Comparative rates of rehydration from soil in two species of toads, Bufo boreas and Bufo punctatus

Comp. Biochem. Physiol., 1970, Vol. 34, pp. 281 to 287. Pergamon Press. Printed in Great Britain

COMPARATIVE RATES OF REHYDRATION FROM SOIL I N T W O S P E C I E S O F T O A D S , BUFO B O R E A S A N D

BUFO P U N C T . 4 T U S JAY W. F A I R Department of Biology, California State College, Fullerton, California, 92631

(Received 30 September 1969) A b s t r a c t - - 1 . The rates of rehydration from a varying soil particle size was investigated for two species of toads, Bufo boreas and Bufo punctatus. 2. Both species of toads were found to gain comparatively equal rates of water per unit of surface area, about 40 mg/cm 2 per hr. However, the larger surface area to volume ratio of B. punctatus aUows this anuran to regain its deficit at a significantly faster rate than B. boreas. B. punctatus showed rates of about 3 per cent standard weight gain per hr while B. boreas showed rates of about 1"5 per cent standard weight gain per hr. 3. It is proposed that a reduced size (i.e. increased surface area to volume ratio) may be an important adaptation to xeric environments. INTRODUCTION

MOST anurans require the presence of free water for reproduction and succeeding larval development; however, the adults of many species may be found considerable distances f r o m free water even under extremely arid conditions. (Warburg, 1965; Dole, 1967; M c C l a n a h a n & Baldwin, 1969). Since anurans are subject to water-loss at rates forty to fifty times that of reptiles (Thorson, 1955; Claussen, 1967, 1968) T h e p r o b l e m of water balance is of major ecological and physiological significance. I t has been stated (Dole, 1967; McClanahan, 1967; Brattstrom, personal communication) and demonstrated (McClanahan & Baldwin, 1969; Ruibal et al., 1969) that anurans hydrate primarily t h r o u g h the pelvic or groin integument. Except for the studies of Ruibal and Dole, all research has been done in the presence of free water. Desert anurans have relatively little contact with free water except during the breeding season. T h e r e f o r e m u c h of the time a toad m u s t acquire water f r o m the soil if it is to maintain its o p t i m u m b o d y fluid concentration. T h e purpose of this research was to determine the rate of rehydration f r o m soil. T w o species of toad were used; Bufo punctatus, the desert canyon toad, considered to be a fairly xeric adapted anuran and Bufo boreas, the western toad, considered to be a less xeric adapted anuran. MATERIALS AND M E T H O D S Specimens of B. boreas were collected at Orange, Orange Co. California and at Elsinore, Riverside Co., California. All specimens were collected during October of 1968. The 281

282

JAY W. FAIR

B. punctatus were collected in Pushawalla Canyon, Riverside Co., California during the spring of 1968. All specimens were maintained at room temperatures (approximately 20 to 25°C) in aquaria filled with 4 to 4 cm of moist sand. They were fed Tenebrio larvae and kept in a healthy condition (visually determined) throughout investigation. Four days prior to standard weight determinations feeding was terminated. Twentyfour hours prior to standard weight determination the animals to be run were put in an aquarium with 1 to 2 cm of water to ensure complete hydration at time of standard weight determination. Determination of standard weight (weight of specimen without bladder water) was made by the method described by Ruibal (1962) using a Mettler P1200 balance with a precision of 0"005 g to obtain weights to 0"1 g. The animals were dehydrated to establish a water deficit and to induce a water-balance response. Dehydration was at room temperature in glass beakers covered with two layers of gauze. Gram deficits of 18 to 25 per cent were reached in 36 to 67 hr at a relative humidity of 37 to 43 per cent. T h e range of deficits, as well as the time of dehydration, have been shown to have little effect on the rates of rehydration (Shoemaker, 1965; Thorson, 1955) as long as a water balance response is initiated (Stille, 1958). A 10 to 15 per cent loss of standard weight is believed to be the inducing stimulus of the water-balance response. T o determine rates of rehydration, the toads were placed in aquaria on 3 to 5 cm of desert sand containing a known percentage of water. The aquaria were covered with aluminium foil to prevent undue water loss from soil or toads. Recordings of weights at 2 to 4-hr intervals were made until hydration was complete or a 16-hr time limit was reached. The rates of rehydration were measured utilizing three different soil particle sizes of 1 to 1"5 mm, 0"5 to 1 m m and less than 0"5 ram. Determination of particle size was by sifting of soil through graduated, round-hold soil screens. The soil was dried in an oven at 100 to 115°C to a near-constant weight. At the time of rehydration, the soil was removed from a sealed container, and water equal to 20 per cent of the recorded dry weight was added to the soil in the aquaria. The aquaria were covered and the water was allowed to diffuse throughout the soil. Never less than 500 g of water were added to the soil mixtures so that uptake by the toads did not appreciably affect the percentage of soil moisture. RESULTS

