Nest construction by túngara frogs is a sophisticated process in terms of nest architecture and construction process. The initial bubble raft phase seems wasteful of material, energy and delayed oviposition, but may be essential to the production of a correctly proportioned mixture of the female's secretions and water. Mixing precursor fluid directly with pond water would result in rapid dilution of the surfactant proteins required to create foam, so the local concentration of foam components would first need to be raised to an appropriate level before the main building phase can begin. The gradual increase in the duration of mixing events in this phase may solve this problem; prolonged mixings initially may disperse and dilute the fluid excessively, whereas a gradual increase would progressively lead to a coalescent bubble raft until a critical concentration is reached for full quality building foam. The gradual increase in duration might also result from a progressive reduction in viscosity of the medium being processed; the males initially mix foam fluid directly with water, and then increasingly with air as the foam develops. So, the increasing duration of mixings could be functionally important in progressing to a critical concentration of foam materials, or a result of the changing effort required, or both. The fact that pairs return to a nest after disturbance may mean that once females begin to release eggs, they cannot regress to the bubble raft phase without wastefully releasing eggs.
A key element in the production of correct proportions of nest foam precursor fluid and its mixing with water will be the positioning of the pair relative to the water surface. The males were always observed to keep their lungs inflated throughout nesting (electronic supplementary material, video S1), which will have the effect of providing buoyancy and lateral stability for correct positioning at the water surface, and accommodate differing depths of water.
The advantages to the egg-free cortex may be manifold. First, protecting the eggs from dehydration by distancing them from exposure to air. Second, shielding the eggs from light damage; the nests are exposed to direct sunlight but the white foam will be scattering light of all visible wavelengths effectively. Ultraviolet light should be similarly scattered or absorbed by protein in the foam. Túngara frogs have unpigmented or faintly pigmented eggs, so the depth of the cortex through which any stray light must pass would be important for light scatter to be adequately protective. Third, the depth of the double cortices of adjacent nests in a communal nest will probably limit cross-fertilization of eggs deposited in the cores. In relation to this, we have noticed that, during initiation of communal nestings, the male of one pair will issue a call that induces an encroaching pair to move away (M. W. Kennedy 2002, 2004, 2008, unpublished data). Lastly, a cortex would reduce access to the eggs by predators or parasites, which would be given added effect by surfactants in the foam as anti-insect defence (Rostás & Blassmann 2009
Colonial nesting appears to be preferred by túngara frogs and allied species (Ryan 1985
; Zina 2006
), but how do the advantages counterbalance the risks? Colonial nests may be more attractive to specialist nest parasites such as frog flies (Downie et al. 1995
; Vonesh 2000
; Menin & Giaretta 2003
; Giaretta & Menin 2004
) because such nests are probably easier to locate, and infestation of one nest could permit access to all. Advantages to the frogs may include a decreased aggregate surface : volume ratio, and hence reduced evaporative water loss (Zina 2006
). Additionally, joining a nesting already in progress allows exploitation of a pre-existing bubble raft and consequent saving of nest material, energy expenditure and time exposed to predators.