Journal of J Comp Physiol B (1987) 157:393M02 Comparative Systemic,~om~.,, and Environ- Physiology B pmheynsfallogy Springer-Verlag 1987

Metabolism, temperature relations, maternal behavior, and reproductive energetics in the ball python (Python regius)

Tamir M. Ellis and Mark A. Chappell Department of Biology, University of California, Riverside, California 92521, USA

Accepted November 16, 1986

Summary. Thermogenic incubation has been docu- Compared to larger python species which are mented in two large species of pythons, but the endothermic during incubation, ball pythons have phenomenon has not been studied in small species similar aerobic scopes and higher mass-specific with concomitantly large heat transfer coefficients. 17o2max. However, effective endothermy in ball py- We describe behavior, metabolic rates, mass chan- thons is precluded by high thermal conductance ges, and temperature relations for adult ball py- and limited energy stores. thons (Python regius), the smallest member of the genus, during the reproductive cycle. Egg and hatchling metabolism and hatchling growth rates were also examined. Rates of oxygen consumption (!7o2) of both Introduction gravid and non-gravid snakes showed typical ecto- thermic responses to changing ambient tempera- Pythons inhabit the tropical and subtropical re- ture (Ta). The Qlo for Ta's of 20-35 ~ was 2.2-2.3. gions of Africa, Asia, and Australia. All are ovi- The 12o2 of gravid females was significantly greater parous and are among the few reptiles showing than that of non-gravid snakes at all Ta. Maximum extensive parental care. Females coil about their oxygen consumption (l/o max ) during forced exer- eggs shortly after oviposition and attend them until cise was about 12 times resting Vo2 at Ta= 30 ~ hatching, a period that may exceed two months. Eggs (5-6 per female) were laid in April. Total At least two species, the Indian python Python mo- clutch mass was approximately 32% of the fe- lurus and the diamond python Morelia spilotes, are males' pre-oviposition mass. After oviposition, able to keep clutch temperature substantially mother snakes coiled tightly around their clutches warmer than ambient temperature (T,) by means and remained in close attendance until the eggs of physiological thermogenesis. Heat production hatched in June. Sudden decreases in Ta elicited is apparently accomplished by spasmodic muscular abrupt but transient 2- to 4-fold increases in the contractions (' shivering'), and is adjusted so as to 12o~ of incubating females. Similar responses were keep body and egg temperature between 30 and not observed in non-incubating snakes. The 34 ~ at Ta of 23-33 ~ At low Ta, rates of oxygen steady-state I?o~ of incubating females was inde- consumption of incubating females may be 10-20 pendent of Ta. In no case was body temperature times greater than those of similar-sized non-incu- (Tb) elevated more than a few tenths of a degree bating individuals (Hutchison et al. 1966; Vinegar above T~ in steady-state conditions. et al. 1970; Van Mierop and Barnard 1978; Har- The 12o~ of developing eggs increased sigmoi- low and Grigg 1984). dally through the 58-70 day incubation period. To- The occurrence, function, and evolution of tal oxygen consumption during incubation at Ta = thermogenic incubation in the python family are 29.2 ~ was about 3.6 1 per egg. Young snakes poorly understood. Endothermy is thought to be quadrupled their mass during their first year of favored by large body size, with concomitant small growth. surface to volume ratios that favor the retention 394 T.M. Ellis and M.A. Chappell: Ball python reproduction

