A hormone that plays a role in regulating body weight may be a key to understanding how hibernating bears can remain inactive for so long and not experience bone loss, according to a research team led by a University of Maine alumna and researcher.
Dr. Rita Seger, a researcher in the University of Maine Department of Animal and Veterinary Sciences, and a team of researchers conducted the bone metabolism study. They compared active and hibernating bears using a suite of 12 serum markers of bone metabolism and X-rays of the bears’ paws. The researchers found greater amounts of leptin in hibernating than in active bears. In addition, leptin levels in hibernating bears correlated with serum markers of bone turnover, leading them to hypothesize that the hormone’s effect on the sympathetic nervous system may help to prevent bone loss.
In essence, the skeleton appeared to perceive that it was “loaded” or supporting an active body, when it was actually “unloaded” during hibernation, the researchers wrote in the journal Bone.
In animals, activity or mechanical strain is an essential stimulus for bone growth. Inactivity for seven to 26 weeks has been found to result in cortical bone loss of 10 percent to 40 percent in humans, beagles and turkeys. Little brown bats and golden hamsters can lose up to 45 percent of their cortical bone during hibernation. Hibernating bears are the only animals that do not experience unloading-induced bone loss.
Researchers hope that greater understanding of the role of leptin in bone biology can contribute to our understanding of — and better treatments of — skeleton-related diseases, such as osteoporosis.
New evidence suggests that Atlantic cod may have the ability to affect entire food webs in both benthic and pelagic marine ecosystems, according to a University of Maine marine scientist, writing in the Proceedings of the National Academy of Sciences (PNAS).
“Not only are (cod) strong interactors capable of limiting the abundance of their prey and their prey’s prey, but also the prey themselves may limit the recovery of this predator,” says Robert Steneck of the large carnivore that, prior to overfishing, was “widespread, abundant and possibly the most important predator throughout the coastal regions of the North Atlantic.”
“In most countries where fisheries management exists, the focus is on the dynamics of single species,” says Steneck, “and often there is no consideration of how two or more managed species interact or how such interactions can affect the entire ecosystem,”
In his PNAS commentary published May 14, Steneck points to an event in which an overabundance of Atlantic Cod in the Baltic Sea spilled over into the Gulf of Riga, as reported by a research team led by Michele Casini of the Swedish Board of Fisheries. The “predator pulse” — in-migration of juvenile and adult cod — into the gulf lasted a decade, causing a trophic cascade in the marine food web. Cod ate the herring, causing the herbivorous zooplankton population normally eaten by herring to increase. Because, zooplankton consumed phytoplankton, water in the Gulf of Riga cleared, but only for the decade when cod spilled into the region.
This example of successful, albeit serendipitous, cod colonization provides clues as to how cod repopulation occurs and why it isn’t as simple as closing large areas to fishing when Atlantic cod stocks collapse, Steneck contends. In the case of Canada and the United States, fishing managers expected a full recovery of cod stocks within a decade after the closures in the early 1990s; nearly two decades later, cod stocks remain historically low.
It is possible that colonization of new or depleted areas occurs by influx of larger cod rather than cod larvae when adjacent populations reach high population densities, which has not happened in New England for decades at least, says Steneck.