Earlier this year I wrote a series of six articles on this website (see below for links) on Ronald Maxwell Savage (1900-1985), a professional scientist and amateur herpetologist who made extensive studies on the breeding of Common Frogs (Rana temporaria) in the wild. Savage’s big idea was that Common Frogs are attracted to ponds for breeding by the odour emitted by algae in the water. This phenomenon, he argued, would explain the timing in response to earlier rainfall, the preference for one pond over another, why frogs do not spawn in every pond and why frogs may spawn in a pond one year but not the next.
One cannot of course extrapolate the triggers for reproduction from one species to another or, for that matter, claim that only one trigger from the environment is necessary even with one species. However, an interesting new paper on research done in French Guiana suggests that another species of amphibian uses the odour of a compound or compounds in water during a key event in its reproductive life.
The species is Allobates femoralis, the Brilliant-Thighed Poison Frog. Clutches of approximately 20 eggs are laid in the leaf litter. After 15-20 days of development the eggs hatch and the tadpoles are carried on the back of (usually) the father in batches of 1-25 to small pools where they continue their development until metamorphosis; the tadpoles are known to be omnivorous but not predatory. Frogs are known to travel hundreds of metres to deposit their tadpoles and to spread the brood between several ponds. Deposition readily occurs in artificial pools.
Choosing the right site to breed or deposit partially developed tadpoles must have been subject to pretty strong selection. Get it wrong, a pond full of predators or likely to dry out, and the result of disastrous. But what cues do these frogs use to optimise the odds of the tadpoles’ survival? Several years earlier the same group found that tadpole-transporting frogs seemed to be attracted to distant ponds containing large numbers of tadpoles of the same species. There seemed to be two possible explanations: the odour of the conspecific tadpoles was the attractant or the odour of the water or something in the water was responsible. The new paper set out to determine which.
In field experiments three sorts artificial pool (plastic plant pot saucers) were placed on the leaf litter and filled with well water. One had nothing else added. The second had tadpoles of the same species in the water. The third had poured around it ‘stagnant’ water made by steeping leaf litter in water for two weeks. All three types of pond had fresh leaf litter placed around them so as to be visually indistinguishable. In all, 60 artificial ponds were prepared.
The results were clear. Stagnant water was an attractant:
Frogs showed a clear differential pool usage: 11 out of 15 occupied pools were pools from the stagnant water condition. Out of the 253 tadpoles deposited, 212 tadpoles were found in the stagnant water condition, 23 in the tadpole water condition and 18 in the control condition. Of the 18 adult frogs observed at the pools (four of which were transporting tadpoles), 15 were at the stagnant water condition. Sixteen frogs captured visiting pools with stagnant water condition had their closest known territory points between 3 and 50 m (median=12 m) from the pools where they were found.
The questions now remaining are: Why do these frogs prefer depositing their tadpoles in the presence of rotting vegetation and what odoriferous constituent(s) of the stagnant water is attracting the frogs?
The authors suggest that ‘stagnant’ water may be an indicator of a pond’s permanence; one that has had time to remain full and therefore not just a temporary rain-filled depression. I do, however, have another suggestion. Could it be that the smell of rotting vegetation is indicative of a nutrient-rich environment ideal for growth of the tadpoles? The leaf-litter steeped water would be expected to contain all manner of algae, protozoa and small aquatic invertebrates—bottom of the food-chain stuff—ideal as tadpole food. A similar mix soon leads to the production of ‘infusoria’ from vegetation and air-born spores that aquarists use to feed their young fish. Identification of the chemical(s) involved will prove interesting: the odour of products of decay or of organisms using the products of decay to multiply? Could it be the case that algae are involved, just as envisaged for the Common Frog by Maxwell Savage?
Serrano-Rojas SJ, Pašukonis A. 2021. Tadpole-transporting frogs use stagnant water odor to find pools in the rainforest. Journal of Experimental Biology 224, jeb243122. doi:10.1242/jeb.243122