of body heat (derived metabolically or gained by Materials and methods solar basking). This concept is applicable to P. mo- Iurus, a robust species which begins to breed at Animals. Adult Python regius (6 gravid females, 2 non-gravid females and 3 males) were purchased in November 1983, shortly a mass of 15-20 kg and attains masses in excess after their importation from Africa. The animals were housed of 60 kg. However, M. spilotes is considerably individually in cages (1.5 x 0.45 x 0.45 m) which were kept in smaller (3-6 kg). Other reports of shivering during a large environmental room maintained at 29.1_+0.2 ~ and incubation have concerned both large and small 70-80% R.H. with a 12 h photoperiod. Drinking water was always available. Mice (Mus and Peromyscus) were offered to species: the blood python P. curtis (Vinegar et al. all snakes weekly. 1970), Timor python P. timorensis (Murphy et al. Gravid females laid eggs in their cages in late April or 1978), green tree python Chondropython viridis early May. After oviposition the female and a subset of the (Kratzer 1962; Van Mierop et al. 1983), and three egg clutch (usually 4 of 6 eggs) were placed into a plastic dish- Australian forms, Aspidites meIanocephalus, Liasus pan (0.33 x 0.30 x 0.12 m) containing 400 g of sterile particulate mica hydrated with 400 ml of chlorinated tap water. Copper- fuscus, and L. amethystinus (Boos 1979). None of constantan thermocouples were positioned in the substrate at these papers contain data on metabolic rates dur- least 0.15 m from the female and also in the air ca. 0.10 m ing incubation, so the degree of heat production above the substrate. The eggs removed from each clutch were is uncertain. Somewhat paradoxically, two very weighed, measured and incubated separately. Some of them were placed individually into 2 1 respirometers containing 300 g large species, the African python (P. sebae) and of a 1 : 1 mixture of sterile mica and water. Other eggs were the reticulated python (P. reticulatus), have not implanted with 36-gauge thermocouple wire with the couple been reported to regulate clutch temperature by positioned approximately in the geometric center of the egg. endothermic means, although they do coil around The leads were secured to the egg shell with methyl methacry- their eggs (Vinegar et al. 1970; Sclater 1862; Wall late adhesive. Thermocouple-equipped eggs were either re- turned to the maternal female or placed separately into plastic 1926). boxes (0.31 x 0.17 x 0.08 m) containing 400 g of 1:1 mica and A second hypothesis explaining the evolution water. Thermocouple implants did not affect subsequent em- of endothermy was proposed by Vinegar et al. bryonic development. Additional thermocouples were also (1970), who suggested that endothermic brooding placed in the substrate and air immediately adjacent to the egg. Eggs, and later hatchlings, were kept in the same environ- allows pythons to extend their geographic range mental room as the adults. into regions where Ta's are too low to support em- bryonic development. At least some species of py- Metabolic measurements. Oxygen consumption (I2o2) and in thons have eggs that require warm temperatures some cases carbon dioxide production (12co2) from post-absorp- tive animals were determined in open-circuit respirometer sys- during incubation; for example, the eggs of P. mo- tems. Pythons were weighed to +_0.1 g on a Mettler PC 4000 lurus and M. spilotes fail to develop at tempera- balance and placed into plastic metabolism chambers. Chamber tures below 27.5 ~ (Vinegar 1973; Harlow and volumes were 10.3, 17.6 and 0.8 1 for adult snakes, incubating Grigg 1984). This model may explain the occur- females with their eggs, and hatchlings, respectively. Metabo- rence of endothermy in P. molurus and M. spilotes, lism chambers were placed into a large temperature-controlled (+ 0.2 ~ environmental cabinet. Dry air was metered through both of which inhabit cool subtropical as well as Applied Materials mass flow controllers (model AFC-550), hu- tropical regions, and its apparent absence in P. midified to 50-80% R.H., and routed into the metabolism reticulatus and P. sebae, which are restricted to chamber via 2 side ports. A portion (40 ml/min) of the excurrent warm habitats. air from ports at the top of the metabolism chamber was dried (Drierite) and analyzed for CO2 (Beckman LB-2 or Applied In this paper we describe the behavior, metabo- Electrochemistry CD-3A), then passed through CO2 absorbent lism, and overall energetics of reproduction in ball (Ascarite), redried and analyzed for oxygen (Applied Electro- pythons, P. regius. This African species is the chemistry S-3A). The CO2 analyzer was standardized against smallest member of the genus Python, attaining calibration gases daily and both analyzers were referenced an adult length of about 1.5 m and a body mass against air diverted from immediately upstream of the flow controllers ca. every 5 rain during measurements. Air flow was of about 2 kg. Because of its small size, and be- adjusted during experiments so that [O~] was not less than cause its range is apparently limited to warm tropi- 20.5% and [CO2] did not exceed 0.5%. This required flow rates cal lowlands (Pitman 1974), P. regius would not between 0.50-1.20 1/min for adults and 0.06-0.20 l/rain for eggs be expected to show a significant endothermic about to hatch and for young snakes. During metabolic measurements on incubating females, brooding ability. Nevertheless, as in larger py- temperature data were collected from several thermocouples thons, female ball pythons closely attend their placed in the air surrounding the animal, affixed to its scales, clutches. We felt that a study of the metabolic and and inserted within the egg mass. Body temperatures (Tb) of behavioral responses of P. regius might produce incubating snakes were determined using temperature sensitive data which, in conjunction with published infor- radio-telemeters (Mini-Mitter, model XM) which were force-fed to the females prior to oviposition. mation on P. molurus and M. spilotes, could pro- Steady-state metabolic measurements were obtained at am- vide useful insights into the evolution of python bient temperatures (T,) between 20 and 36 ~ We assumed incubation behavior. that an animal was in steady-state if it was quiescent and T,, T.M. Ellis and M.A. Chappell: Ball python reproduction 395

