Aspects of tile breeding biology of the Common Frog, Rana temporaria temporaria L., were studied in relation to altitude. Work centred on 55 breeding ponds in northern England, ranging in altitude from 86 to 838m. A chemical temperature integration technique was used to measure pond and soil temperatures at a series of sites. The temperature fell by approximately 0.4 C for every l00m increase in altitude. Between 1974 and 1977 spawning was always later at higher altitudes. By delaying spawning, highland embryos were less likely to encounter lethally low temperatures. Highland female frogs were about five per cent smaller in snout-vent length than lowland females and produced 707 eggs on average, less than half the mean number of 1,586 eggs produced by lowland frogs. These differences were thought to be due to the short growing season and possibly to the lack of food at higher altitudes. At 6 C the eggs from highland females developed four per cent faster than those from lowland females. Highland eggs had a lower lethal limit for normal development of 2.8 C; one Celsius degree below the limit for lowland eggs. The mucopolysaccharide capsules covering the eggs of Common Frogs act as insulators, keeping the embryos warmer on average than the surrounding water. This is thought to be of importance as Common Frogs breed early in the year when pond temperatures are often close to the lower lethal limit. The volume of the capsules and their insulating efficiency varied in different pond waters. The concentration of the ions in the pond water used as the culture medium was found to be the major factor influencing capsular swelling, but the valency of the ions, the temperature and pH of the water were also important. These findings were discussed in terms of their adaptive significance
