Although the majority of amphibians lay their eggs in water, some species choose to breed in a more terrestrial environment. For these land-loving species, their eggs have to contend with more extreme fluctuations in environmental conditions than their aquatic-based friends, and the eggs run the risk of dehydrating. To help alleviate this problem, some frogs make bubble nests to lay their eggs in. However, as both terrestrial and aquatic breeding frogs produce bubble nests, their exact function had remained a mystery. It was long assumed that the bubbles helped protect developing eggs from predators, reduced egg dehydration and improved the oxygen supply to the developing embryos. However, a recent study by Méndez-Narváez and colleagues, at the Universidad de los Andes, Colombia, has identified an additional function behind these bubble nests, and the special properties they possess. The secret of the bubble nest's success, it seems, is how they insulate the developing embryos from extreme fluctuations in ambient temperatures.

The study, published in Physiological and Biochemical Zoology, examined sympatric leptodactylid frogs that habitually build bubble nests but inhabit different nesting environments. The nesting habitats of the frogs range from entirely aquatic (Physalaemus fischeri) to nesting well over 20 m away from the nearest water source (Leptodactylus fuscus). Working in the Reserva Wisirare, situated in North East Colombia, the team used thermocouples to measure the temperature deep inside the bubble nests and the ambient temperature of the air immediately surrounding it. Repeating this at set times across a 24 h period, Méndez-Narváez and co-authors discovered that temperatures within the bubble nests in the mornings and afternoons were lower than that of the tropical ambient air, while temperatures within the nest were higher at night when compared with their surroundings. Furthermore, these differences between nest and ambient temperatures were more pronounced in the species that build nests at greater distances from the water, suggesting superior insulation in the nests of the more terrestrial breeding species.

The bubble nests were acting to buffer changes in ambient temperature fluctuations, as the variation in nest temperature was lower than corresponding variations in ambient temperature. Moreover, by maintaining higher temperatures within the nest at night, embryonic development was accelerated. To understand the functional significance of this thermal buffering, the team translocated nests of the most aquatic species, P. fischeri, from traditional nest locations floating on the water surface to sites several metres from the water edge. Relocating the nests to a terrestrial site led to a large increase in embryo mortality, due to the poorer thermal properties of bubble nests made by aquatic breeding species.

The nesting preference for each species is reflected in the thermal properties of their respective bubble nests, with species nesting further from water having more thermally insulated nests compared with those species nesting directly on the water surface. It is highly likely, therefore, that the increased insulation that the bubble nests provide would have played a pivotal role during the early colonisation of thermally variable habitats in frogs and other vertebrate groups. Indeed, the authors conclude that the thermal properties of the bubble nests are particularly advantageous in environments that experience strong fluctuations in temperature throughout the day, and where the timing of events such as the rainy season is highly variable and unpredictable.

S. V.
Foam nests provide context-dependent thermal insulation to embryos of three Leptodactylid frogs
Physiol. Biochem. Zool