Tb and Vo2 were stable during several hours of observation. id females and males weighed 1523 _+ 136 g (range: During experiments data were recorded and stored by a com- 1427-1619g; n=2) and 1571_+146g (range: puter. The ~2o2 and Vco2 were calculated using the equations 1408 1618 g; n=3) respectively. Prior to oviposi- in Hill (1972). For brooding animals, these values were cor- rected for the contributions of the eggs by subtracting egg Vo2 tion, there was little difference in the behavior of (obtained from closed-system measurements as described be- gravid and non-gravid snakes. Both sexes were qui- low, and adjusted for the effects of temperature using a Qlo escent during the day, but considerable activity was of 2.2) from the total oxygen consumption. observed at night. Non-gravid females and males Maximum oxygen consumption (12o max) was determined ate sporadically throughout the period of observa- for non-gravid adult pythons in a 5.71 chamber held at 30 ~ with an air flow rate of 1.2 l/min. The l)'o2max was elicited tions. In contrast, all gravid and incubating fe- by continually turning the chamber so that the python struggled males refused food until the eggs hatched in June. to right itself. This regime was maintained for 15 min. However, At this time all animals fed regularly. snakes were exhausted and unable to maintain a normal posture In early April, prior to egg laying, the mass after 5-10 rain of agitation. Because these Vo2 measurements were short term, we computed 'instantaneous' Vo~ according of gravid females had decreased to 1492_+149 g to Bartholomew et al. (1981). This procedure incorporates the (n = 4). Oviposition occurred in mid-April and was washout characteristics of the metabolism chamber and there- observed in one instance, with eggs being deposited fore increases the resolution of short-term metabolic events. at ca. 40 rain intervals. After egg deposition, The "effective volume" of the metabolism system during these mother snakes weighed 1085 +_ 95 g (n = 4), or 28% measurements was 5.0-5.41, depending upon the size of the snake being studied. less than non-gravid females. The mode clutch size The metabolism of two freshly laid eggs was followed was 6 eggs (range: 5-6; n = 5 clutches). Mean mass throughout development. Egg respirometers and an empty re- of the egg clutch shortly after oviposition was spirometer used as a control were sealed with gas-tight lids 485.0 _+ 47.0 g (n = 3), or about 32% of the females' equipped with rubber stoppers. A tube was attached to one stopper so that gas samples could be withdrawn from the imme- pre-laying mass. Eggs were cream colored, ovoid, diate vicinity of the egg. Gas samples (60 ml) were removed and smooth. Eggs weighed 85.5+14.7 g (n=13) as the respirometers were sealed and again after intervals (vary- and were 6.9 +_ 0.3 cm through the longest axis and ing from 4-24 h) calculated to produce a reduction in initial 4.2_+0.3 cm in diameter through the geometric oxygen concentration of about 1%. After each sample, the re- center. spirometers were opened and room air was drawn through the chambers with a vacuum pump to preclude gas trapping by the mica substrate. Duplicate 25 ml dry, CO2-free sub-samples were assayed for O2 with a Beckman E-2 paramagnetic ana- Behavior of incubating females lyzer. The CO2 content of other sub-samples was determined Females with eggs formed a turban-like coil using a Beckman LB-2 analyzer. The metabolism of a third egg was measured by open flow around their clutches within an hour after oviposi- respirometry during hatching. Observations began 12 h prior tion (Fig. 1 a). Their posture while coiled around to pipping (the appearance of slits in the egg shell) and con- the egg mass was unusual: the posterior third of cluded 12 h after the snake emerged from the egg. When no the snake was rotated 90 ~ so that the ribs and activity within the egg was apparent, metabolic data were col- lected for about 30 rain at intervals of approximately 60 rain. ventral scales projected laterally. The ventral body surface was concave and the eggs were partially Thermal conductance. Heating and cooling rates of females at- accommodated within this cavity (Fig. I b; see also tending eggs were determined by equilibrating the females plus Pitman 1974). In most cases (including periods egg clutch in saturated air at constant Zd, and then rapidly when Ta exceeded 34 ~ the egg masses were com- heating or cooling the metabolism chamber to a new Ta. Body temperatures were measured by radiotelemetry. Air, substrate pletely covered by the females' coils. Brooding fe- and intra-egg temperatures were measured by thermocouples. males loosened their coils every 3-4 h, and ap- Simultaneous 1)'o2data were also obtained. Measurements were peared to inspect the eggs for 10-30 min before taken every minute, until T, approximated Tb. Incubating fe- recoiling. Inspection activities consisted of nosing males did not become active or change posture during these manipulations. Conductance was calculated from cooling rates among the eggs accompanied by considerable and metabolic heat production, using a specific heat of 3.43 J/g tongue-flicking, but involved only the proximal and an assumed heat production of 20.1 J/ml O2. third of the animal's body and did not disrupt the All gas volume data were corrected to STPD. Data are ex- arrangement of the eggs within the distal coils. Less pressed as mean + standard deviation. frequently, a female would uncoil to the extent that the eggs were no longer in contact with her ventral surface. During uncoiling, movement by the female Results usually resulted in the repositioning of the eggs Body mass and behavior of adults prior to the resumption of brooding posture. Occa- sionally an egg would become dislodged from the Gravid Python regius weighed 1863 _+ 123 g (range clutch during these maneuvers. A loose egg would 1744-2077 g; n=6) in November, while non-grav- be retrieved either by looping a coil over it or by 396 T.M. Ellis and M.A. Chappell : Ball python reproduction

