Comparative metabolic rate-temperature curves of Phrynocephalus arabicus Anderson, 1894 and Agama [Stellio] stellio brachydactyla Hass, 1951 (Agamidae, Sauria, Reptilia)

Journal of Arid Environments(1994)28:249-256

Comparative metabolic rate-temperature curves of PhrynocephMus arabicus Anderson, 1894 and Agama [Stellio] stellio brachydactyla Hass, 1951 (Agamidae, Sauria, Reptilia)

M. K. AI-Sadoon & N. M. Abdo Department of Zoology, College of Science, P . O . Box 2455, King Saud University, Riyadh 11451, S audi Arabia (Received 5 November 1992, accepted 19 March 1993) Resting oxygen consumption (~rO2) rates of two desert agamid lizards,

Phrynocephalus arabicus and Agama [Stellio] stellio brachydactyla, were determined in relation to ambient temperatures ranging from 10 to 40°C using a double-chambered, volumetric, closed system. Both species show relatively similar activity and preferred body temperature (PBT) ranges. Thermal dependence co-efficient (Qlo) value shifts reflect that A. s. brachydactyla has a slightly lower mean PBT than that ofP. arabicus. However, the metabolic ratetemperature (M-T) curves display the enhanced body mass-specific 702 compared with the resultant obscured effect of the cooler microhabitats that A. s. brachydactyla seeks and adapts to.

Keywords: oxygen consumption; temperature; Agama; Phrynocephalus Introduction Thermal resources are important because many chemical, physiological and physical processes vital to life are temperature dependent. Extremes of temperature are lethal (Sturbaum, 1982), and the extensive works of Cloudsley-Thompson (1971, 1972, 1991) on the temperature and water relationship of reptiles that live in arid regions have added much to this field. Ectotherms usually have preferred body temperatures (PBT) or voluntary body temperatures (VBT) which reflect the range within which the body temperature of each is regulated; and Bogert (1949) reported that this situation was species specific and an important factor in ecological isolation. Animals subjected to changes in environmental temperature must be able to regulate their body temperatures adequately to survive (Sturbaum, 1982). The vital requirement for oxygen, and the rates at which it is consumed are influenced by several factors, including sex, nutritional status, activity, body mass and hormonal secretions (AI-Sadoon, 1983; A1-Sadoon, 1986; A1-Sadoon & Spellerberg, 1987; Bennett & Dawson, 1972; Gupta, 1982; John-Adler, 1983; Thapliyal & Gupta, 1984a; 1984b). Several studies have been undertaken to determine the behavioural and physiological responses of lizards to temperature (AI-Sadoon, 1986; Bogert, 1959; Cloudsley-Thompson, 1972, 1991; Cowles & Bogert, 1944; Dawson & Templeton, 1963; Hutchison, 1979; Licht et al., 1966). Variations in thermal resources are associated both macroclimatically and microclimatically with various habitat parameters, including latitude, altitude, topography, soil and vegetation type (Davies et al., 1981). Thermal homeostasis in desert 0140-1963/44/030249 + 08 $08"00/0

(~) 1994AcademicPress Limited

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reptiles is achieved by a combination of behavioural responses, cyclical processes and physiological reactions (Cloudsley-Thompson, 1972). The agamid lizards (Loumbourdis & Hailey, 1991) are widely distributed throughout the arid regions of the Old World. Agama stellio, in particular, could be regarded as a typical inhabitant of arid environments, as it is common from Egypt to Asia Minor (Cloudsley-Thompson & Chadwick, 1964), and one of its sub-species is the only agamid that occurs in Europe, inhabiting rocky areas (Beutler, 1981). According to this relatively wide range of distribution, Agama stellio, is a useful model for the study of climatic adaptation (Loumbourdis & Hailey, 1991) and hence could be undertaken in comparison with the more localized P. arabicus. In the present study, these two tropical desert agamid lizard species have been investigated to verify their resting oxidative metabolism vs. ambient temperature in relation to mean body mass (~ w). Differences in thermal dependence (Qlo) values and shifts, relative to their microhabitats, were also manipulated.

