Maxwell 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.
His hypothesis was based on his earlier observations on the food, or at least the gut contents, of tadpoles of which algae formed a large part; no point in spawning in a pond with no algae. In addition he found that frogs spawned in ponds with higher phosphate and potassium concentrations, in other words water ideal for the growth of plants. He suggested that in summer ‘higher’ plants grow rapidly and deplete the water of minerals. Runoff from rain in the winter then carries minerals, particularly phosphate, into the ponds which then leads, with increasing length and intensity of daylight, to an algal bloom and it is the smell of the algae that frogs take as their cue to migrate to the source of the odour. In essence he provided an explanation for the effects of amounts of rainfall in the months before spawning on the timing and direction of migration as well as the choice of pond.
|Cartoon illustrating Maxwell Savage's hypothesis
Savage realised that a problem arose with frogs moving to ponds against the direction of the wind. He suggested that various physical phenomena involving different movement of air at ground level, in ditches which frogs often use to reach ponds, and eddying could overcome the objection.
Savage further argued that it is possible to dispense with some substances in or produced by ponds as possible attractants. The first is water itself or the direct effect of rainfall since many frogs hibernate in ponds or very damp surroundings from which they migrate to a breeding pond. Then there were carbon dioxide (ubiquitous), ammonia (frogs would be ‘drawn to manure heaps’) and hydrogen sulphide (probably not released into the air), methane (again ponds would not be the only source). Instead he firmly came down on the side of volatile organic compounds produced by algae:
…It is, however, not the higher plants that do this [impart the characteristic of pond water], but the algae. There is much information on this matter, for it is of economic importance. If certain species of algae grow in drinking-water reservoirs to too large an extent, the consumers complain that the water tastes or smells. The odours are not always unpleasant, but people do not like water that has a strong smell or taste, whatever it is. The odours are due to essential oils elaborated by the algae, and the smells are so characteristic that a skilled person can detect and identify the species of alga sometimes before it can be located under the microscope. I once detected a smell from a pond (Large Totteridge) many yards from the bank, and suspected from the textbook description that it was due to Synura livella[*]. Microscopic examination showed that this species was abundant in the water
The fit of this hypothesis with most of the facts in the field is very good. The smells are found in ponds—nowhere else in the whole countryside. Any particular smell is probably only found in a few ponds for there are so many species of algae that, in a limited area, there are hardly any two ponds with the same flora. Ponds tend to have the same species in successive years, but this is not invariably so…
The observational and statistical associations that Savage unearthed make a compelling case for the central role of aquatic algae; they tie everything together. As good as Savage was in observing and drawing evidence from lots of different fields into a plausible hypothesis, the fact remains that there have been no experimental tests of what remains a fascinating possibility at least and a high probability at best.
There are two points to stress at this stage. The first is that Savage’s hypothesis concerns one species, the Common Frog, Rana temporaria. Various species of amphibian are now known to use a variety of mechanisms for navigation during migration. The evidence that frogs head for the pond in which they grew as tadpoles as their breeding pond (‘homing’) now seems to be strong and it is here that evidence suggests that Savage’s algal hypothesis cannot be the whole story. I have seen frogs accumulating on the earth of three filled-in ponds at the normal time of breeding; in two cases spawn was laid with no hope of its survival. (A similar phenomenon has been observed in toads.) Were these frogs that had not strayed far and using a local memory map to return home? Would frogs from further away not have been drawn to the bare earth of a filled-in pond?
Savage was clearly disappointed by the reception given to his hypothesis while realising the difficulties in taking things further, as demonstrated by the following extract:
It would be quite wrong to conclude this chapter leaving the reader with the impression that the algal hypothesis has been universally accepted. In fact, it is probably true to say that the general attitude has been one of polite incredulity…
Frogs live their aquatic life invariably among algae, which dominate the life of a pond. It has been said that if all the higher plants in a pond were to be removed and replaced by glass models of the same shape and size, the animal life in the pond would go on just the same. Remove the algae, and life would be vastly different. Knowing the number of parallels between the behaviour of frogs and the behaviour of algae, and that no two essential oils have the same chemical composition or the same smell, I have always thought the hypothesis suffered from the difficulty of proof, rather than from any improbability. But there is no need to despair. After all, it was only in late 1957 that we had experimental proof that satellites were kept in their orbits by gravitation. Up till then the whole thing had been a hypothesis, based on a number of parallels!
What Savage did not deserve was to be ignored by many of those who came after him. For example, in one chapter of a book published in 2005 which I will not name since it does not deserve even adverse publicity, Savage does not get a mention even though such factors as rainfall and odours in triggering spring migration in amphibians are discussed at considerable length. Like Trevor Beebee before me I find the omission of Savage—and not just in that one case—both astonishing and inexcusable.
However, all is not last since last week the popular BBC programme Countryfile included an item on frogs and a contributor said they were attracted to their breeding ponds by the smell of algae.
In the next article I will discuss how Maxwell Savage tried in his final research paper to take his algal hypothesis further but had to swap species in order to do so.
*the smell of Synura and the chemical composition of the odours produced is described in this YouTube video.