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 i t is also a component of local adaptation to geographical variation in ecol ogical conditions. 2, Major body organs were measured in five species of Icelandic seabirds, s ampled from two areas where birds experience slightly different ecological conditions. Between-area differences in ecological conditions were consiste nt 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 kid neys were larger in locations where daily energy expenditure was expected t o be higher; small intestines were heavier where food was of lower energy d ensity; stomachs were heavier where food was less digestible; heart and fli ght muscle were larger where flight costs were greater. 4, It is concluded that adaptive variation in organ size may be an importan t means by which animals optimize exploitation of their local environment, and may be a proximate factor in intraspecific life-history and metabolic v ariations between geographically separate populations.