The ~7o~ and l/co2 of males and non-gravid fe- males, and of post-incubating females, showed re- sponses to changing ambient (and body) tempera- tures that are typical for ectothermic vertebrates (Fig. 2a). Both Vo~ and l/co2 increased exponen- tially with temperature with a Q~.o of approximate- ly 2.2 The regressions of log Vo2 against Ta for non-gravid pythons and for post-incubating fe- males were both significantly different from zero, but there were no significant differences in slope between these groups (P=0.358; ANCOVA). The respiratory exchange ratio (R) varied between 0.7 and 0.85. Gravid females also showed a normal response to changing temperature, with a Qso of 2.1 over the temperature range tested. However, they were characterized by mass-specific l/o'S 27-33% greater than their own post-incubation values (Fig. 2 b), or the combined data from males and non-gravid females. The differences are highly sig- nificant (P=0.0003 and P<0.0001, respectively; ANCOVA). Throughout the gestation period (De- cember until April), I/o2 was elevated by an incre- ment that remained approximately constant. In striking contrast to data from non-incubat- ing snakes, the 12o2 of female snakes incubating eggs and in thermal equilibrium with the environ- ment did not show any relationship to Ta (Fig. 2 c). Fig. 1. A Python regius incubating a clutch of 5 eggs. The tight turban-like coiling posture was assumed by all incubating fe- The regression of log 12o~ against T~ for incubating males. B Lifting the top coils reveals that the posterior third snakes has a slope not significantly different from of the female is rotated and the ventral scales face laterally. zero. There is considerable scatter in the data due Eggs fit within a cavity between the ribs to both inter- and intra-individual variation. Nev- ertheless, the regression slope for incubating fe- males is significantly different from the slopes for encircling the entire egg mass and recoiling. Fe- non-incubating adults (Fig. 2d; ANCOVA). In males left their clutches unattended only to drink general, l~o~ was elevated above the levels observed or shed. Excursions from the eggs were markedly in non-incubating females of similar mass. The dif- less frequent at night. ference was particularly large at low T,'s, where rTo~ of incubating snakes was often more than twice that of non-incubating animals. Neverthe- Metabolism of adults less, egg temperatures greater than 0.2 ~ above Regressions of resting metabolic rates (measured T, were never recorded during steady state-condi- as I/o 2 and lZco~) as a function of T~ were com- tions. At high Ta (31-35 ~ the !~o2 of incubating puted for snakes in 4 reproductive categories: females was occasionally considerably less than for males and non-gravid females, gravid females, in- non-incubating snakes. We found no evidence sug- cubating females, and post-incubating females gesting that I2o2 of incubating females followed a (mother snakes after their eggs had hatched). daily rhythm. Within these categories, significant differences be- Incubating females also contrast with non-in- tween individuals were observed only once (one cubating animals in their metabolic responses to non-gravid female's 1)92 was slightly but signifi- sudden drops in T~. A typical pattern from a non- cantly lower than the Vo2'S of the 4 other animals incubating snake is shown in Fig. 3 a. This animal in this group; ANCOVA). Exclusion of data from had been maintained at 31 ~ for several hours this individual did not affect the significance of and its 12o~ was stable. Rapid reduction of Ta to between-group comparisons. Accordingly, within- 27 ~ elicited a very small and transient increase group data were lumped for further analysis. in l/o 2 (after several hours both Tb and lYo~ had T.M. Ellis and M.A. Chappell: Ball python reproduction 397