K n o w l e d g e of the rates of w a t e r u p t a k e for a g i v e n t i m e period, as well as k n o w l e d g e of t h e surface area of the h y d r a t i n g surface (i.e. pelvic region), allows for c a l c u l a t i o n of t h e w a t e r u p t a k e p e r u n i t area p e r u n i t time. T h e surface area of t h e pelvic r e g i o n is g i v e n b y the regression line f o r m u l a y = 0 . 1 3 x + 5 - 6 5 w h e r e y is surface area i n c m 2 a n d x is s t a n d a r d weight i n g (Fig. 1). T h e coefficient of correlation is 0.93. T h e weights of t h e B. boreas were selected so as to r e d u c e t h e specific size differential b e t w e e n B. boreas a n d B. punctatus. E v e n so, the m e a n s t a n d a r d weight for t h e t e s t e d B. boreas was a p p r o x i m a t e l y 30 g while t h e m e a n s t a n d a r d weight for t h e t e s t e d B. punctatus was a p p r o x i m a t e l y 10 g. Since s u c h size difference can lead to p r o b l e m s w i t h t h e surface area to v o l u m e ratio, a factor to a c c o u n t for the difference i n v o l u m e s h o u l d be used for c o m p a r i s o n to o t h e r results. S u c h a factor u s e d is p e r cent s t a n d a r d weight gained. T h e r e h y d r a t i o n rates of B. boreas a n d B. punctatus o n v a r y i n g soil particle size a n d s t a n d a r d i z e d p e r cent m o i s t u r e c o n t e n t are g i v e n in T a b l e 1 i n t e r m s of m g u p t a k e / c m ~ per h r a n d p e r cent standard weight gain/hr.

REHYDRATION

FROM

SOIL

283

IN TOADS

T h e significance of the differences of the rehydration rates between B. boreas and B. punctatus is shown in Table 2. Figure 2 presents a visual presentation of the difference in rates of rehydration between B. boreas and B. punctatus using mean standard weight gained per increasing time.

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Fro. 1. Graphic illustration of the relationship between pelvic surface area and standard weight. In the formula, x is standard weight and y is surface area. The coefficient of correlation is 0"93. DISCUSSION T h e use of standard weight as an indication of surface area has been almost uniformly applied throughout the research on the rates of rehydration. Such use of standard weight has an associated error in the form of variable standard weight with unvarying surface area. This is to say, a 50 g standard weight animal has a surface area of y, while the same animal dehydrated to 80 per cent of its standard weight will have a surface area "less than y " . T h e fallacy of such application is obvious. Methods have been employed to reduce such discrepancies or at least minimize the differences. In addition, the computation of pelvic surface area by the formula y = 0.13x+ 5.65 shows that a very large difference in standard weight is necessary to appreciably affect the surface area. It would seem that rapid uptake of water would be advantageous for terrestrial anuran. In spite of this, most studies have demonstrated that aquatic or semiaquatic anurans gain water at an equal or faster rate than most terrestrial anurans. T h e r e appears to be no significant difference in rates of rehydration, both anurans tested have a similar uptake per cm ~ assuming that each cm ~ of skin II

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JAY W. FAIR

p a s s e s an a p p r o x i m a t e l y e q u a l v o l u m e w h e n c o m p a r e d to e v e r y o t h e r c m a in t h e s a m e area. ( R e s u l t s f o r soil of less t h a n 0-5 m m will b e d i s c u s s e d later.) TABLE 1--COMPILED STATISTICSOF ALL REHYDRATIONRUNS ON THE VARYINGSOIL PARTICLE SIZES Uptake (mg/cm 2 per hr)

B. boreas

n Mean Range S.D. S.E.

34 38"5 12'9-52"0 7"2 1"3

Standard wt. (% gain/hr)

B. punctatus

B. boreas

B. punctatus

Soil particle 1'5-1 m m 14 34 43"0 1"4 36"0-51 "0 0"6-2"6 5"3 0"5 1"5 0"1

14 3"1 2"3-3'8 0"5 0"1

Soil particle 1-0"5 m m n

26

9

26

9

Mean Range S.D. S.E.

