Jan-Peter Hildebrandt

• Hyperplasia and hypertrophy, or their counterparts hypoplasia and hypotrophy, are elements of the adjustment of organ size and function in animals according to their needs under altered environmental conditions. As such processes are costly in terms of energy and biomaterials, it is assumed that they are beneficial for the survival of the individual. The ability of animals to perform such adjustments and the limitations in the scope of the adjustments are considered to be adaptive genetic traits which enable individual animals to survive regularly occurring changes in the environmental conditions in their habitats as long as such changes stay within critical limits. The restructuring of mono-functional glands in ducklings, which serve the animals in getting rid of excess amounts of ingested salt from the body, is presented as an example of complex plastic changes in organ structure. Phenotypic adjustments in these salt glands encompass both reversible processes, when environmental conditions switch back to the original state (‘phenotypic elasticity’), and irreversible ones (‘phenotypic plasticity’ in the narrow sense). As more information on genomes or transcriptomes of non-model animal species becomes available, we will better understand the biological significance of such phenotypic adjustments in animals in their natural environments and the underlying molecular mechanisms.