8O 1.= A , Males and Non-Gravid Females I B Gravid Females ~ P~ Females " l~ ~L 5~ '~. 1.6 .%8~ ," /l ,, ", o /L 1.4 . o'=" l~jq-. : / r o o,~j..~ , .o j / " ~ ~ "'S~. -o" IL ../. _,/ 2o

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.J / 1.0 Incubating Females ~G All Data Combined lO 2'2 24I 26I 2~ 30I 32I 3'~ 2'~ 2', ~'6 2~ 3'0 32I 3',

Ambient Temperature oc Fig. 2A-D. Oxygen consumption (Vo2) of adult ball pythons at different ambient temperatures (Ta). In all four plots, the dashed line is the regression for mother snakes after their eggs had hatched. A Non-gravid adults (solid regression line) and mother snakes after their eggs had hatched. The two regressions are not significantly different. B Gravid females (solid regression line). The slopes of the two regressions are not different but the intercepts differ significantly. C Incubating females (solid regression line). The slope for incubating females is not significantly different from zero. D Regression lines for non-gravid adults (NG), gravid females (G), incubating females (/), and mother snakes after their eggs had hatched (PH). The regression equations are: NG: log V.'o2 (ml/kg-h)= 0.0350(T~)+0.2706; n=53, rZ=0.690 G: log V% (ml/kg.h)= 0.0299(T,)+0.5461; n=22, r2=0.603 I: log Vo2 (ml/kg.h) = -0.0063(T,)+ 1.6758; n=53, r2=0.017 PH: log Vo2 (ml/kg.h)= 0.0332(T~)+0.3446; n=27, r2=0.766 declined). Similar small to nonexistent responses 219.6 _+ 36.0 ml/kg, h (n=4) when averaged over were characteristic of both males and non-gravid 8 min. These values are approximately 11-12 times females. However, the 1?o2 of incubating females the resting I)92 of non-gravid animals at 30 ~ always increased markedly when T= was lowered The Vo2 and Vco2 remained elevated above normal (Fig. 3 b, 3 c, 3 d). The response occurred rapidly resting levels for at least 40 min after exercise had (ca. 3-6 min), even after small (ca. 0.4 ~ changes ceased. However, pythons were able to locomote in Ta and before any decrease in Tb was apparent. normally within 10-20 min after testing. In most cases 1)'o2 increased by 100-300% within 10 min following a decrease in ira, and began to decline about 40 min later. After several hours, r Thermal conductance remained elevated above the premanipulation lev- el, even though Tb had decreased substantially. In Adequate cooling curves (n=4) were obtained no instance did we observe spasmodic muscular from three incubating females. The thermal con- contractions or "shivering", as reported for P. mo- ductance computed from these data showed con- lurus and M. spilotes. No unusual metabolic re- siderable variation, possibly because of differences sponses occurred in any group when Ta was in- among the animals in the number of eggs and the creased. arrangement of the coils. Values ranged from 0.43 The Vomax of four adult pythons to 1.02 W/[kg-~ which is equivalent to 78 to (1123+159g) at 30 ~ was 257.4_+37.2ml/kg.h 183 ml O2/[kg-h~ The mean conductance was (n = 4) when averaged over 2 rain and 0.65+0.11 W/[kg- ~ 398 T.M. Ellis and M.A. Chappetl: Ball python reproduction

