Backyard Biology

Nature stories from my backyard and beyond

Nature Stories
ringlet by Bug Eric

Solar Energy

added Friday, June 3, 2011

It is a well-known scientific fact that animals in the cold northern climes are larger than their brethren in warmer southern climes. For instance, New England raccoons can be twice the size of their Florida counterparts. The reason for this has to do with surface area vs. volume. A small Florida raccoon has more surface area ( body surface) compared to its mass than a large New England raccoon. More surface area per mass equates to faster heat loss (or heat gain in hot temperatures). In a cold climate like New England with extremes in temperature, a raccoon that looses heat faster is at a disadvantage. Thus – larger raccoons.
If a small animal looses heat faster than a large animal, imagine how fast a butterfly will cool off in cold temperatures.
In order to fly, a butterfly’s flight muscles must be at a temperature of 80—110 degrees F. That is about the same temperature your muscles need to work effectively. Remember that the body temperature of humans is about 98.6 degrees F. That is the temperature at which your muscles work most efficiently.

To see the effect of cold temperatures on your muscles, try this experiment:

  • Extend your fingers out straight.
  • As fast as you can, close your fist then extend your fingers again. Time how long it takes you to do this twenty times.
  • Next: submerge your hand in a bucket of ice water and leave it there for one minute to cool down your muscles.
  • Repeat the experiment, timing how long it took to extend your fingers twenty times when your muscles are cold.

Some of our native butterflies hibernate during the winter and can be seen flying on a relatively warm, late winter day. The daytime temperature may only reach 50 degrees, but the butterflies are active nonetheless. Butterflies are cold-blooded – that is they take on the temperature of their environment . So if these active butterflies need a body temperature of 80 degrees or more to fly, but the air temperature is only 50 degrees, then they must have raised their temperature by at least 30 degrees. And since butterflies are so small and loose temperature quickly, they must continually heat their body to keep that elevated temperature. How is it that a cold-blooded butterfly can raise its temperature 30 degrees?

The answer is they sunbathe. They use solar energy to gain the needed heat.
During the cold night, the butterfly is tucked away in a sheltered spot, out of the wind. Its temperature drops with that of the night, but because it is in a sheltered spot, does not drop below freezing. As the day starts to warm, the butterfly must move to a sunny area where it can bask in the sun’s warm rays. But before it can fly, it must warm its body to 80 degrees. So it begins to shiver, contracting its flight muscles in a way that does not move the wings. Slowly at first, the muscles move faster and faster, generating more heat until it is able to move to a sunny spot. Once in the sun, it will bask until flight temperature has been reached.
Because they loose heat so quickly, butterflies spend a lot of time basking, especially on cold days. And since basking involves sitting still for a period of time, it is an easy behavior to observe.

Butterflies have a number of different methods of basking. Some butterflies will bask one way while others use a different method. It is not always easy, however, to put a label on a particular basking strategy. Even butterfly experts can’t agree.
Some experts say there are four types of basking. Others only recognize two. So I will list all four basking strategies and let you make up your mind from your own observations. The first two are pretty well agreed upon by the experts. It is the latter two that some question.

  1. Dorsal basking. In dorsal basking, the wings are held out flat to the ground, sometimes with the wingtips actually touching the ground. Dorsal basking is typical of dark-winged butterflies. The sun shining on their wings generates heat much the same as sun shining on any dark surface. This heat warms the air trapped under the butterflys’ wings which in turn heats the body.
  2. Lateral basking. Lateral baskers sit with their wings held closed above their body. They will orient themselves with the sun to their side and lean away from the sun, so the suns rays shine directly on the exposed underwing.
  3. Reflectance baskers. Reflectance basking is found in some of our light-colored butterflies – the whites and sulphurs. These butterflies hold their wings at a 60-120 degree angle to each other – in a V pattern, and orient themselves so the sunlight reflects off their wings directly onto their body.
  4. Body baskers. Sunlight only reflects off light colored wings. It is absorbed by dark colors. Some dark colored butterflies will bask with their wings at an angle like reflectance baskers, but since the sunlight doesn’t reflect off their wings, they must orient themselves so that the sunlight shines directly on to their body to heat it directly.

Regardless of the actual method of basking, it is an amazing feat of nature that butterflies can raise and maintain their body temperature 30 degrees above ambient air temperature on a cold day. Not bad for a so-called “cold-blooded” insect.

So the next time you are out on a cold late winter morning watching a solar powered butterfly warming itself in preparation for flight, you might ask yourself this question. If butterflies rely on solar energy to warm themselves for flight, how do night-flying moths raise their temperature?