Osmoregulatory ability predicts geographical range size in marine amniotes

Abstract

Species that are distributed over wide geographical ranges are likely to encounter a greater diversity of environmental conditions than do narrowly distributed taxa, and thus we expect a correlation between size of geographical range and breadth of physiological tolerances to abiotic challenges. That correlation could arise either because higher physiological capacity enables range expansion, or because widely distributed taxa experience more intense (but spatially variable) selection on physiological tolerances. The invasion of oceanic habitats by amniotic vertebrates provides an ideal system with which to test the predicted correlation between range size and physiological tolerances, because all three lineages that have secondarily moved into marine habitats (mammals, birds, reptiles) exhibit morphological and physiological adaptations to excrete excess salt. Our analyses of data on 62 species (19 mammals, 18 birds, 24 reptiles) confirm that more-widely distributed taxa encounter habitats with a wider range of salinities, and that they have higher osmoregulatory ability as determined by sodium concentrations in fluids expelled from salt-excreting organs. This result remains highly significant even in models that incorporate additional explanatory variables such as metabolic mode, body size and dietary habits. Physiological data thus may help to predict potential range size and perhaps a species' vulnerability to anthropogenic disturbance.

Keywords: distribution; marine; osmoregulation; salinity; tetrapod.

Figure 1.

Relationships between maximum (a), minimum (b), range (c) and mean (d) salinity and geographical marine range size across the species included in our study. Each point represents data for a single species.

Figure 2.

The relationship between osmoregulatory ability (standardized sodium concentration in the urine for mammals (a) or secreted by the salt gland for birds and reptiles (b,c)) and the size of the geographical marine range in tetrapods. For mammals (a), symbols as follows: black for Mustelids, light grey for Pinnipeds, dark grey for Cetaceans and white for Sirenia. For birds (b), symbols as follows: circles: black for Anseriforms, white for Gruiforms, light grey for Pelecaniforms, dark grey for Phalocrocoracidae; squares: black for Procelariforms, white for Sphenisciforms, light grey for Charadriiforms. For reptiles (c), symbols as follows: circles: black for sea snakes, white for sea kraits, light grey for Homalopsid snakes, dark grey for Acrochordid snakes; squares: black for Crocodilians, white for Emydidae turtles, light grey for Cheloniidae sea turtles and dark grey for Dermochelyidae. Each point represents data for a single species.