The Absence of Snakes in Ireland

According to legend, lreland in northern Europe has no snakes because a fifth-century monk named Patrick drove the island’s snakes into the sea. In reality, lreland has no snakes because of the interplay between organisms and their physical environment.

Snakes, like all reptiles and amphibians, are ectotherms (cold-blooded): their body temperature and functionality are dictated by the temperature of their external environment. Species diversity of reptiles decreases rapidly from the equator toward the higher latitudes. In North America, the number of lizard species is highest in the warm desert regions of the southwest and declines continuously as you move northward. The same pattern of species diversity is evident in Europe, where the number of reptile species declines markedly as you move from the warmer Mediterranean coast toward northern Europe and the British Isles. Although one snake species, the European adder (viper), is found above the Arctic Circle in Scandinavia, its unusually northern distribution comes at a cost. The adder has a very limited period of activity, often only three to four months a year. In addition, the species may take up to four years to attain sexual maturity, and females may breed only once every three or four years, using the intervening period to build up the fat reserves necessary to produce offspring.

The progressive decline in solar radiation and temperatures from the tropics to the poles not only reduces the abundance and diversity of reptiles but also has a direct influence on their body size. The reason for these patterns is that heat exchange occurs across the surface of a body, but warming usually occurs throughout the entire body’s mass or volume. Large bodies, because of their low surface area to volume ratio, take longer to warm than smaller ones. This physical reality results in upper limits to the size of snakes (and other reptiles) depending on the distance of the snake’s habitat from the tropics. All of the large snakes, such as the anaconda and python, are found within the tropic and subtropical regions. Other large reptiles- such as the iguanas, monitor lizards, and the crocodilians (alligators, caimans, and crocodiles)- -are likewise limited in their distribution to the warm, aseasonal environments of the subtropics and tropics. The maximum body size for ectotherms declines as you move north and south from the equator. This pattern is the exact opposite of that observed for endotherms (warm-blooded animals), where average body size increases from the tropics to the poles, a pattern referred to as Bergmann’s rule. The environmental constraint on the upper limit of body size for ectotherms is at the very heart of the current debate over whether the dinosaurs were cold- or warm-blooded. The fossil record places the dinosaurs well into the northern latitudes, with specimens found in Alaska and Siberia.

As for snakes, the fossil record reveals that the diversity of reptiles and amphibians (terrestrial ectotherms) has always been low in northern Europe. During the Pleistocene epoch (from 2.6 million to 11,700 years ago), a time when the temperature of Earth was considerably lower than that now observed, much of the Northern Hemisphere was covered by glaciers. Although certain refugia (small pockets of land not covered by ice) existed, the massive ice sheet virtually obliterated all life in northern Europe, including Ireland and Britain. As temperatures rose and the glaciers retreated during the Holocene epoch (11,700 years to the present), the region was recolonized by plants and animals, but the cool temperatures of northern Europe proved inhospitable to most snakes. Nevertheless, the island of Britain, similar in climate to lreland, ended up with three species of snakes that survive there to this day.

Three species of snakes is a paltry sum, but nonetheless, it is more than lreland has. Why the difference? After the retreat of the continental glaciers at the end of the last ice age, Britain had a land bridge connecting it with the mainland of Europe, whereas lreland did not. The limited diversity of snakes on the mainland meant that the pool of species available to recolonize both Britain and Ireland was small in the first place. However, lreland’s lack of a land bridge with continental Europe coupled with the limited dispersal abilities of most snake species have to date prohibited their successful recolonization there.

1.Snakes, like all reptiles and amphibians, are ectotherms (cold-blooded): their body temperature and functionality are dictated by the temperature of their external environment. Species diversity of reptiles decreases rapidly from the equator toward the higher latitudes.?In North America, the number of lizard species is highest in the warm desert regions of the southwest and declines continuously as you move northward. The same pattern of species diversity is evident in Europe, where the number of reptile species declines markedly as you move from the warmer Mediterranean coast toward northern Europe and the British Isles. Although one snake species, the European adder (viper), is found above the Arctic Circle in Scandinavia, its unusually northern distribution comes at a cost. The adder has a very limited period of activity, often only three to four months a year. In addition, the species may take up to four years to attain sexual maturity, and females may breed only once every three or four years, using the intervening period to build up the fat reserves necessary to produce offspring.