The nickname `lounge lizards' is more appropriate than you might think for some reptiles, given that certain lizards seem to avoid activity as they have difficulties getting air into their lungs when they move. While it might be acceptable for some species to sit and wait for dinner to pass by, limited respiration poses a challenge for livelier reptiles that actively forage for their food. One such lizard is the South American tegu, which caught the interest of Wilfried Klein and Steven Perry. They found that tegus have a primitive diaphragm, the post hepatic septum, which divides their body cavity in two, separating the lungs and liver from the rest of their internal organs. Smaller or lazier lizards lack this membrane, which made Klein wonder whether the post hepatic septum might help tegus to breathe during activity. So Klein left his laboratory in Bonn and travelled to the pleasanter climes of Brazil to work with Augusto Abe's colony of tegu lizards, where he could measure the animals' respiration while they walked(p. 2135).
The experiments were simple in principle. Klein monitored the breathing of groups of animals that either had an intact post hepatic septum or had the septum surgically removed, as they walked slowly on a treadmill.
However, they had not bargained on the tegus' lack of cooperation on the treadmill. According to Klein, tegus are very intelligent animals. `If they are handled regularly most of them will be tame after a short while and they cooperate easily. However, some animals display a very strong character... and always have to be handled with great care as they have very sharp teeth!' To add to the team's frustrations, tegus hibernate during the winter; so the race was on to get the data before the animals became dormant. But having come to terms with the lizard's recalcitrance, Klein was able to measure the animals'respiratory rates and volumes, as well as their endurance as they walked at speeds up to 0.28 m s-1.
Klein and his colleagues found that while the animals with a post hepatic septum were exercising, they breathed deeper and faster then when they were resting. However the tegus that lacked a post hepatic septum took much smaller breaths as they walked, although they made up for this smaller tidal volume by breathing faster. The team also noticed that the tidal volume of the tegus that had been operated on only increased immediately after exercise, not during, and then returned to resting levels. Klein explains that during exercise, the lizards' internal organs had shifted, effectively `squashing'the lungs. So by restraining the animal's internal organs as it moves, the tegu's proto-diaphragm gives it the extra lung space it needs to chase its next meal.
Klein explains that this is extremely interesting given the active lifestyle of the animal, as its lungs are more like those of lazier lizard relatives, which are relatively stiff and requiring a lot of work to operate them. Tegu lungs therefore seem to be better equipped for life as a `sit and wait' predator, so they must have developed their post hepatic septum to cope with this limitation. This finding may also have important evolutionary implications, as the true diaphragm in mammals is the main muscle, which supports breathing. Perhaps the mammalian diaphragm also started out as a membrane similar to the post hepatic septum, but evolved from an assisting role, to become respiration's major muscle.