Bubble nests of foam-nesting frogs are constructed with precision using a three phase process.
“Frogs that build foam nests floating on water face the problems of over-dispersion of the secretions used and eggs being dangerously exposed at the foam : air interface. Nest construction behaviour of túngara frogs, Engystomops pustulosus [formerly Physalaemus pustulosus], has features that may circumvent these problems. Pairs build nests in periodic bursts of foam production and egg deposition, three discrete phases being discernible. The first is characterized by a bubble raft without egg deposition and an approximately linear increase in duration of mixing events with time. This phase may reduce initial over-dispersion of foam precursor materials until a critical concentration is achieved. The main building phase is marked by mixing events and start-to-start intervals being nearly constant in duration. During the final phase, mixing events do not change in duration but intervals between them increase in an exponential-like fashion. Pairs joining a colonial nesting abbreviate their initial phase, presumably by exploiting a pioneer pair’s bubble raft, thereby reducing energy and material expenditure, and time exposed to predators. Finally, eggs are deposited only in the centre of nests with a continuously produced, approximately 1 cm deep egg-free cortex that protectively encloses hatched larvae in stranded nests.” (Dalgetty & Kennedy 2010:293)
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“Several tropical frogs, known as foam-nesters, also build a nest of bubbles. The mother exudes a fluid and beats it into microscopic bubbles with her hind legs. She then lays her eggs inside, and her mate, who has clung to her back throughout, fertilizes them. As the parents leave, the outer bubbles harden to form a protective case that encloses a foamy core of several thousand eggs. This foam nursery provides shelter from predators, bacteria, and sunlight, as well as preventing dehydration. Because the foam is mostly air it supplies all the embryos’ oxygen needs until well after hatching. The nest then disintegrates, and the young emerge from the crowded apartment and, all being well, drop into the water below.” (Downer 2002:54)
“The foam nests of the túngara frog (Engystomops pustulosus) [formerly Physalaemus pustulosus] form a biocompatible incubation medium for eggs and sperm while resisting considerable environmental and microbiological assault. We have shown that much of this behaviour can be attributed to a cocktail of six s, designated ranaspumins (Rsn-1 to Rsn-6), which predominate in the foam. These fall into two discernable classes based on sequence analysis and biophysical properties. Rsn-2, with an amphiphilic amino acid sequence unlike any hitherto reported, exhibits substantial detergent-like surfactant activity necessary for production of foam, yet is harmless to the membranes of eggs and spermatozoa. A further four (Rsn-3 to Rsn-6) are lectins, three of which are similar to fucolectins found in teleosts but not previously identified in a land vertebrate, though with a carbohydrate binding specificity different from previously described fucolectins. The sixth, Rsn-1, is structurally similar to proteinase inhibitors of the cystatin class, but does not itself appear to exhibit any such activity. The nest foam itself, however, does exhibit potent cystatin activity. Rsn-encoding genes are transcribed in many tissues of the adult frogs, but the full cocktail is present only in oviduct s. Combinations of lectins and cystatins have known roles in plants and animals for defence against microbial colonization and insect attack. Túngara nest foam displays a novel synergy of selected elements of innate defence plus a specialized surfactant protein, comprising a previously unreported strategy for protection of unattended reproductive stages of animals.” (Fleming et al. 2009:1787)
