More Foam Nesting Frogs

Another species of frog we found in Lopinot Valley on Thursday morning was the Whistling Frog, Leptodactylus fuscus. Whistling frogs lay their eggs in foam nests within small burrows at locations that will flood with increased rainfall. Locating nests is tricky. But, Roger Downie has developed a technique that includes probing the soil with small sticks until a nest is found, removing the mud cover, and exposing the foam nest. Below is a photo of a whistling frog foam nest and an adult Leptodactylus fuscus. As rainfall increases, the nest will be covered with water and the tadpoles will escape into the pool, or be washed into a pool by runoff. Should the rains be delayed the tadpoles will stay in the nest and produce more foam (undoubtedly with a different chemical composition) to prevent desiccation and deter predators. Which is why Dr Paul Hoskisson is a microbiologist of Strathclyde University is here. Hoskisson is examining the structure of proteins in frog foam, both foam made by the parent frogs initially building the nest and the foam mad by the tadpoles.

The Foam Nest of the Tungara Frog

This morning a walk in Lopinot Valley produced several frog nests after last night's rain. Below is a photo of a nest made by the túngara frog, Engystomops pustulosus.  My companions were interested in studying the proteins involved in making these nests. So, here is some background on nest construction and function of the foam.

The widespread and abundant túngara frog, builds foam nest on the surface of small temporary pools of water in the Neotropics. The male collects eggs and foam precursor fluid from the female's cloaca with their feet, and create a rounded mound of foam incorporating pond water, the eggs, and proteins with a mixing action of their legs. Construction of the nest is a series of mixing events that takes about an hour. At first observation nest construction appears to be a simple process of regular, periodic bouts of egg release and foam production that should result in eggs being disorderedly distributed throughout the foam. But, Dalgetty and Kennedy (2010) showed the nesting process is sophisticated, occurs in three phases, and that the nests have a differentiated internal structure.

They found that a semi-dispersed raft of bubbles is produced initially, during which no eggs are released, followed by the appearance of a growing mound into which eggs are incorporated. The males keep their lungs inflated throughout the nesting process, and arch their spines considerably to accommodate the height of the nest as it enlarges. Time lapse filming revealed that they move regularly from side to side, probably to ensure the round mound shaping of the finished nest.

They measured the duration of each mixing for 16 nesting events, and found that mixings are initially short, but increase in duration linearly from about 1 second to a plateau of approximately 4.5 seconds which is then sustained with little variation until nest completion

The time between the beginnings of successive mixing events is irregular at first, but then becomes regular with intervals of about 14 seconds that are maintained throughout the main building phase. Towards the end of nesting, the time between mixings increases in an exponential-like manner until cessation, though the duration of mixings does not change.

When the frogs are disturbed (in the wild or in the laboratory) and leave the nest, they tend to return to it, reversing into their original position to resume construction. When disturbance occurs, the periods between mixing event beginnings are irregular but soon return approximately to the original periodicity.

Thus, they found nesting occurs in three discrete, overlapping phases. Phase 1 is characterized by the production of a bubble raft with no egg release and mixing events increasing in duration. Phase 2 is the main building phase, marked by both event duration and start-to-start intervals being roughly constant as the foam hemisphere develops. Phase 3 is the termination phase, when the duration of mixings does not change but the time between them increases in an exponential-like fashion to cessation of the construction.

Túngara frogs may build nest colonially so that many nests are laid in contact with one another, but the eggs and foam are not co-mixed. Vertical cross sections of nests show eggs concentrated close to the base in the core of the nest, with a distinct cortex of egg-free foam about 1 cm deep. When a completed nest is placed on a dry surface, mimicking post-spawning drying of the nesting pond, tadpoles descend to the base of the nest and remain confined to its core, and do not breach the egg-free cortex despite the core's foam becoming liquefied, and the authors found the larvae can communicate between the cores of two nests that are in contact without breaching the protective cortex.

The advantages to the egg-free cortexseem to include protecting the eggs from dehydration by distancing them from exposure to air; shielding the eggs from light damage; and the cortex reduces access to the eggs and tadpoles by predators or parasites. The advantages of colonial nesting may be a decreased in the surface to volume ratio, and thus a reduction in the evaporative water loss. Also, joining a nesting already in progress allows exploitation of a pre-existing bubble raft and thus saving of nest material, energy expenditure and time exposed to possible predators.

Citation
Laura Dalgetty and Malcolm W. Kennedy. 2010. Building a home from foam—túngara frog foam nest architecture and three-phase construction process. Biology Letters doi:10.1098/rsbl.2009.0934