Water Uptake and Moulting in Bufo Regularis Reuss

Jørgensen (1949) has shown that in Bufo bufo and Rana temporaria moulting is accompanied by an increased rate of water uptake through the skin. The experiments of Bufo regularis Reuss about to be described confirm and in some respects extend Jørgensen’s findings. In this species moulting takes place very rapidly, and it is therefore easy to follow the changes in the rate of water uptake over the whole period of moulting. The old skin starting to come away from the lower surface of the legs is usually the first sign of a moult. When the animal is kept in water at 26° C. the old skin may be almost completely shed within an hour, but usually this takes 2 hr. and occasionally as much as 3 hr.

The present work arose out of studies on the water balance of B. regularis which involved, measurement of the water absorbed through the skin and of the urine output at hourly intervals (Ewer, 1950). Occasionally an animal moulted during the course of an experiment, and if this happened the rates of water uptake and urine production were measured at hourly intervals until the moult had been completed. All experiments were carried out at 26° C.

Seven moulting animals were studied. In all cases changes in the rate of water uptake were found to accompany moulting. Fig. 1 shows the results of a typical experiment; in this case moulting was rapid and was completed within an hour. The rate of water uptake rises as moulting begins, and falls rapidly immediately afterwards, reaching in 2 hr. a value which is normal for an animal of about 50 g. weight.

In order to sum the results the figures are treated as follows. The level to which the water uptake sinks after moulting is taken as 100 and the rates of uptake for the previous hourly periods are expressed as percentages of this value. These 100% values are in each case found to fall within the normal range for nonmoulting toads of corresponding weight. Each set of readings is then arranged with the point at which moulting was first observed to have started as zero time, and the other values arranged 1, 2, 3, etc. hours before or after the commencement of moulting: mean values are then calculated for each hourly period. Table 1 shows the results arranged thus. Moulting is not necessarily detected with equal ease in every case and a second method of summing the results is possible, in which the time at which water uptake is at a maximum in each experiment is taken as zero time. Fig. 2 shows the results of both these methods of treating the data. The true picture probably lies somewhere between the two graphs. The first method of summing spreads the peak values too widely, since it is not possible to determine exactly the instant at which moulting starts; while the second method takes all the peak values as simultaneous, regardless of any individual variation which may exist.

The increased rate of water uptake is not due to the new skin being more delicate and more permeable than the old, for the water uptake starts to rise before the old skin shows any signs of being shed, and falls rapidly as soon as the old skin has come away. Moulting in the Amphibia is under endocrine control, and it seems reasonable to suppose that the increased rate of water uptake is a reflexion of the hormonal changes responsible for the initiation of moulting. In Triturus viridescens moulting is controlled by the thyroid, which in turn is controlled by the anterior pituitary (Adams, Richards & Kuder, 1930; Adams, Kuder & Richards, 1932). In the Anura the matter is less simple. Although the thyroid may be involved it appears to play a less important role than it does in Triturus (Ungar, 1933; Aubrun, 1935). Heller (1941) has shown that anterior pituitary extracts have no effect on water balance in Rana, while previous work on the effect of thyroxine on water balance in Anura is inconclusive (Biasotti, 1923; Heller, 1930). It thus seemed necessary to find out whether in Bufo regularis thyroxine increases the rate of water uptake, and experiments to test this point were therefore carried out. The experimental animals were given 0-25 mg. thyroxine by mouth on 4 successive days, and the rate of water uptake was then measured for 3 hr. periods starting 2 hr. after the last thyroxine feeding. There was no indication that thyroxine feeding caused the animals to moult more frequently than usual, but during the course of the experiments two animals did moult. These two behaved exactly like normal moulting animals and showed the usual associated changes in water uptake. For ten animals which did not moult during the course of the experiments the mean rate of water uptake was found to be 5·25 ±0·23 %/hr. This is not significantly different from the value of 5·18 ± 0·18%/hr. found for normal animals of the same weight range. The increase in water uptake accompanying moulting in B. regularis cannot therefore be due to increased thyroid activity.

Jørgensen (1947) finds that adrenaline increases the permeability to salt of isolated frog skin, and suggests (1949) that the increased water uptake of moulting Anura may possibly be due to adrenaline. Brunn (1921) and Adolph (1935), however, found that adrenaline injection did not affect the rate of water uptake in whole animals. That posterior pituitary extracts do increase the rate of water uptake in Anura is well established, and it is therefore possible that the water balance changes accompanying moulting are a reflexion of increased posterior pituitary activity. In this connexion the values found in the present experiments for rate of urine production are of interest. Injection of mammalian posterior pituitary extract causes not only an increased water uptake, but also a marked anuresis in B. regularis (Ewer, 1950). In the present experiments there was no indication of any diminution of urine flow during or immediately before moulting. Fig. 1 shows’ the rate of urine production in a typical experiment; far from falling it actually increases considerably as moulting becomes imminent. The graph for urine production lags somewhat behind that for water uptake. As a consequence the weight of the animals when kept in water increases just before moulting and does not return to normal until after the moult has been completed. An injection of mammalian pituitrin sufficient to increase the rate of water uptake to the degree shown in Fig. 1 would have been accompanied by an almost complete anuresis lasting at least 2 hr. One might therefore assume that the absence of anuresis indicates that posterior pituitary activity cannot be the factor responsible for the observed increase in water uptake. This does not necessarily follow. Heller (1941) has shown that the antidiuretic activity of anuran posterior pituitary extracts is very slight compared with that of mammalian extracts, and Jørgensen (1950) finds that while in some Anura extracts of the animal’s own pituitary are anti-diuretic in others there is no such effect. In the present case the increased water load would tend to produce an increased urine flow, and this might be sufficient to override and mask a slight antidiuretic effect. The absence of anuresis cannot therefore be taken as a definitive proof that posterior pituitary activity is not responsible for the increased rate of water uptake accompanying moulting. The factor responsible for this phenomenon cannot yet be identified with certainty, and further experiments on the subject are planned.

This work was carried out during the tenure of a research grant from the South African Council for Scientific and Industrial Research.