Materials and m e t h o d s

Animals The experimental animals comprised two groups of desert species of Agamidae. Specimens ofA. s. brachydactyla were collected from crevices and slopes on rocky terrain in the desert and semi-desert coast near Alexandria, Egypt. Individuals of the same species occupy similar crevices among rocky microhabitats around wadis and escarpment slopes in Riyadh Province, Central Region of Saudi Arabia. Nine specimens having a mean body mass (~ w = 30"4 g), were selected for experiment. Ten specimens o f P . arabicus with a mean body mass (~ w = 5"2 g) were selected from the specimens collected in Riyadh Province. Here they inhabit sand dunes and terrain where they can be seen moving very fast or diving into sand. In the laboratory the animals were maintained in glass tanks (measuring 100 x 50 x 45 cm) with wire-net sliding tops and substrata of sand containing dry herbs, vegetation and stones, to simulate the lizards' habitat. A 100-W lamp was turned on automatically for 9 h per day to enable the lizards to thermoregulate behaviourally. The soil temperature gradient available to the lizards was from 20 to 40°C. Although the two species belong to the same family (Agamidae) and live in similar conditions, they differ in their microhabitats and, to a marked degree, in the method of thermoregulation. P. arabicus is a strict heliotherm, whilst A. s. brachydactyla is mainly a heliotherm, but also exhibits thigmothermic behaviour. The lizards were fed exclusively on a diet of mealworms and adult insects, water was provided ad lib. In order to achieve a post-absorptive state, the animals were not fed over a 5-day period prior to experimentation, although water was still provided.

Resting metabolic rate The restin~ metabolic rate (RMR) was measured in terms of the resting VO2 rate (ml g-1 h - ) at different temperatures range from 10 to 40°C. A double-chambered, volumetric closed system, as described by A1-Sadoon (1983), and A1-Sadoon & Spellerberg (1985, 1987) was used. All measurements of VO2 were made at times when the lizards would normally be active. Comparisons of mean oxygen consumption between the experimental groups were made using two-tailed t-tests. The level of significance adopted was p < 0"05.

METABOLIC

RATE-TEMPERATURE

CURVES

251

Results Metabolic rate-temperature ( M - T ) curves The resting 702 rates, or the R M R for the two groups of lizards (~ w = 30"4 g and 5"2 g) and the mean ~rO2 for each at the different metabolic rate-temperature ( M - T ) curves are presented in Fig. 1. The M - T curves clearly demonstrate that the 7 0 2 rate of each species increased directly with temperature. On the other hand, the values for the small P. arabicus (~ w = 5"2 g) were significantly (p < 0"05) higher than for the heavier A. s. brachydactyla (R w = 30"4 g). Variations in oxygen consumption rates were observed within the M - T curves of the two species. Three patte .ms were observed in each of them. For A. s. brachydactyla there was a gradual increase in VO2 from 10°C up to 25°C. A nonsignificant increase in 7 0 2 was noticed between 25 and 30°C. For P. arabicus there was a similar increase in '¢O2 up to 30°C. A non-significant increase in ~rO 2 was recorded above 30°C, with an inconspicuous increase between 35 and 40°C. At all temperatures, however, the 7 0 2 levels o f P . arabicus were higher than those ofA. s. brachydactyla (p < 0"05). Differences between the observed and predicted values of ~/O2 (Table 1) for A. s. brachydactyla were less than unity by 0"2, except at 20°C where they were less than unity by 0"6. For P. arabicus the observed values of 7 0 2 were varied widely from the predicted, except at 35°C where the ratio was 0"95. Q lo

T h e calculated Q,o values for resting

values

702 at each interval of 5°C, ranging from 10 to 35°C

0-40

0.20

"7 7 t~ "~

0.10 0.08 0-06

6~ 0.04

0.02

0.010

(3-

I 10

I 15

I 20

I 25

"--7--7 I 30

35

.

.

.

.

.

.

.

I 40

0

'

45

T e m p e r a t u r e (°C)

Figure 1. Relationship between oxygen consumption rate and experimental temperature for the two agamid species, P. arabicus ( 0 O) and A. s. brachydactyla (O-------O). Each point represents the mean of different individual lizards. Vertical lines represent standard errors. The reported (see discussion) PBT range of P. arabicus (© . . . . . ©) (~ PBT = 39"31) and A. stellio ( 0 . . . . . Q) (~ PBT = 33).

