THE AVIAN SPLEEN - A NEGLECTED ORGAN

The unctions of the little-studied avian spleen are reviewed and compa red with those of its better known mammalian counterpart, which is gen erally larger in proportion to body size than in birds. A role in immu nity similar to that in mammals is evident, but the organ's contributi on do oxygen supply seems less extensive; splenic storage of erythrocy tes, for example, is unrecorded for birds. The spleen is a principal o rgan of systemic immunity, and its importance in disease resistance is presumably accentuated by the scarcity of avian lymph nodes. The stri king intraspecific: variation in size partly reflects seasonal changes in spleen morphology and activity. Several explanations, principally based on changing oxygen demand, have been proposed previously for the se periodical cycles. But seasonally small spleens could sometimes sim ply stem from a combination of (1) a cessation of active splenomegaly as seasonally patent infections recede, and (2) a seasonal lymphoid in volution, occurring even if an individual has not recently responded t o, and recovered from, an infection. Possible determinants for these a nd other processes are discussed from evolutionary and ecological pers pectives. There is a pressing need for a thorough investigation of bot h hematological and immunological functions, using a phylogenetically and ecologically broad range of species, as well as modern histologica l and experimental techniques.