Veggies in the making

MANY plant-eating species descend from meat-eating lineages. Indeed, adorable bamboo-munching giant pandas belong to the order Carnivora. However, it is rare to catch this evolutionary transition to vegetarianism in action. Now a team of biologists has done so with a lizard.

Balkan green lizards (pictured below) inhabit the Greek archipelago as well as the mainland. Formally, they are an insectivore that occasionally nibbles on plants. But Kostas Sagonas of the University of Athens suspected that food scarcity on the Greek islands may be driving lizards which live there to feed on plants more often than mainland kin.

To find out, Dr Sagonas measured the bite force of the lizards. He knew from previous studies that herbivorous species tended to have stronger jaws to chop through tough plant material than carnivorous ones. His suspicions were fuelled when he found that the island lizards had an average bite force 18% stronger than mainland lizards. An analysis of what the lizards ate, by studying their stomach contents and faecal material, showed the island dwellers were consuming 30% more plant material than mainland ones.

These discoveries, reported in Science of Nature, led Dr Sagonas to wonder if increased herbivory might be driving other changes in the island lizards. He and his colleagues collected 74 of them, half from the islands of Andros and Skyros, and half from two mainland locations, the Stymphalia Lake area and the Karditsa Plateau. The lizards were given a couple of weeks to acclimatise in a laboratory on a diet of meal worms before food was withheld for four to five days. Each lizard was then fed a single worm with a small plastic marker embedded inside it. The team timed how long it took for the marker to travel through the lizards’ digestive systems: 23% longer on average for the island lizards. This suggested that the guts of the island lizards might actually be longer. So, the researchers took a look inside them.

Seven lizards from each of the four locations were dissected and, despite all being the same size, their organs differed markedly, with the island dwellers having hindguts and intestines around a quarter longer than those of mainland lizards. However, what really surprised Dr Sagonas was the discovery of specialised chambers called caecal valves.

Somewhat like a human appendix, caecal valves function as a place where symbiotic bacteria can help break down plant material so that it can be more fully digested. Such valves have been found in some other herbivorous reptiles, like the green iguana, but have never been seen before in the Balkan green lizard. Although the valves turned up in both island- and mainland-dwelling lizards, they were common in the islanders, appearing 62% of the time, but in the mainlanders just 19% of the time.

All of these findings lead Dr Sagonas to argue that insect scarcity in the Greek archipelago is causing lizards there to rely more heavily on eating plants and that this, in turn, is causing natural selection to favour anatomical traits that support herbivory. The lizards still interbreed and are thus one species. It seems unlikely that will remain the case for very long.