492 LAND ANIMALS 



Low temperature. — The air temperature decreases with altitude 

 at the rate of 1° for 140 m. in the Alps, 1° for 165 m. in the Caucasus, 

 and 1° in 195 m. in the equatorial Andes. In the Swiss Alps, the 

 average minimum in January is — 7.7°, and the average maximum in 

 July is +7.9° at St. Gotthard's Pass (2093 m.). On the Sonnblick, 

 1000 m. higher, the January minimum is — 13.3°, the maximum in 

 summer +9°. The long persistence of the snow and the shortened 

 warmer season leave only two seasons in the alpine zone, a long winter 

 and a short summer. The length of the season with mean temperature 

 above 0° varies from six months at 2000 m. to only two months at 

 3100 in. in the Alps. 4 



These temperature relations exclude many animals from the alpine 

 habitat. The fact that relatively large numbers of poikilothermal 

 animals range up to the snow zone depends on the difference between 

 ground and air temperatures, for, in consequence of reduced atmos- 

 pheric absorption, the temperature of rocks and soil on which the sun 

 shines is considerably higher than that of the air. The difference be- 

 tween ground and air temperature increases with altitude, amounting 

 to 1.5° at 1000 m., to 2.4° at 1600 m., and to 3.6° at 2200 m. This 

 radiant heat is of special importance to the invertebrates which live 

 on or in the soil. North slopes, consequently, are much more im- 

 poverished in their animal life than are south slopes. 4 Snow affords 

 protection against the extreme cold of winter to the hibernating in- 

 vertebrates under it, for when the air temperature is — 17°, the tem- 

 perature of the snow varies from — 15° at the surface to — 1.6° at 

 52 cm. depth. 5 



The number of species present is steadily reduced with increasing 

 altitude in every mountain range. Of the 240 species of snails known 

 from Tirol, 80 range into the alpine zone and only 8 are known from 

 the snow zone. 6 There are 96 species of butterflies in the coniferous 

 forests of the Swiss Alps, while only 27 range into the shrub and 

 meadow zones, and only 8 into the sub-snow zone. Insects with com- 

 plete metamorphosis, which appear to resist temperature changes better 

 than those with incomplete metamorphosis, predominate. Reptiles are 

 very sparingly represented. 



Many animals have their rate of development reduced by the low 

 temperatures. All the insects of the alpine zone are accordingly small 

 species, which develop in a short period, such as the podurids (Col- 

 lembola) which hatch in 8 to 10 days in the lowlands. Many but- 

 terflies, which regularly have two generations at low altitudes have 

 only one in the mountains — Pieris brassicae and Papilio machaon 



