Report – ZL2103 – Desiccation of frogs

ZL2103, 2000-11-16

Study Of Desiccation in the species Litoria caerulia and Bufo marinus

Abstract

Comparisons of rates of cutaneous evaporative water loss in Litoria caerulea, Bufo marinus, and agar models, specimens of each were placed in desiccating chambers and their weights recorded over time. The experiment showed that neither species desiccated as fast as its agar model, implying that neither loses water as fast as a free water surface. L. caerulea lost water more slowly than B. marinus, most likely because of its relatively impermeable dorsal skin, and to the secretion of a protective lipid membrane.

B. marinus were examined of its behaviour for water-conservation by placing toads in enclosures containing a number of artificial shelters, with one irrigated and one dry half.

By observing the toads after a couple of days of acclimatisation we could determine their preference. We found that toads tend to form aggregations, and seeking shelter sites on the irrigated soil to reduce the evaporative water loss.

Lastly, the desiccating power of various microhabitats were examined by placing agar models of frogs and toads into four different conditions and monitoring their mass loss over seven days. We found that sunny microhabitats caused faster evaporative water loss than shady ones, and that open microhabitats caused faster evaporative water loss than covered ones.

Introduction

Most amphibians lack morphological or physiological mechanisms to limit cutaneous permeability (Wygoda 1984). Therefore, amphibians tend to be highly susceptible to desiccation through cutaneous evaporative water loss (Schwarzkopf and Alford 1996). Wygoda (1984) states that a free water surface having the same surface area as a ‘typical’ amphibian exposed to the same environmental conditions have the same rate of evaporative water loss equivalent.

Pough et. al (1983) suggest that it is very important for terrestrial amphibians to have behavioural responses for reducing water loss. Such behaviours may include forming aggregations, seeking diurnal shelter, and restricting activities to night-time.

Because arboreal habitats are considered to be more desiccating than non-arboreal ones, arboreal frog species have become more adapted to and are able to lose water by evaporation much more slowly than non-arboreal species (Wygoda 1984).

Two species common to Townsville in Tropical North Queensland are the arboreal Green Tree Frog, Litoria caerulea and the ground-dwelling Cane Toad, Bufo marinus (hereafter referred to as frogs and toads). During the tropical dry season, the stresses of low water availability is often high (Cohen and Alford 1996). However, L. caerulea and B. marinus survives.

This report aims:

to compare desiccation in amphibians with different ecologies (ie, arboreal and ground-dwelling), and to determine if they differ from that of a free water surface;

to examine the behavioural mechanisms toads use for controlling water loss; and

to compare the effects of different microhabitat conditions on water loss.

Materials and methods

We conducted the study between 9.30am to 12noon, the 19th of October 2000, beside the aquaculture research facilities and in the biological sciences building at James Cook University, Townsville, in tropical north Queensland, (146.80 E, 19.30 S).

We measured evaporative water loss in the frog, Litoria caerulea, and in the locally abundant cane toad, Bufo marinus, to see if there was any difference in evaporative water loss between the frog and the toad’s adaptability to live on dry land.

The shelter site selection in toads was observed to examine the behavioral mechanisms that amphibians use to avoid desiccation using two shelters, ca. 5m long and 3m wide, with about 20 cane toads (Bufo marinus) in each shelter. Half the shelter was irrigated while the other half was left dry, when this was conducted the air temperature was 29ºC.

By placing agar models of frogs and toads in various locations we were able to measure the “desiccation power” of different kinds of environmental locations, four in each of the following environments; sunny, protected; sunny, open; shady, protected; shady, open. By weighing them we obtained a measure of evaporative water loss.

Evaporative water loss was measured by placing the frogs and toads, both live animals and agar models of each species/model, in a desiccation chamber which is an confined chamber which passes dry air over the frogs/toads/models at a constant speed. The air flow through the chamber was controlled by a flow regulator and a flow meter. The air was passed through a column of desiccant (Dri-Rite or silica gel) to remove any moisture from the air before the air enters the desiccation chamber.

We measured and recorded the change in weight during this experiment. Evaporative water loss is directly proportional to the weight loss under these circumstances, therefore the weight loss recorded reflect the evaporative water loss.

