The ecology and life history of the common frog (Rana temporaria temporaria)

ponds, as will be seen from the following account of the jelly mass in R.temporaria (Savage, 1950).The jelly mass is not an amorphous lump, in which are embedded the vitelli.On the contrary, it possesses a structure, which may be likened to a bunch of grapes glued together only where they touch, and having wide channels (the intercapsular channels) between each egg.Indian ink dropped on a freely floating cluster falls right through in a very short time, and small swimming animals can be seen under the micro- scope to emerge from the depths.The diffusion of gases so essential for respiration does not therefore take place over distances of, say, five or ten centimetres but only over the semi-diameter of the envelope, a matter of a few milhmetres.The structure cannot act like this unless the clump is floating or at least supported under water on some irregular surface, because the jelly is so weak that, if it has to support its own weight, it collapses and obliterates the channels.It is for this reason that large clumps of spawn brought into the laboratory often develop irregularly.The clumps are commonly placed in only enough water to cover them, in a dish with an impervious surface, and the lower ends of the channels are then blocked by the collapse of the mass.In the field, not only are the clumps laid in a greater depth of water, but there is almost always enough wind to disturb the water and produce a gentle oscillation of the clumps which produces a current of water through them.Now, in all this discussion of the probable effect of the jelly mass and temperature on the distribution of frogs, there has been one feature missing-confirmation from the field.All laboratory experiments that are performed, not for the intrinsic interest of the facts discovered, but for the hght they may throw on ecology, must satisfy the requirement that besides being valid, they must also be relevant.This point comes up over and over again in ecology, so that it may be as well to make it clear now what is meant by this distinction.An experiment is vahd if it is competently performed and correctly analysed so that it is clearly significant, whether or not a statistical test is actually apphed.It is, however, only evidence of what happened in the experiment, and this remains true even though it is successfully repeated many times by many people.The conclusions drawn from the experiment can only be transferred to other conditions by a process of inference that corresponds to the graphical process of extrapolation.It is well known that the extrapolation of even a straight line far beyond the THE EGGS AND YOUNG TADPOLES f points on which it is based is a risky procedure.If the line is curved or has some comphcated shape, then extrapolation is quite unjustified, for no one knows what twists and turns the line really takes beyond the points that have been determined.Mathematics can often help to decide the validity of experiments or observations, but is of no use in deciding relevance, a matter that depends on the exercise ofjudgment in the light of all available facts.Great caution is therefore always needed before accepting that the results of experimental zoology or physiology really apply to ecology, for however important they may be in themselves, as additions to scientific knowledge, they may be quite irrelevant to ecology, if they are merely accurate descriptions of events that never happen in nature.The test of relevance is to try and find something in the field that corroborates the laboratory results.It need not be as extensive evidence as that provided in the laboratory but the two sets of observations must fit.If there is no fit, then the laboratory results are not applicable to the field.It is never the other way round, £or, provided a field observation is correctly reported, and is therefore vahd, it must be relevant, for no process o£ extrapolation is involved-it is direct evidence.These remarks are inserted here not because they apply especially to the work of the authors who have studied this special subject, but simply because this is Chapter i, and a suitable place to emphasize a general principle.To return now to the effect of lethal temperatures on distribution, no one seems to have reported the death of eggs and embryos in the field under circumstances that could be attributed to high temperatures.Dickerson (1906) stated that in Texas, tadpoles and water frogs are often killed in large numbers in the shallow pools, and considered that death occurs at 40°C.This is not quite the evidence that is being sought, but it shows the kind of tiling that is wanted.For R. temporaria there is a httle evidence, not enough to establish the case, but at least sug- gestive.The spring of 1949 was exceptionally warm in south-eastern England, and in a pond on the borders of Middlesex and Hertfordshire, some spawn reached a temperature of 2i'5°C.This is perilously near Douglas's figure of 24°C, or