The insular lizards’ astonishing behavior
José Pichel Andrés/DICYT Every year, Valentín Pérez Mellado, Zoology Professor of University of Salamanca, spends a great deal of his time at Isla del Aire, a small island situated in the South-east of Menorca, where he has been studying the endemic black lizard’s fascinating behavior and the interactions that it establishes with its surroundings for decades.
In 1997, Valentín Pérez Mellado described, at the same time that a CSIC team who worked independently, the first well-known case in the world of a reptile which pollinated a plant. It was the Balearic lizard (Podarcis lilfordi), which climbs at the marine fennel (Crithmum maritimum) in order to sip the nectar of its flowers. The grains of pollen get stuck and, while they move from plant to plant, they pollinate them. Nowadays, there are other similar cases, but that “spectacular result” was only the first of a series of breakthroughs related to this reptile.
Podarcis lilfordi has only survived in the islets that surround Mallorca and Menorca since the romans introduced predators which killed it in the larger islands. In fact, it is one of three unique endemic species of Baleares, along with another lizard, Podarcis pityusensis, which lives in Ibiza and Formentera, and the Balearic toad (Alytes muletensis) of Sierra de Tramontana. The insular ecosystems offer “quite peculiar phenomena”, comments the researcher in an interview with DiCYT. The shortages of fauna make many animals live without the predator’s menace, therefore there is high population density, though they lose their defense mechanisms and, sometimes, have few resources to feed, as a consequence these lizards became omnivores.
Especially in April, the subspecies that inhabits the Isla del Aire, Podarcis lilfordi lilfordi, has an astonishing relationship with a plant which probably arrived in the island a few decades ago, the rapa mosquera (Drucunculus muscivorus). “It has quite sophisticated pollination mechanisms” compared to those that deceive some insects, since it “imitates a mammal corpse” through a big-pinkish-hairy leaf” and, above all, an orifice from which a thermogenic floral axis comes out, that is, capable of producing heat, which has a strong smell of rotting meat. Therefore, it attracts flies that lay their eggs on the rotting meat, enter the orifice and get stuck. That is why it is also known as “fly swallower”, but it is not a carnivorous plant. “The male and female flowers are inside the orifice, the fly arrives full with other plants’ pollen and, when it tries to escape, it lets the pollen go and fertilizes the female flowers; afterwards, these flowers close their stigmas and the male flowers begin producing pollen which falls on the fly, after many hours, the fly is released in order to attend to another plant and the process starts again”, explains Valentín Pérez Mellado.
If this mechanism is fascinating, the relationship between Drucunculus muscivorus and the Balearic lizard observed by scientists in the past years is still more fascinating. The researchers believe that the plant arrived at Isla del Aire less than 70 years ago. In fact, it had been seen there just a short time ago; nevertheless, suddenly, the islet was invaded by the rapa mosquera.
Temperature and food
What has really happened? The interaction with the lizard was essential. The reptiles need thermoregulation and, in fact, some of Valentín Pérez Mellado’s team’s experiments have demonstrated that they always maintain a temperature that varies one degree centigrade, something which they accomplish looking for the sun or the shadow, although they use other heat sources. “The rapa mosquera blossoms in April and the thermogenic floral axis may be until 14 degrees above the environment temperature”, explains. That is why the Balearic lizard approaches the plant to regulate its own temperature. This way, the reptile has also discovered a source of food in the insects that get closer to the plant, to such an extent that they are able to hear the flies while trapped, as some of the experiments have demonstrated. “We have put the flies in recipients in a way that the flies could neither see nor smell them, but hear them, and they have always approached the ones which contained the fly”, explains.
In conclusion, the presence of the plant is extremely beneficial for the reptile, but, in a certain way, this relationship has also helped a lot the Drucunculus muscivorus dispersion. In 1999, there were about 3.000 plants and now there are about 35.000 in an island with only 32 hectares: “It is impressive, the whole island smells of corpse in April”, says the herpetologist. The scientists have asked themselves how it was possible and have discovered that in June, when the floral axis fructifies in the form of cluster, the lizards eat them and up to 95% of their excrements contain the rapa mosquera’s seed. This way, the dispersion intensity is formidable, an extra fast evolutionary phenomena. “We thought before that these things have been happening during many generations and for thousands of years and now we know that they may happen extremely fast. The relationship between the plant and the lizard has caused such an impact that it has appeared in one of the chapters of BBC television series Life in Cold Blood of David Attenborough, the famous broadcaster.
Shy individuals and bold individuals
In the last years, the scientists have been asking themselves what made the lizards have a behavior like that towards a new plant in the island and, this way, they have made more unique discoveries in reptiles about what science calls the continuous shy-bold, that is, in animal populations there are many shy individuals and few bold ones. This idea explains a lot how evolution works. “The brave ones are the first that go after a new resource, what looks good, but also risky, while the shy ones imitate them and go on safe”, points out.
Many experiments with the Balearic lizard are confirming the existence of these types of individuals in a quite different number, with an adaptation value. For example, when the scientists place two pieces of fruit, the reptiles always choose the one that is already being eaten by other members of their species.
Other more elaborate experiments have shown that the bold ones are capable of solving problems and, if they are able to solve them satisfactorily, the rest of the individuals imitate them. One of them was a complex ramp to obtain food. “If one of the lizards is able to find the way, the other animals are capable of understanding the mechanics through visual observation”, claims the researcher. Besides, it has been proved that even if the problems are different, they are always solved by the same individuals.
“Cultural transmission”
The implications of this research to understand behavior and evolution are huge. “The imitative behavior is cultural transmission, it could not be adaptive if it was at the animals’ genome, because the food available in an island is unpredictable, for example, a storm may attract jellyfish and these lizards eat them if there is an individual who dares to do it for the first time”, explains. In fact, “human society works likewise” and the distribution of shy and bold has a great adaptive value.
In cooperation with University of las Islas Baleares, the researchers of University of Salamanca have been making tests in order to understand these behaviors and, above all, the key that distinguishes the shy and the bold individuals. A small island with a great population density is the ideal laboratory to make progress in a passionate line of research which has been contributing with unique information about the animal kingdom.