All down to size
MORE than 175 years ago, biologist Carl Bergmann noticed that creatures living at colder latitudes tended to be larger than those at warmer ones. He postulated ‘Bergmann’s Rule’, which states: “In cold climates, large body mass increases the ratio of volumeto-surface area and provides for maximum metabolic heat retention in mammals and birds.” This rule has been successfully applied to many bird species and appears most significant for resident, sedentary species, as well as to those which have open nests. But there are always exceptions, such as Redwing and Northern Wheatear.
One recent study looked at Eurasian Wren populations in the UK as these are known to fare badly during cold winters. Birds from the milder south-west of England were 5% lighter than those in eastern Scotland. In other words, northern birds are larger and heavier, and island subspecies (think the likes of Shetland and St Kilda) can more easily survive the winters there.
Climate change is likely to have effects which conform with Bergmann’s Rule. Research in North America, looking at birds over a 38-year period, showed that they had decreased in size by an average of 2-4%. This reduction in size is likely due to warmer conditions, which will enable smaller birds to survive more readily. A similar study in the Amazon rainforest showed species getting smaller by up to 2% per decade.
In North America, one species which illustrates Bergmann’s Rule well is Song Sparrow. With at least 25 subspecies over its range, which vary widely in both size and appearance, it is an ideal species to study. Recently its whole genome has been sequenced, from the small subspecies of California (17 g) to the much larger birds of Alaska (50 g). These revealed candidate genes linked to body mass. This suggests there is a genetic basis for Bergmann’s Rule, which would fit with natural processes such as climate influencing the evolution of species.
Two East Asian species, Oriental Magpie and Japanese Tit, were studied to see how climate affected them. The tits were shown to have the smallest bodies in the warmest climates, in line with Bergmann, while the magpies showed little difference. However, the magpies in northern regions had smaller bills and shorter legs than those in the south, which fits with Allen’s Rule, relating to the size of limbs and appendages. It predicts that in cold climates animals will have shorter and thicker limbs and appendages.