42'6 31-9-61.5 9"2 1"8

39-6 30"1-48'2 5"5 2.0

1"6 0'8-2"6 0'6 0-1

2.7 2'4-3"2 0"2 0'1

n Mean Range S.D. S.E.

23 33"8 31" 1-46" 1 5"1 1.1

Soil particle less than 0"5 m m 12 23 47"5 1"6 42" 6-51"0 1"1-2" 3 2"8 0"4 0"9 0.1

12 3"3 2"7-4" 0 0"5 0.1

TABLE 2--RESULTS OF t-TESTS ON THE SIMILARILEHYDRATIONRUNS OF B. boreas AND B. punctatus

mg/cm ~ per hr Standard wt. (% gain/hr)

1-5-1 m m

1"0-0"5 m m

Less than 0"5 m m

0"08 0"001

0"20 0"001

0"01 0"001

D e s p i t e e q u a l rates of u p t a k e p e r c m 2, it was seen t h a t B. punctatus r e g a i n s its deficit s i g n i f i c a n t l y f a s t e r t h a n B. boreas (Fig. 1). Bufo boreas is a s s o c i a t e d w i t h free w a t e r a n d / o r m o i s t soils. W h e n f o u n d in t h e desert, it is g e n e r a l l y a s s o c i a t e d w i t h p e r m a n e n t s o u r c e s o f water. I t is s u g g e s t e d t h a t r a p i d gains of s t a n d a r d w e i g h t m i g h t b e o f less selective v a l u e for B. boreas t h a n for B. punctatus since t h e h a b i t a t of B. boreas is u s u a l l y a s s o c i a t e d w i t h sufficient m o i s t u r e to m a i n t a i n

285

R E H Y D R A T I O N FROI~I S O I L I N T O A D S

optimum body fluid concentrations. Bufo punctatus, however, is generally associated with a habitat lacking free water; in fact, they are almost solely dependent on precipitation trapped on rocks or in the soil. Rapid rates of uptake could be extremely advantageous for B. punctatus since time-intervals between rainfalls in the desert are often lengthy and the presence of trapped moisture is ephemeral. Bufo punctatus emerges during or soon after a brief rainfall. If the toad is to capitalize on the rainfall, it must do so before the percentage of soil moisture is B puncla/us

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FIG. 2. Relationship between the different rehydration rates of B. boreas and B. punctatus on the varying soil particle size. A, soil smaller than 0"5 mm diameter; B, 1 to 0"5 mm; C, 1"5 to 1 mm. below that which it can extract. Since percolation rates are comparatively rapid in sand due to the relatively weak attraction of the water molecules for the large soil particle size, a rapid uptake would be advantageous in maintaining body fluid concentrations. Bufo punctatus regains its water deficit significantly faster than B. boreas. This faster gain may be attributed to equal rates of uptake per cm 2 area coupled with a larger surface area per unit deficit which is found in smaller organisms (Table 3). Fast gains of per cent standard weight plus observed behavior of rapid emergence would suggest a strong correlation and adaptation to the habitat. In the presence of free water, B. boreas gains water at an average of 700 mg/cm * per hr while B. punctatus gains water at about 400 mg/cm z per hr with per cent standard weight gains about equal. (McClanahan & Baldwin, 1969). These agree with results reported by Thorson (1955) and Claussen (1969) in which no correlation of rehydration rates to environment was found. In general, an almost

286

JAY W . F A I R

TABLE 3--A

COI~IPARISON OF HYDRATING SURFACE AREA TO A STANDARDIZED WEIGHT LOSS OF 20 PER CENT

Standard weight

20 % deficit (g)

Hydrating surface (cm ~)