Moore's of 25X, but not quite there.The spawn in two other ponds in the same area on the same day did not nearly reach this temperature, but was in each case at about 1 5°C.When values show a range as wide as this, it often indicates that even the extreme value actually observed would be exceeded if more observations had been available, and it is rather likely that somewhere 8 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG in England on that day, spawn was in danger from the heat.England is a long way north of the southern hmits of distribution, and it is even more hkely that somewhere in Europe, some spawn was killed.A spring like that may not happen again in England for a long time (three weeks later, the hot weather continued, and the tadpoles in this pond were in water at 29°C), and the best chance of verifying the hypothesis of Moore and Douglas would be at the southern limits of the species.Even a few cases of heat death in eggs laid naturally in a pond would be of much interest.There is, however, another more comphcated effect of high tempera- tures at spawning time.If spawning is delayed for any reason, the eggs within the frog may become over-ripe and develop abnormally, or even die altogether.Nussbaum (1897) considered that under these conditions R. esculenta absorbs the eggs without laying them, but that R. temporaria under the same circumstances lays infertile eggs.Witschi (1952) has pointed out that spoiled eggs of this species are rarely found in the field, and I agree with him.He made a detailed study of the abnormalities that can be produced by delayed spawning.He found that over-ripe eggs are much more sensitive to the deleterious effects of high temperatures than those that have only been in the ovisacs for a few days.If they remain there for a week at 20°C, they die.Now, delayed spawning can be produced by several conditions, but hot weather at spawning time is one of them.It is therefore possible that at the southern limits of the species, spawning will sometimes be delayed by the hot weather, and that many of the eggs will as a result be infertile.Moreover, the delay itself makes it more likely that, as the season advances, lethally hot weather may kill those eggs that are alive when laid, so that the effects of Moore and Douglas become reinforced.I have seen events in the field that support this suggestion.In 1958, spawning in my area was delayed.The cause of the delay was not in this case high temperature at spawning time, so that only the effect of the delay could be observed.In one pond, there were thirteen clumps of eggs.Of these, six contained spoiled eggs in an abnormally high proportion, and there was in addition a clump of jelly with no vitelli at all in it.It seems almost certain that here was a field instance of the effects that Nussbaum and Witschi have described, and that these frogs were breeding so late that they only just succeeded in doing so at all.Very few ponds contained spawn and the breeding population was much reduced.In i960 this pond again had spawn later THE EGGS AND YOUNG TADPOLES 9 than other ponds, and of the seventeen clumps, seven had spoiled eggs in them.It is, however, rather unusual in biology for a simple explanation such as this to account for the whole of the facts of a complicated matter hke the geographical distribution of an animal.We do not, for example, know that this frog, regarded by taxonomists as one species, has the same thermal physiology in different parts of its range.Volpe (1953) has shown that, in the case of Bufo americanus, the eggs from toads coming from Oklahoma were less tolerant of cold than those of toads from Wisconsin or North Carolina.Volpe does not think that the upper lethal temperature, 3i°C, is a limiting factor, because he doubts whether this temperature is reached in the breeding ponds, although he says that there are no temperature readings to confirm this view.It will be recalled that a temperature only two degrees less than this has been found in England, not a very hot country, and Dickerson's remarks suggest that in Texas, 40°C may be reached.Spallanzani in the eighteenth century (from Daudin, 1802)was interested in this problem and put the thermal death point of the eggs of a species that was probably R. esculenta as high as 35°R, approximately 43 °C, and has an account, given him by a friend, of hving frogs in the Baths of Pisa in a temperature of 37°R (46^C).Zoologists in the eighteenth century made many excellent field observations and these results differ by only i°-2°C from observations of Seurat (1922), who found R. esculenta and its tadpoles in a hot spring in North Africa at 44°-45°C.For all we know at present, the eggs of R. temporaria from the extreme southern parts of its range may withstand higher temperatures than those of the frogs used by Douglas, taken in the neighbourhood of London, or those of Moore, from north-east France and Belgium.A slight change of habit, such as a tendency to lay eggs in the shade, rather than in full sun as i