Geographical Variation in the Size of Body Organs in Seabirds

1. The size of body organs shows adaptive temporal variation in many animal species. The variation in the size of body organs was examined to see if it is also a component of local adaptation to geographical variation in ecological conditions. 2. Major body organs were measured in five species of Icelandic seabirds, sampled from two areas where birds experience slightly different ecological conditions. Between-area differences in ecological conditions were consistent among the study species, allowing tests of the generality of ecological effects on organ size. 3. All major body organs showed geographical size variation. Livers and kidneys were larger in locations where daily energy expenditure was expected to be higher, small intestines were heavier where food was of lower energy density; stomachs were heavier where food was less digestible; heart and flight muscle were larger where flight costs were greater. 4. It is concluded that adaptive variation in organ size may be an important means by which animals optimize exploitation of their local environment, and may be a proximate factor in intraspecific life-history and metabolic variations between geographically separate populations.