It is easy for us relate to the mammalian strategies for winter survival that I wrote about last week, but when you look at the invertebrate crowd, things start to get a little crazy. Invertebrates can use any of the methods that we discussed last week to survive the winter, but they do not have the advantage of an internal furnace to keep them warm. In order to hibernate, invertebrates must either tolerate being frozen, or avoid it.
One example of a freeze tolerant insect is the goldenrod gall fly larvae. You may remember from the fall, these are the critters that overwinter in a little round house that they prompted the stem of a goldenrod plant to form around them. Fall temperatures and shorter days trigger the accumulation of cryoprotectants (glycerol and sorbitol), which provide safe, non-destructive places for the ice crystals to form in the larva’s cells. Then when the outdoor temperature falls below 17 degrees F, the larvae freeze solid, and happily survive with as much as 65% of their body water turned to ice.
Although gall fly larvae can endure multiple freeze-thaw cycles over the winter, warmer temperatures increase their metabolism, and reduce their body size. When they hatch in the spring, the resulting adults – which do not feed – will not be able to lay as many eggs. As our winters become more variable with climate change, will we see a decrease in goldenrod galls?
In contrast, frost intolerant invertebrates go to great lengths to prevent ice crystals from forming in their bodily fluids. They use special anti-freeze chemicals that allow them to “supercool.” Supercooling is when a liquid is cooled to a temperature below its freezing point yet does not freeze. However, if the critter cools below its supercooling point, or if another disturbance promotes ice crystal formation, it will freeze solid and die.
One of the most hated creepy-crawlies in the Northwoods is frost intolerant and uses supercooling—the deer tick. In North America, larval deer ticks hatch in the late summer and fall, quest for a bloodmeal, and molt into nymphs after feeding. These newly molted nymphs must overwinter before emerging in the late spring or early summer the following year to quest for their next (nymphal) bloodmeal host. Nymphal deer ticks are the most troublesome life stage for humans, because they are often infected with Lyme disease and other similar diseases. What’s more, the nymphs are tiny, and thus difficult to detect, and they are active in the late spring through summer, when people spend the most time outside.
Currently, ticks seem to have a high rate of winter survival (85%). Snow cover helps to insulate them, so scientists predict that one effect of cold, dry winters like this one might be lower tick survival rates. In addition, wetter winters might foil their attempts at supercooling, because moisture can instigate crystal formation. Unfortunately, scientists have not found evidence that tick survival rates are decreasing due to our weird winters. It seems that ticks are just very good at finding cold, dry places in the leaf litter where they can supercool and not freeze.
Ticks are crafty, too. A study published in 2010 by Yale School of Medicine and others found that ticks infected with Anaplasma phagocytophilum (which causes the disease anaplasmosis in humans) have an enhanced ability to survive the cold. The Anaplasma helps ticks produce an antifreeze protein. The protein bonds to crystal structures to prevent them from growing. Scientifically, this is a fantastic example of a symbiotic relationship where both creatures benefit. Personally, it just seems unfair that what makes ticks more dangerous to us also makes them more numerous.
As our winters become less predictable, less like “the good old days” we remember, the strategies that various critters have devised for winter survival may become insufficient. Global climate change has been described as the greatest natural experiment in the history of science. We may not always be happy with the creatures who are able to come out of the experiment ahead, and it may not always be fair.
For over 44 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 13470 County Highway M. The current exhibit, STAR POWER: Energy from the Sun, opened in May 2012 and will remain open until April, 2013.
Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com/.