Biology Reference
In-Depth Information
Although it is important to be clear about the distinction between causal and
functional explanations, it is equally valuable to realize that the two kinds of question
are complementary and that asking 'why?' questions can often help to understand the
answers to 'how?' questions, or vice versa. An example of how causal and functional
explanations go hand in hand is illustrated in Fig. 15.4. Black-tailed prairie dogs
(
Cynomys ludovicianus
) are colonial and live in underground tunnels which may be up
to 15 m long. The tunnels are usually simple U-shaped passages with an opening to the
surface at either end. It has been known for a long time that prairie dogs build little
mounds of soil around the two entrances of the tunnel. These mounds were considered
to function either as lookout points or to protect the tunnel against floods. However, a
closer inspection revealed that the two ends of the burrow have different kinds of
mounds. At one end there is a high steep sided 'crater' mound while at the other end
there is a low rounded 'dome' (Fig. 15.4). If the mounds are simple lookouts or flood
barriers, why should they be different shapes? The answer to this 'why?' question comes
from an understanding of how air is exchanged in the tunnel (Vogel
et al
., 1973). A
prairie dog living in the long underground tunnel cannot survive without a regular
supply of fresh air and it appears that the mounds around the two entrances are
designed to ensure a continuous flow of air through the tunnel. The crater mound is
higher and has steeper sides than the dome; as a consequence, air is sucked out of the
crater end of the tunnel and into the dome end.
The forces causing the air flow are viscous sucking and the Bernoulli effect. Viscous
sucking refers to the fact that when moving air passes a region of stationary air the still
air is dragged along with the current. The effect is larger at the crater end because the
crater is higher than the dome and so it is exposed to faster winds. The Bernoulli effect
Causal and
functional
explanations
complement each
other
Fig. 15.4
A diagrammatic section of a prairie dog burrow. A typical burrow has two entrances, one with a
low, round 'dome' at its entrance and the other with a taller, steeper-sided 'crater'. The different heights and
shapes of the burrow entrances cause air to be sucked out of the crater end and, therefore, in through the
dome. From Vogel
et al
. (1973). Photo © Elaine Miller Bond.
