1 INTRODUCTION
Amphibians can be found in habitats ranging from fully aquatic, to fully terrestrial or arboreal (Hillman et al. 2009). Some habitats are xeric and can potentially cause rapid dehydration, and death due to desiccation. However, some amphibians employ physiological and behavioral mechanisms both to prevent water loss, and to increase water uptake (Bentley 1971). Understanding the successes of amphibians in environments differing in degree of terrestriality requires understanding the intricacies of adaptations for water exchange in different hydric environments (Tracy et al. 2014).
Nearly all anurans (frogs and toads) have a very thin stratum corneum relative to other terrestrial vertebrates (Shoemaker and McClanahan 1975; Drewes et al. 1977) and must maintain hydrated skin to survive (Duellman and Trueb 1994; Jorgensen 1997a ). Water loss rates from amphibians are high relative to other terrestrial vertebrates, and can be lethally rapid (Jorgensen 1991, 1997a , 1997b ; Young et al. 2005; Hillman et al. 2009). Rather than drinking, anurans replace water losses by efficiently absorbing water through their skin (McClanahan and Baldwin 1968; Sinsch 1991; Jorgensen 1994; Hillman et al. 2009; Tracy et al. 2013).
Many frogs are able to absorb water rapidly through a highly vascularized area of ventral skin called the “seat patch”, which is associated with both behavioral and physiological adaptations for water uptake (McClanahan and Baldwin 1969; Bentley and Main 1972). The size and location of this site of facilitated water absorption varies among species of terrestrial frogs and toads (Ogushi et al. 2010a). Behavioral adaptations consist of body posturing, so that the seat patch is adpressed against a wet environmental surface from which water can be absorbed (Heatwole et al. 1969). Physiological adaptations of the seat patch include controlling rates of water uptake via antidiuretic hormones (Bentley 1969; Tracy and Rubink 1978; Cree 1988; Hillman et al. 2009), which can influence the permeability of the seat patch largely by poorly-understood mechanisms that include changes of skin conductance (Tracy 1976, 1982), changes to blood flow (Viborg and Hillyard 2005), and seat patch water potential (Tracy and Rubink 1978; Hillman et al. 2009).
In addition to the need of anurans in terrestrial environments to absorb water from their desiccating environments, anurans in aquatic environments face the challenge of restricting osmotic absorption of water from their environments. If the driving force to water absorption is the osmotic gradient between blood osmotic potential and a fresh water environment, then physiological adaptations are required to prevent excess water uptake for both completely aquatic frogs and other anurans that are unavoidably in water for extended periods. Completely aquatic frogs (e.g. Pipidae) do not have a specialized seat patch (Ogushi et al. 2010a). Nevertheless, the frogs do take up water across their ventral skin (as evidenced below). For the sake of simple expression, we will use the term “seat patch” in reference to the ventral sites of physiological control of water absorption whether or not they are specialized regions with high capillary density.
We studied the seat patch water potentials of six species that differ in use of habitat. The African clawed frog, Xenopus laevis , is a fully aquatic species while the American bullfrog, Rana catesbeiana , is semi-aquatic. The Northern leopard frog, Rana pipiens, and the Western toad, Bufo boreas , are terrestrial species. The Pacific chorus frog, Pseudacris regilla , which can be found both in trees and under logs, and the California tree frog,Pseudacris cadaverina , which is often found on rocks near streams, are considered arboreal species. We hypothesized that because the six frog species are found in a wide range of hydric environments, there would be advantages to them to be able to regulate their water uptake in ways that are adapted for their different hydric environments. Thus, highly terrestrial species would benefit from a seat patch water potential that is much lower than that of the environment to facilitate rapid water uptake or uptake from moist soils rather than free water (Tracy 1976), but fully-aquatic frogs would benefit from a seat patch water potential that is closer to that of water as a way to avoid excessive water uptake. Here, we report on experiments to measure seat patch water potentials of several frog species that differ in their ecological habit.