24 February 2009

How to take a slug's temperature and what you can learn from it

They don't put drawings like these into scientific papers anymore. Whenever I see such figures in old publications, I usually think of a "mad scientist" working in a cluttered basement lab late in the evening amid bottles of chemicals, glass vials, spark generators and a caged animal or two. Perhaps, one reason why editors now stay away from printing graphic descriptions of experimental setups is to prevent the association of such imaginary scenes with modern scientists.

This is Figure 1 from a paper by Hogben & Kirk (1944) with a bit of yellow color added by me to bring attention to their experimental subject, which happens to be a slug. But, no, they were not trying to reanimate a dead (or a half-dead) slug by infusing it with a secret elixir or passing thru it a high-voltage current derived from thunderstorm clouds. Their slug was alive, but had a thermocouple inserted into its foot (ouch!).

Hogben & Kirk were trying to demonstrate that the body temperatures of animals like slugs and earthworms whose external surfaces are always wet are not necessarily in equilibrium with the temperature of their surroundings, despite the fact that they are poikilothermic or cold-blooded. They did that by measuring the body temperatures of slugs and earthworms at different air temperatures and humidities.

In the experimental setup depicted in Figure 1, humidity was controlled by circulating the air above an appropriate solution of sulfuric acid in flask V thru chamber C where the slug was. The chamber and the flask appear to have been inside a constant temperature bath (B); the air temperature inside the chamber C was measured by a thermometer (D) and the humidity by a wet-bulb thermocouple (W).

I am summarizing their findings for the slug Arion ater in the graph below.

Data from Table 2 of Hogben & Kirk (1944). The green arrow is pointing at a barely visible point.

This is a "bubble plot": the areas of the circles ("bubbles") are proportional to the numerical value of the difference between the air temperatures and the body temperatures of slugs. There are 3 regions in the graph, which I marked with 3 different colors:

1. The orange circles show that when the humidity is low, there is a large difference (>10 °C) between the air temperature and the slug's body temperature more or less regardless of the air temperature. This difference arises from the cooling of the slug's body as its slime evaporates.
2. The yellow circles show that at moderate humidities, there is a moderate difference between the air temperature and the slug's body temperature.
3. The green circles show that when the humidity is high (>80%), there is a small, almost negligible, difference (<2 °C) between the air temperature and the slug's body temperature.

As Hogben & Kirk note, evaporative cooling of their bodies enables slugs to survive air temperatures that would otherwise kill them. They can keep their bodies cool as long as they have access to water to replenish what they lose thru evaporation. If they run out of water, however, they quickly succumb to heat and desiccation. That is exactly what happens when they get trapped on hot and dry sidewalks in the summer (example).

L. Hogben & R. L. Kirk. 1944. Studies on Temperature Regulation. I. The Pulmonata and Oligochaeta. Proceedings of the Royal Society of London. Series B, Biological Sciences, 132:239–252. Download a free copy before the end of March.


Anonymous said...


Anonymous said...

I LOVE SLUGS!!!!!!!!!!!