Lizards

In 1971, biologists moved 5 adult pairs of Italian Wall lizards from their home island of Pod Kopiste, in the South Adriatic sea to the neighboring island of Pod Mrcaru. Podarcis Sicula which mainly eats insects, was present on Pod Kopiste but there were none on Pod Mrcaru. Largely insect eaters, the half foot long reptiles would find themselves in a terrain that had insects in short supply.

Because of political upheaval in the region (the adriatic sea borders croatia), it was not until 2004 that Duncan J. Irkshik, a professor of biology at University of Massachusetts and a team of international biologists could return to find out what had happened to the lizards.

THE LIZARDS OF POD MRCARU

- a real life study of Evolution

Duncan says:

"As a scientist, it was a once in a lifetime opportunity. Historical circumstances prevented people from going back to the island for a very long time. So when we first went out there in 2004, we didnt know what we would find. We chartered a boat out to the island, and it was amazing. The island was swarming with lizards."

Pod Mrcaru Lizards

Duncan:

"Pod Mrcaru is a tiny rock in the middle of the water. You could walk from one end of the rock to the other in 5 minutes. It has a Mediterranean climate - hot and very dry. Its mainly rock with little shrubby plants sticking up out here and there. It looks very inhospitable."

Researches returned to the island twice a year for 3 years, in the spring and summer of 2004, 2005 and 2006. Captured lizards were transported to a field laboratory where their snout-vent length, head dimensions and body mass were measured. Tail clips taken for DNA analysis confirmed that the Pod Mrcaru lizards were genetically identical to the source population on Pod Kopiste.

Two of the most striking changes that were observed in the lizards were increase in head size and change in head shape. Observed differences in head morphology were caused by adaptation to a different food source. Lizards on Pod Kopiste were well suited to catching mobile prey, feasting mainly on insects. Life on Pod Mrcaru offered them an abundant supply of plant foods including the leaves and stems of native shrubs.

Italian Wall Lizard - Source populant of Pod Kopiste

Analysis of the stomach content of the lizards of Pod Mrcaru showed that their diet included two-third plants, depending on the season, a large increase over the population of lizards on Pod Kopiste.

Graphs illustrating difference in diet between population in spring (A) and summer (B). Differences in the proportion of plants (black bars), invertebrate prey (white), and the rest fraction (gray) are highly significant between populations. Seasonal differences in diet that were highly significant on Pod Mrcaru but not on Pod Kopiste. Error bars depict 1 standard deviation.

Duncan:

"Individuals on Pod Mrcaru have heads that are longer, wider and taller than those on Pod Kopiste, which translates into a big increase in bite force. Because plants are tough and fibrous, high bite force allows the lizard to crop smaller pieces from plants, which can help them break down indigestible cell walls."

Examination of the lizard's digestive tracts revealed something even more surprising. Eating more plants caused the development of new structures called cecal valves, designed to slow the passage of food by creating fermentation chambers in the gut, where microbes can break down the difficult to digest portion of plants. Cecal valves, which were found in hatchlings, juveniles and adults on Pod Mrcaru has never been reported for this species including the source population on Pod Kopiste.

Photographs illustrating the cecal valves in a male (A), a female (B), and a hatchling (C) P. sicula from Pod Mrcaru. Note the thick cecal wall and pronounced ridges. The arrow in C indicates the position of the cecal valve in a hatchling as seen from the outside

Duncan:

"These structures occur less than 1% of all known species of scaled reptiles. Our data shows that evolution of novel structures can occur in extremely short time scales. Cecal valve evolution went hand in hand with a novel association between the lizards on Pod Mrcaru and microorganisms called nematodes that break down cellulose which were found in their hindguts."

Change in diet also affected the population density and social structure of the Pod Mrcaru population. Because plants offer a larger and more predictable food supply, there were more lizards in a given area in Pod Mrcaru. Food was obtained through browsing than active pursuit prey and the lizard has given up defending territories.

Oddly, Pod Mrcaru's original lizard residents, Podarcis Melisellensis were nowhere to be found. There used to thrive a healthy population of lizards which were less agressive than the new implants. The new species apparently wiped out the indigenous lizard populations, although how it happened is not known.