Statistical calculations (ANOVA) was made in SPSS v.10.0 for Windows and MS Excel 97.

Results

Desiccation Rates of Frogs, Toads and Agar Models

It was found that rates of evaporative water loss differed between specimens. As can be seen in Figure 1, the model toad lost weight fastest, followed by the real toad, the model frog and the real frog, respectively. In all cases, rates of water loss tended to decrease over time.

Figure 1. Percentage of mass lost by real and agar frogs and toads models held in desiccating chambers in 80 minutes.

Habitat Selection in Toads

The chi-square value of homogeneity was calculated with 1 degree of freedom to 26.298, which is highly significant (p<0.001). This means that the toads did not disperse randomly within the enclosures, but displayed strong habitat selection. There was a strong preference for the wetter areas, with 79% of toads choosing wet positions over dry ones.

Most toads were aggregated into small groups, sometimes as many as 12 individuals. The few individuals that where by themselves tended to partially bury themselves in the soil. Most toads were found in shelters, and perhaps as a result of our disturbance the few that were not, were hopping around. The active toads were the only ones that did not sit in a water-conserving posture.

Table 1. The distribution of cane toads in enclosures.

Habitat

Enclosure 1

Enclosure 2

Wet area

25

36

Dry area

9

7

Desiccating Power of the Environment

Both toad (Figure 2) and frog models (Figure 3) was found to be affected differently of the microhabitat that had different powers of desiccation. Both toads and frogs experienced the fastest rates of desiccation in the sun/exposed microhabitat. In frogs, sun/covered was the least desiccating environment while for toads, sun/covered, was the second most desiccating environment. As predicted both species models left in the shade/exposed microhabitat desiccated faster than those in shade/covered positions.

Figure 2. Mass specific mass loss due to evaporative water loss for model frogs in various microhabitats, over a period of seven days.

Figure 3. Mass specific mass loss due to evaporative water loss for model toads in various microhabitats, over a period of seven days.

Table 2. – ANOVA – The differences in the desiccating powers between the different microhabitats were suggested by the both single factor ANOVAs to be not statistically significant (Toads: F3,7=0.830, P=0.543; Frogs: F3,7=1.298, P=0.390).

Sum of Squares

df

Mean Square

F

Sig.

TOAD

Between Groups

2141.429

3

713.810

.830

.543

Within Groups

3440.761

4

860.190

Total

5582.190

7

FROG

Between Groups

2584.179

3

861.393

1.298

.390

Within Groups

2654.501

4

663.625

Total

5238.681

7

Discussion

We would have expected that if the frogs and toads had the same rate of evaporative water loss as a free body of water they would had lost as much water as the agar model replicas. It was found however, that for both species the agar models had faster rates of evaporative water loss than the real animals (Fig 1.). As can be seen in Figure 1, the model toad lost weight fastest, followed by the real toad, the model frog and the real frog, respectively. In all cases, rates of water loss tended to decrease over time.

Toads loose water through cutaneous evaporation much faster than frogs do. As an arboreal species, L. caerulea is exposed to more drying conditions than species in the understory (Pough et. al 1983) Therefore, must frogs be better to resist evaporative water loss than toads. This has been found to be the case in a number of studies (Buttemer 1990; Buttemer et. al 1996; Christian and Parry 1997; Wygoda 1984),

and tend to be a result of the different skin properties in the Litoria genus (Buttemer et. al 1996). Buttemer et. al (1996), states that the skin of Litoria spp. range from moderately to highly water-proof water vapour permeabilities.

It is also believed that L. caerulea wipes the secretions of specialised dermal glands, generally described as a simple lipid layer, over its body to retain water from evaporating. The secretions’ chemical nature however, is unclear (Dr. L. Schwartzkopf, School of Tropical Biology, J.C.U. 2000 pers. comm.).

The toads however, (B. Marinus) have very little water-conserving ability (Cohen and Alford 1996). Toads are able to tolerate large amounts of water loss, up to 50% of their body mass in order to survive the hot, dry conditions of the environment (Dr. R. Alford, School of Tropical Biology, J.C.U. 2000 pers. comm.; Cohen and Alford 1996).

