Its cold out there!

   Unless you are in the Southern Hemisphere right now, winter is coming on strong this December, causing the temperatures to drop significantly as the days get darker. All of the college students on the University of Oklahoma Campus are bundled up in scarves, hats, and whatever layers so they can to beat the cold and trudge from study location to study location, dorm to library, or stop at the occasional coffee shop or dining halls to refuel for more studying. From above, one can imagine that the campus looks like an ant colony with everyone going to their respective "nest" to stay warm and prepare for the finals next week. Accumulating a proverbial "academic fat layer" to rely on while staring at the blue books, scantrons, and other miscellaneous test forms thrown at them. Then the mass exodus of everyone heading home for the holidays, leaving the OU campus empty and drained until January...and still cold.

Not OU campus, but might as well be! Image from South Dakota State University Collegian Newspaper

   Which brings us to this week's topic...why is it that most invertebrates aren't really active in the cold? After all, us humans and other animals are out and about (maybe not happily in the cold...but still out there). The reason is that, unlike other animals, invertebrates are poikilothermic, or cold-blooded. Humans and other mammals have an internal thermostat that keeps our body temperatures up (we are homeotherms, or warm-blooded) so that all of the enzymes, muscle contractions, and nerve signals that keep our bodies running can keep going normally, whatever the temperature is outside. Invertebrates instead have their bodies (and all of the functions contained in them) regulated to the temperature outside...when its colder, the enzymes for things like digestion slow down. Their muscle contractions slow down...all of the molecules that signal nerve pathways slow down. If you find an ant crawling on the sidewalk in December, even though its out in the cold environment walking around, its probably going VERY slow.
      This slowing down, or even stopping of functioning in the cold has its benefits. For one, if functioning in the body slows down because its cold, it also means that energy consumption slows down. All cells need energy, which is obtained through food that is digested, processed, and passed through the body. Winter, especially for animals that feed on plants, is not a good time for obtaining food because most plants die back and drop their leaves. Or, if the plant can tolerate the cold, usually they have various different modifications to survive the lower temperatures that make them fairly unpalatable for insects, like waxes or resins (think of trying to eat a Holly tree leaf or a pine-needle!). As a result, if the food source is not easily available, it might be beneficial to scale back energy usage in response. Other vertebrates do this too to some extent, including animals that hibernate (bats, bears, etc.).
    The second benefit is that most invertebrates have fairly short lifespans, and often the goal is: eat, breed, die. Most insects, for example, do not live much past a season and very rarely past a few years (though there are notable exceptions with periodic cicadas, some aquatic insects, spiders and beetles). Again, this is somewhat tied to the resources they depend on for food and energy. Invertebrates that rely on short-lived plants that only last a season usually have similar lifespans, often dying as adults before winter and the eggs for the next generation waiting for the next year's plants or going into hibernation (diapause) as adults to lay eggs in the spring.

Lady bug (Coccinellidae) in diapause. From http://iowabuglife.blogspot.com/

     Several of the longer-lived species live in habitats where the resources are around for longer periods of time (rotting logs, creek beds), so there is a benefit for possibly extending the ability to tolerate cold as an adult to continue to have access to that resource in the winter. Ants are a good example because as scavengers they constantly are on the move to look for a variety of food. Many are not limited to particular plants, and can even find some of the other insects that are immobilized by the cold and bring it back to the nest as a protein source. College campuses, in fact, probably make a great habitat because of all of those students eating on the run: providing plenty of dropped Burger King crumbs, potato chip bags, and other debris that ants can utilize no matter what the season if the temperature is right.
    So while moving much slower than they would in the summer, ants (like college students) continue to brave the cold, doing what they have to survive in their respective worlds this December.
      Good luck on finals OU students!