Surface : deficit ratio

Per cent decrease

10 20 40 80

2 4 8 16

7'0 8-3 10'8 16"0

3"5 : 1 2"1 : 1 1"4 : 1 1:1

-40 60 70

Per cent decrease is a reflection of the decreasing surface area to volume ratio from an initial 10 g standard weight. inverse relationship of water uptake to environment was reported. T h e more terrestrial the species of anuran, the slower the rate of uptake It is proposed that a possible negative correlation exists. It has been shown by Schmid (1965) that aquatic amphibians withstand prolonged hydration for significantly longer time-periods than more terrestrial amphibians. He suggests that close association with water during winter hibernation has led to the ability to withstand extremely reduced blood ion concentrations. In other words, intimate association with water has led to aquatic adaptation. Therefore, a proposal that rapid rates of rehydration from free water indicates a correlation to an aquatic or semi-aquatic habitat might not be without some validity. An explanation is necessary for the significant difference in the uptake/cm 2 per hr on soil of less than 0.5 m m since the data indicate that B. boreas gains water at a slower rate on the smaller size soil than B. punctatus. As the soil particle size decreases, the hygroscopic attraction of the water molecules for the soil increases. This leads to the possible conclusion that the increased rate of uptake for B. punctatus is a sampling error. It seems inconsistent that B. punctatus can acquire water faster from a smaller particle size than a larger particle size. However, that B. boreas gains water slower from a smaller particle size would be consistent with our knowledge of hydroscopic pressure (Ruibal, 1969), and possibly suggest that B. punctatus is better adapted to removal of water from soil than B. boreas. Following the same line, Dole (1967) published some mean values on the rate of rehydration for Rana pipiens. Utilizing the formula for pelvic surface area, it was calculated that the mean value for weight gain was approximately 32 mg/cm 2 per hr while the mean standard weight gain was approximately 1.2 per cent/hr. Variability of rates was great, but it can still be seen that B. boreas and B. punctatus gain water at a mean rate of 6 to 10 mg/cm 2 per hr faster. Note, however, that B. boreas and R. pipiens gains in per cent standard weight per hour are very similar, which might be said to correlate with their respective habitats--B, boreas being semi-aquatic and R. pipiens being aquatic. T h e relatively small size of B. punctatus, coupled with its comparatively equal uptake of water, has led to a selective advantage in the replacement of body-water

R E H Y D R A T I O N F R O M S O I L I N TOADS

287

lost to the environment. It is noted that m a n y of the more desert-adapted anurans are of relatively small size. A m o n g these are B. punctatus, B. deblis, B. compactilis, B. microscaphus, Scaphiopus couchi, S. hammondi and S. bombifrons. It is proposed that in the anuran progression to terrestrial inhabitation, a selective pressure for reduced size has possibly been operative. Such smaller size would enable the toad to regain lost body water more rapidly due to a larger surface area to volume ratio (Table 3). T h e larger surface area to volume ratio also increases the rate of evaporative water loss. This increase is probably compensated for by the development of behavioral (burrowing, nocturnal activity etc.) and physiological (ability to withstand high body fluid concentrations) characteristics. In a burrow under the soil or surrounded by rocks, a large surface-to-volume ratio would probably be insignificant since water loss would be at a m i n i m u m due to a reduced contact with air and an increased humidity. A multitude of factors undoubtedly affect the ability of an anuran to survive in xeric habitats. T h e relative significance of the roles of the integumental water uptake or body size are not known. It is only suggested that they are contributing factors in the ability to survive.

Acknowledgements--I would like to express my thanks to Dr. Bayard H. Brattstrom for his most helpful suggestions on the research and manuscript. Also I wish to thank Miss Karen Fogg who supplied the key word index. REFERENCES

CLAUSSEN D. L. (1967) Studies of water loss in two species of lizards. Comp. Biochem. Physiol. 20, 115-130. CLAUSSEN D. L. (1969) Studies of water loss and rehydration in anurans. Physiol. Zo61.

61,226-241. DOLE J. W. (1967) The role of substrate moisture and dew in the water economy of leopard frogs, Rana pipiens. Copeia 1, 141-148. McCLANAHANL. L. (1967) Adaptations of the spadefoot toad, Scaphiopus couchi, to desert environments. Comp. Biochem. Physiol. 20, 73-99. MCCLANAHAN L. L. and BALDWINR. (1969) Rate of water uptake through the integument of the desert toad, Bufo punctatus. Comp. Biochem. Physiol. 24, 381-389. RUIBALR. (1962) The adaptive value of bladder water in the toad, Bufo cognatus. Physiol. Zo61. 35, 218-233. RUIBAL R., TEVIS L. N: ROIG V. (1969) The terrestrial ecology of the spadefoot toad, Scaphiopus hammondii. Copeia 3, 571-584. SCHMID W. D. (1965) Some aspects of the water economy of nine species of amphibians. Ecology 46, 261-269. SHOEMAKERV. H. (1965) The effects of dehydration on the electrolyte concentration in a toad, Bufo marinus. Comp. Biochem. Physiol. 13, 261-271. STILLEW. T. (1958) The water absorption response of an anuran. Copeia 2, 217-218. THORSON T. B. (1955) The relationship of water economy to terrestrialism in amphibians. Ecology 36, 100-116. WARBURGM. R. (1965) Studies on water economy of Australian frogs. Must. ft. Zool. 13, 317-330.

Key Word Index--Hydration ; toads; Bufo boreas; surface area/volume ; Bufo punctatus ; water regulation.