The are also able to rapidly absorb water from damp substrate through thin skin on their ventral side (Dr. R. Alford, School of Tropical Biology, J.C.U. 2000 pers. comm.; Cohen and Alford 1996), and they also have the ability to reabsorb water from the bladder (Cohen and Alford 1996) as do some frogs (Pough et. al 1983).

Interestingly the agar frogs conserved water better than did the agar toads. This tend to imply that toads have a greater ‘surface area’ to ‘volume ratio’ than frogs, which could be the reason for the higher rates of evaporative water loss in toads. The known bias in the study design was the use of mass rather than surface area when comparing rates of evaporative water loss. Although the assumptions of that mass would give a good estimate of surface area was clearly false, as toads are more angular, while frogs are rounder in shape.

Habitat Selection in Toads

The toads showed a strong tendency to use shelters, on the basis of soil moisture where the moister the more preference the toads tend to have. These observations are same as for previous studies, in which toads were found to select shelter sites that provided the maximum protection from desiccation (Cohen and Alford 1996).

It also was observed that toads had a tendency to form aggregations. This tendency was also observed in the study of Cohen and Alford (1996), in which the authors contended that aggregation may serve to reduce effective ‘surface area’ to ‘volume ratios’, and may allow toads to take advantage of increased soil moisture caused by urination of their fellows.

Desiccating Power of the Environment

The ANOVAs showed that there was no significant difference between the gathered data of model toad and frog microhabitats. Also, examination of raw data replicates reveals considerable differences. It is likely that these differences were due to a biasing variation in selection of microhabitats for the placement of the agar models.

The lack of clarity in the descriptions were as simple as “shade/open” and “open/covered” resulted in models being placed in very different locations, also the factors such as intensity of sun/shade influenced, texture and thickness of ‘cover’, and degree of exposure to air currents.

Another influencing factor could have been exposure water sources such as irrigation and rainfall, as these cause models to soak up water and therefore increase their mass. On days 5 and 6 it did actually rain in Townsville which actually influenced the toads mass slightly (see Fig. 3).

Giving more specific descriptions of the microhabitats and perhaps the people doing the agar model experiment should had discussed the locations with each other and compared them so they where similar and perhaps also make sure that all models were equally exposed to factors such as air currents and rain.

Even though we had some sampling problems, some points were clear. Open, uncovered models desiccated faster than covered ones, and models exposed to sun tended to be more desiccating than those in a shady habitats. These conclusions support the results of the other experiment where toads were tended to use shelter sites.

Conclusion

A free water surface as found to evaporate faster than both L. caerulea and B. marinus. The frog sp. L. caerulea was also better at conserving water than B. marinus were, this tend to be due to the almost impermeable skin of L. caerulea and its secretion of protective lipid membrane.

Because the toad B. marinus do not have these physiological adaptations as the frog does, it is forced to rely on behavioural mechanisms such as forming aggregations, seeking shelter (preferably moist shelters) during the day.

References

Buttemer, W. A. 1990. Effect of temperature on evaporative water loss of the Australian tree frogs Litoria caerulea and Litoria chloris. Physiological Zoology 63(5):1043-1057.

Buttemer, W. A., M. van der Wielen, S. Dain and M. Christy. 1996. Cutaneous properties of the Green and Golden Bell Frog Litoria aurea. Australian Zoologist 30(2):134-138.

Christian, K. and D. Parry. 1997. Reduced rates of water loss and chemical properties of skin secretions of the frogs Litoria caerulea and Cyclorana australis. Australian Journal of Zoology 45:13-20.

Cohen M. P. and R. A. Alford. 1996. Factors affecting diurnal shelter use by the cane toad, Bufo marinus. Herpetologica 52(2):172-181.

Pough, F. H., T. L. Taigen, M. M. Stewart and P. F. Brussard. 1983. Behavioural modification of evaporative water loss by a Puerto Rican frog. Ecology 64(2):244-252.

Schwarzkopf, L. and R. A. Alford. 1996. Desiccation and shelter-site use in a tropical amphibian: comparing toads with physical models. Functional Ecology 10: 193-200.

Wygoda, M. 1984. Low Cutaneous Evaporative Water Loss in Arboreal Frogs. Physiological Zoology 57(3):329-337.

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