A black and white tegu. Photo credit: Bernard Dupont, CC BY-SA 2.0, via Wikimedia Commons.
Sitting and waiting for a meal to pass by is a common tactic among reptiles. But many lizards can't wait around all day for their food, so they must search out their meals. However, this poses a problem as lizards have trouble breathing while they walk – the bending movement can cause their internal organs to compress their lungs, making breathing difficult. Some lizards may have evolved a solution to this problem. Separating their liver and lungs from the rest of their internal organs is a thin membrane called the post-hepatic septum which gives the lungs room to expand without being crowded by other internal organs when the lizard is walking. But what happens when the digestive system is full of food, or a female is full of eggs before she lays them? This tantalizing question led Alan Marini, Wilfried Klein and colleagues of the Universidade de São Paulo, Brazil, to examine whether having the post-hepatic septum really helped with breathing and whether a full stomach would hinder the breathing of South America's largest lizard, the black and white tegu (Salvator merianae).
Working with six tegus, Marini and Klein began the painstaking process of measuring the volume of each tegu's lungs and how expandable they were by slowly filling them with air. They then carefully surgically inserted a balloon in the tegu's abdominal cavity which they could fill with water so that it weighed 15% of the tegu's body mass. ‘Tegus can ingest great amounts of food at once (up to 12% of their body mass), or in the case of a female tegu, carry eggs within their reproductive tract representing about 40% of a female's body mass’, says Klein, explaining that this amount of added weight is not unusual for tegus. Unsurprisingly, the water-filled balloon decreased the amount of air left in their lungs after a normal breath and how much the tegus could inflate their lungs, as there was less room for the lungs to expand. Having the balloon in the way also made it harder for the reptiles to inflate their lungs, ‘just as it takes more effort to inflate a small balloon compared with a large one’, explains Klein.
The researchers then removed the post-hepatic septum and repeated their experiments. Compared with when the septum was intact, removing the thin membrane further restricted the lung volume of the tegus when the balloon was filled, suggesting that keeping the lungs in a separate compartment from the other internal organs helps the tegus to breathe more normally when they are full of food. But could the post-hepatic septum also reduce the effort it takes these full tegus to breathe?
When the balloon was inflated, the researchers saw a very small increase in the amount of work that it took for the tegus to breathe. If the post-hepatic septum was removed, the amount of work nearly doubled, and if the balloon was inflated at the same time, the effort it took to breathe tripled. This was especially true when the number of breaths per minute increased but it depended more on the amount of air in each breath. This suggests that the post-hepatic septum is an effective barrier against the encroaching abdominal organs, helping the tegus expend less energy on filling their lungs. While other lizard species have different adaptations that help them cope with breathing after eating a gluttonous meal, there is no doubt that the post-hepatic septum found in black and white tegus is effective at making full and/or pregnant tegus breathe a little easier.