252

M.K. AL-SADOON & N. M. ABDO

Table 1. The observed and predicted mean resting oxygen consumption values (mlg-1 h-1) of the two lizard species. The predicted values at three temperatures were calculated using regression equations derived by Bennett and Dawson ( l 976) and Bennett (1982 )

Species

20°Cobserved predictedratio(O/P)

30°Cobserved predictedrafio(O/P)

35°Cobserved predictedrafio(O/P)

P. arabicus A. steHiobrachydac~la

0'146 0"079

0 " 2 9 4 0"182 1"62 0 " 1 0 9 0"1343 0"81

0 " 3 0 9 0"316 0"95 0"165 0"212 0"80

0"070 2 " 0 9 0"049 1"61

for the two groups of lizards, are shown in Table 2. The Qlo values show variations within the M - T curves of the two groups. These variations in the Qlo values were noticed both at low and high temperatures. All Qlo values were found to be positive, though an observed value almost equal to unity (1"1) showed a shift or a constant 9 0 2 value in the range 2530°C and the range 30-35°C (extended to 40°C) for A. s. brachydactyla and P. arabicus (respectively). In general, Qlo values ofA. s. brachdactyla were higher than those recorded for P. arabicus throughout the experimental temperature range, except at 25-30°C, where P. arabicus scored a Qlo of 2"4 compared with 1"1 for A. s. brachydactyla (Table 2). The overall Q~o of P. arabicus was 1"6; again lower than the 1"8 overall Q~o value o f A . s. brachydactyla.

Discussion

The M - T curves of both species appear to be similar in shape, in spite of the slightly different position of the thermal dependence shifts noticed within the curves. It can also be seen that the M - T curve o f P . arabicus is above that ofA. s. brachydactyla, which indicates a higher metabolic rate. This may be explained by body mass-specific metabolism which varies inversely with body mass (~ w of P. arabicus and A. s. brachydactyla were 5"2 and 30"4 g, respectively). Similar observations have been made by A1-Sadoon (1983, 1986) and other workers (AI-Sadoon & Spellerberg, 1986, 1987; AI-Sadoon & Abdo, 1988, 1989; 1991). The results demonstrate the evolution of low body mass-specific VO2 within a family (Agamidae), correlated with the aridity of the niche, in comparison with the reported values of species inhabiting cooler climates (A1-Sadoon 1986; Davies & Bennett, 1981; Duvdevani & Borut 1974; Wheeler, 1984; Zari 1991). This supports the concept that low 9 0 2 is advantageous for desert reptiles. Such support is important, as accumulating data on small lizards (1-25 g body mass) tend to obscure the concept although low body massspecific ~zO2 seems to be common (Duvdevani & Borut, 1974). In the tropical and subtropical desert regions, such animals are rarely subjected to the long winters that would deter them from having a lengthy period of seasonal activity. Hence their lower metabolic

Table 2. The thermal dependence (Q l o values)of resting oxygen consumption for the species P. arabicus and A. stellio brachydactyla Qlo values of resting oxygen consumption for the indicated range of temperature

