to in the wild. A small threat of infection might explain why they don’t wake up right away. So far, no one’s looked at that.” Matthew Kluger, who researches fever responses at the Medical College of Georgia, says the study highlights a tantalizing new direction for hibernation research. “The concept that arousal may activate a dormant immune system is an interesting hypothesis.” However, he adds, it’s now incumbent upon the researchers to investigate the phenomenon further with additional experiments. Ant Supercolonies The history books of Europe are heavy with the stories of great empires that stretched clear across the continent. And from Alexander the Great to the Romans and Ottoman Turks, many anchored their far-reaching powers in the fertile lands and mild climate that ring the Mediterranean Sea. Today, a new type of conqueror has founded an empire along those same shores. A supercolony of invasive Argentine ants (Linepithema humile) now occupies the southern coasts of Italy and France, and the entire coastline of the Iberian Peninsula. With borders stretching for some 6,000 kilometers, its billions of members make up the largest cooperative unit known, reports a team of European scientists in the Proceedings of the National Academy of Sciences. After staging encounters between individuals from 33 colonies in the area, they found only one pocket of anthills, which they call the Catalonian supercolony, whose inhabitants would fight ants from the larger group. Large groupings of nonaggressive ant colonies are very rare in nature. Researchers surmise that the success of the Southern European supercolony is similar to an ant version of the famous Pax Romana. In Caesar’s time, the absorption of warring tribes into the empire suspended the constant skirmishing that had sapped local wealth and resources. Money once spent fortifying walls and arming soldiers could be put towards building cities and improving living standards. The same principle likely applies to the Argentine ants. Normally, “the number one biggest enemy of an ant colony is another ant,” says Brian Fisher, an entomologist at the California Academy of Sciences. “But that’s not true in this case.” “There is a kind of ecological advantage to eliminating the costs of territoriality. United we stand, divided we fall,” says ant researcher Andrew Suarez of the University of California at Berkeley. Suarez, Neil Tsutsui of the University of California at Davis, and David Holway and Ted Case of the University of California at San Diego have found another supercolony of Argentine ants in another region with a Mediterranean climate—California. Here, a single colony inhabits a 1,000-kilometer section of the state’s temperate coastline from San Diego to Ukiah. Of more than 40 California colonies sampled, only five, near San Diego, didn’t belong to the supercolony. Meanwhile, in their South American homeland, Argentine ants behave with far more circumspection. Like the vast majority of ant species, they live in widely scattered individual colonies that regularly battle each other and the colonies of other ant species. So why do some ants fight while others don’t? The relatively few ant species that invaded California and Southern Europe in agricultural shipments at the turn of the twentieth century had likely passed through a severe genetic bottleneck to get there. Sure enough, the Davis researchers found that individuals from the California supercolony showed a whopping 50 percent drop in genetic diversity when compared with wild Argentine ants. “We suspect that, as a result, they all behave like close relatives in a colony,” Tsutsui says. Moreover, environmental differences such as food and habitat appear to play a negligible role in determining recognition among ants. When the researchers raised groups of ants from battling colonies in similar lab conditions for more than a year, and then staged fights between them, the same colonies that had fought in the past continued to show hostility toward one another. The researchers are now delving into exactly how ants distinguish colony friend from colony foe. They have taken a cue for their next avenue of inquiry from ant social behavior. When one ant meets another, it may pause a moment to tap the stranger’s body with its antennae. The “taste” of the chemicals, or cuticular hydrocarbons, which cover the exoskeletons of all ants, tells an ant whether to walk on by or to get ready to rumble. Tsutsui and Suarez are in the first stages of analyzing the characteristics of these chemicals to find which ones are important in kin recognition. “Finding the genes that code for the chemicals they are using to discriminate nestmates from non-nestmates is what we would ultimately have to do” to prove that genes, and not the environment, are the driving force behind the formation of ant supercolonies, Suarez says. As in Europe, the spread of the Argentine ant through California has decimated populations of native ants. “It’s easy to think, ‘well, it’s just an ant—we’ll just replace it with an exotic ant.’ But it’s not providing the ecological services native species do,” Fisher says. Suarez has already linked the decline of coastal horned lizards (Phrynosoma coronatum)to the disappearance of the native acrobat ant, Crematogaster californica, on which it depends for food. Yet the reign of the supercolonies is unlikely to last forever. “In the long-term, it’s probably not evolutionarily stable,” Tsutsui says. The lack of competition between colonies, Suarez says, also means that ants from colonies carrying the most beneficial genes are no longer more likely to survive than any other colony. Over the generations, the overall genetic fitness of the species will degenerate. In addition, “colonies with less genetic diversity are more susceptible to pathogens and parasites. In California, they may be teetering on the brink,” Tsutsui says. The right pathogen sweeping through the population could eliminate the lot. For the state’s native ants, horned lizards, and other native species, that day can’t come soon enough. Kathleen M. Wong is Senior Editor of California Wild. |
