This story appears in the April 2011 issue of National Geographic magazine.
Emerging from a partially frozen pond more than 6,000 feet high in the Alps, European common frogs (Rana temporaria) set out to find mates and begin breeding activities. These frogs have adapted to a wide variety of conditions, increasing their range over most of Europe. Here on the Massif de Beaufort in the French region of Savoy, ice may not thaw until June, leaving only a brief window of warm weather for females to lay eggs and tadpoles to metamorphose into juveniles, known as froglets. Cold-climate frogs grow more slowly than their relatives in temperate areas, but live longer (12 years, compared to 5 for lowland frogs) and so grow larger. They're also active during warmer, daylight hours, unlike their kin elsewhere.
Frogs mate in a position called amplexus, with the smaller male clasping the female from behind in a ride that can last two days or more. As she lays eggs, he expels sperm to fertilize them. Though egg laying takes place in spring, frog pairs in mountain ponds can begin hibernation in amplexus—a months-long embrace that may provide a breeding advantage by allowing mating as quickly as possible once warm weather arrives. Eggs of high-elevation frogs may be 30 percent larger than those of lowland females, giving tadpoles a head start. Eggs and tadpoles of mountain frogs have developed resistance to genetic damage from ultraviolet radiation, a component of sunlight that is more intense in the thinner air of high altitude.