P. arabicus A. steUio brachydactyla

10-15°C

15-20°C

20-25°C

25-30°C

30-35°C

35-40°C

OverallQlo

2"0

1"4

1"7

2"4

1"1

1"3

1'6

2"3

1"7

1"8

1"1

2"3

3"0

1'8

METABOLIC RATE-TEMPERATURE CURVES

253

rate could well be an adaptation to desert existence. It has been reported that this situation prolongs the time over which an animal can live on a limited amount of food (Gordon et al., 1982), and can economize on its energy and water balance relations (Ambrose & Bradshaw, 1988). The oxygen level ratios consumed by the species with the higher body mass (A. s. brachydactyla) were closer to the predicted values of Bennett & Dawson (1976) and Bennett (1982), in comparison with the ratios of tOE of the lighter species (P. arabicus) (Table 1). These variations in the observed and predicted ~rO2 values may be attributed to the fact that the equations computed by Bennett & Dawson (1976) were obtained from interspecific relationships between large lizards, compared with the present data obtained from smaller animals (~ w = 5"2; 30"4 g). Hence, this variation in body mass could affect the body mass-specific RMR relationship in lizards. The present results are similar to those works on desert reptiles reported by AI-Sadoon (1986), AI-Sadoon & Abdo (1988, 1989, 1991); AI-Sadoon & Spellerberg (1987). The relatively high level M-T curves of P. arabicus may be a necessity for its life in this desert habitat. The Qlo values obtained (Table 2) were higher in the case of A. s. brachydactyla (~ w = 30"4g) than in P. arabicus (~ w = 5"2g). This situation is in agreement with previous work (Bennett & Dawson, 1976; Rao & Bullock, 1954). The single exception, recorded in the temperature range 25-30°C, could be explained by the fact that A. s. brachydactyla became active at a lower temperature and approached the PBT earlier than did P. arabicus, and a degree of homeostasis resulted in a shift in the Qlo value at 25-30°C. As the microhabitat ofP. arabicus is more hot and arid than that ofA. s. brachydactyla, it is to be expected that P. arabicus approaches PBT at a higher rate. The M-T curves show that the Qlo value shift ofA. s. brachydactyla in the temperature range 25-30°C (1"1) is well below the temperature range (30-35, 35-40) that gives the low Qlo value of P. arabicus (1"1, 1"3). These low thermal dependence values, which correspond with VBT or PBT ranges, suggest that P. arabicus which inhibits warmer microhabitats is active and has a higher PBT than A. s. brachydactyla which inhabits relatively cooler microhabitats. Previous work is consistent with this deduction (AlSadoon, 1986; Patterson & Davies, 1989; Pough, 1980; Spellerberg, 1982). From the results obtained by Rao & Bullock (1954) it appears that large animals are heat depressed at 22°C and higher, though such a situation could also occur with winter active animals. In general, the same workers accepted the slopes produced by Edwards (1946), who explained that between 7 and 17°C, Qlo increased with body mass, but at 23 and 32°C steep regression lines indicated heat depression in animals with large body masses. It is apparent from the Qlo values that there are two phases of increasing metabolism preceeding, and following, the low Qlo period equivalent to the PBT range. Several previous works have speculated on this situation (Andrews & Pough, 1985; Avery, 1976; Bennett & Dawson, 1976; Gatten, 1985; Patterson & Davies, 1978, 1984, 1989; Zari, 1991). Though the two species belong to the same family, and share several common habitats having similar climatic conditions in Arabia, many previous reports about localities of the two species point to differences in microhabitats according to latitudes, altitudes and thermal resources. Arnold (1986) observed specimens ofA. s. brachydactyla in the western montane and the northern Hail mountains of Arabia, in Egypt and Palestine, where communities of A. s. brachydactyla have invaded relatively cooler and damper northern latitudes. In Egypt, specimens of A. s. brachydactyla taken from around the coastal salt ponds of Alexandria were introduced to the banks of the Nile around the year 1896, and a population is now well established there (Flower, 1933). AI-Sadoon (1988) recently recorded a population of the same species that inhabited rocky escaprments near Riyadh in Central Arabia. On the other hand, he observed P. arabicus established on the highly arid sand dunes of the open desert. AI-Ogiley & Hussein (1983) described the regions in Arabia which are inhabited by agamids as being extremely hot and dry during summer, alternating with some rain and cold during winter. The atmospheric temperature

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M.K. AL-SADOON & N. M. ABDO

oscillated between 24 and 45"4°C, whilst the variation in crevices and burrows was only 2-8°C (32"5-35"3°C) during summer and autumn. It seems that burrows and crevices provide a suitable microhabitat within the VBT range of lizards (Zari, 1991), in particular for those species that are mostly burrow and crevice dwellers, e . g . U . a , microlepis and A. s. brachydactyla. (See also Cloudsley-Thompson, 1991). Diurnal heliothermic agamids usually maintain high (38°C) activity temperatures (Spellerberg, 1977; Avery, 1982; Philips, 1986). The extremely high temperature basker, P. arabicus has a ~ P B T of 39" 3 I°C (Arnold, 1984) while A. s. brachydactyla has a relatively lower £ P B T of 33°C (Scortecci, 1940). However, Loumbourdis & Halley (1985) reported the P B T range of A. stellio as 30-37°C; i.e. a ~ P B T of 33"5°C. The observations of AI-Sadoon (1983); Davies & Bennett (1981); Dawson & Templeton (1963); Halley & Davies (1986); Patterson & Davies (1989); Wheeler (1984); Zari (1987), on the climatic effect of metabolic rate on reptiles are reversed, or not clearly relevant here. The reversed position of the M - T curves and the overall Q~o values of A. s. brachydactyla (1"8) and P. arabicus (1"6) point to the vast body mass difference (5"2g; 30"4g), and indicate that the body mass-specific metabolic rate effect is more conspicuous and enhanced than the climate-habitat effect. The observations of Rao & Bullock (1954) strengthen the present view. Further studies of the metabolism and thermal gradient relations of A. s. brachydactyla populations from rocky desert microhabitats in Central Arabia, compared with populations from northern mediterranean regions, should provide interesting information. We wish to express our deep thanks to J. A. Tulba, and A. M. Fateh EI-Bab for technical assistance.

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