The Cane Toad - now a Top End resident
Although we might not like it, the cane toad is now a resident of the Top End, and this handsome specimen (right) appeared on the cover of the very first issue of the new journal Biology Open. Previous work (from researchers at the University of Sydney) has shown that cane toads at the invasion front are different from those in populations that have been around for a long time. Their behaviour is different (they move more), and they have slightly longer hind legs, and these characteristics are consistent with being good dispersers. RIEL staff (with colleagues from the University of Sydney and Monash University) surmised that these differences may also be associated with physiological differences.
A wide range of physiological and biochemical measurements were made on toads from Cairns (a population with a very long history - 1936), Borroloola (a population with about a 20 year history), and Timber Creek (which was the invasive front at the time the work was done). Although there were some very significant differences among the populations with respect to some of the physiological parameters measured (including the time the toads were able to hop on a treadmill and their resulting metabolic rate), none of these were consistent with the idea that toads at the front are better suited for dispersal.
The authors concluded that natural selection will act most directly on whole organism performance, not underlying physiological capacities, so whole organism performance should correlate with behavioural ecology more closely than physiological abilities. Thus, behaviour can act as a buffer for selective pressures on physiology, resulting in relatively weak selective pressure on the underlying physiology associated with characteristics such as locomotion.
The "water holding" frog
Meanwhile, this handsome specimen (left) is a native "water-holding" Cyclorana frog that inhabits central deserts, but these were studied at the northern-most part of their range (the bottom of the Top End) along with four of its close relatives from the same area. These five species of frogs are active in the wet season, but during the dry season they go underground and form cocoons. Although cocoons slow the loss of water to the air, the air underground is pretty humid even when the soil is dry. A recent RIEL PhD graduate, Steve Reynolds (along with myself and RIEL colleague Chris Tracy) showed in 2010 that the main function of the cocoon was to prevent water loss to the soil. In the recent paper, RIEL staff, along with colleagues from Ohio State University, looked at the biochemical composition of the cocoons and the frog skin to see exactly how cocoons form a "waterproof" barrier to the environment. It has long been known that the cocoon is made of multiple layers of shed skin, and it has been assumed that the dead skin is responsible for creating a resistance to water loss.
In this recent paper, however, it was shown that the frogs secrete lipids that are not normally found in the skin when they produce the cocoon. So the cocoon is made of layers of skin that have specific lipids in between the shed layers, and these lipids greatly enhance the water resistance of the cocoons. Furthermore, the kinds of lipids they produce are the same sorts of lipids that reduce water loss the skin of birds and mammals.