For insect-eating lizards, island living means going hungry every now and then. To make the most of scarce food resources on isolated Greek isles, Balkan green lizards (Lacerta trilineata) have evolved a special digestive system that allows them to absorb nutrients better. The work, published in The Science of Nature, helps explain how animals are able to successfully colonize islands.
Reptiles have been known to adjust their food preferences in response to challenging conditions like low rainfall or food supply that’s limited (in time and quantity). Previous studies found that Balkan green lizards on Greek islands widened their diet to include more plants: about 30% of their diet consists of plant material, compared to the 10% for mainland lizards.
To study the impact on digestion, a trio of researchers led by Kostas Sagonas from the University of Athens compared adult male Balkan green lizards from populations on mainland Greece with those living on the islands of Andros and Skyros. Island lizards, they found, had longer gastrointestinal tracts and gut passage times. This exposes food to digestive enzymes for longer, resulting in higher digestive efficiency. In addition to a longer small intestine and hindgut, lizards from Skyros also have longer stomachs.
Then, when the team dissected their hindgut, they discovered that structures called cecal valves appear more frequently in island-dwelling lizards. Typically found in plant-eating lizards, cecal valves help slow the passage of food and provide chambers for microbes that ferment and break down plant material into fatty acids. The researchers found cecal valves in 62% of the island lizards and just 19% of the mainland ones – which reflects how these insect-eaters have started to consume a more varied diet that includes plants.
Because of these cecal valves and their longer digestive tracts, food eaten by island lizards takes 26% longer to pass through. Retaining food for longer maximizes energy income and increases the amount of nutrients extracted. "Such adaptations allow insular populations to take advantage of the limited food resources of the islands and, eventually, overcome food dearth," Sagonas said in a statement.