=

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20 1345 g 40 B ~v~*g~ 40 ,0 0g] O :oO:O = o/oli';''''i

00%0o00 o= o*% oO~CooooOoOOooco ,,~e J o o o 0Oo o,o o o~.,.-,....,.~. @ 0') C e- Carbon Dioxide U =r X uJ Is ...... 60 o i" 0 ol

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o~O ~ Elapsed Time (black bar equals 10 rain) Fig. 3A-D. Responses to decreases in T~. A Non-gravid adult, B-D incubating females. The black bars represent 10-min inter- I I I I I vals. Heavy lower line is l/o2, thin upper line is Ta, dashed 10 20 30 40 50 6~0 70 line is Tb (obtained from a telemeter in the digestive tract), Days After Oviposition and dotted line is Tegg (obtained from implanted thermocou- ples) Fig. 4. Closed system ~2ozand carbon dioxide production (12co2) during development in two eggs (open and closed symbols) maintained at 29.2 ~ Triangles indicate data gathered shortly after pipping (day 70). Squares represent data measured after Egg metabolism and hatchting growth rates emergence (day 71) Python regius eggs attended by the females pipped after 58-59 days (n = 7), while those eggs isolated from the females required between 71-76 days (n = At this time the embryo was covered by fluids with- 8) to pip at similar temperatures (29-30 ~ How- in the extra-embryonic membranes, which were en- ever, there was no differential mortality between gorged with extra-corporeal circulation (particu- the two methods of incubation. larly those which adhered to the shell). No p.ulmo- A sigmoid relationship of T2o~ and l/co 2 over nary breathing was evident. The 12o2 and Vco2 at time was obtained from two developing P. regius pipping were similar to the values measured during eggs (Fig. 4). During the first 50 to 55 days of the last week of incubation. At 12 to 30 h after incubation at 29.2 ~ the increase in 1/o2 and l?co 2 pipping, most of the fluids within the shell had with time was approximately exponential. Both evaporated, blood had coagulated within the extra- l?o~ and l/co ~ plateaued for 10 to 15 days prior embryonic membranes, and pulmonary ventilation to pipping. Transient increases in gas exchange was obvious. The 1/o2 and l?co~ measured after were observed during the plateau phase of 12o2 the onset of lung breathing were about 40% prior to pipping; these may have been associated greater than the rates present during pipping and with embryonic activity. However, plateau-phase correspond to the 12o2 measured from emerged fluctuations produced at most an 8% increase in hatchlings. Table 1 gives metabolic data gathered Vo2 and Pco~. in an open flow system from an individual egg Respiratory exchange ratios (R) throughout in- and its hatchling. cubation were about 0.73, which is indicative of Hatchlings emerged from the eggs 24-48 h after fat catabolism. The total Po~ during the 70-day pipping. At this time, their mass was 50.2_+ 6.7 g incubation period was about 3.6 1 per egg at Ta = (n=15) and they measured 39.6_+1.5 cm (n=15) 29.2 ~ representing about 72 kJ of energy metab- from snout to vent and 43.0_+].7 cm in total olism. Assuming that all of this energy was derived length. All but one of 15 young snakes refused from fat, each egg metabolized about 1.8 g of fat food for 20-40 days after hatching and then began over the interval from oviposition to hatching. to feed avidly (the single exception fed on day 6). Several eggs were opened shortly after pipping. Six months after hatching, mass had increased T.M. Ellis and M.A. Chappell: Ball python reproduction 399

Table 1. Mass-specific 12o2, l?co2, and respiratory exchange ratio eral families of snakes at 20 and 30 ~ Their equa- (R) from an individual egg and its hatchling maintained at tions indicate scaling to mass ~176 which is signifi- 29.1 ~ Gas exchange was measured using an open flow sys- cantly different from the b values we obtained. The tem. The hatchling weighed 59.5 g at emergence and 43.1 g at age 30 days. Values of 12o2 and lZco2 (ml/[kg. h]) represent Bennett-Dawson equations slightly underestimate a series of measurements in each condition, and are given as the metabolism of a 40-50 g hatchling P. regius, the mean_+ standard deviation but overestimate the metabolism of a I kg adult by about 2-fold. Condition 1/o2 Vco~ R Incubation periods and the overall rates of oxy- Pre-pipping 62.6_+ 3.5 59.2 + 3.1 0.946 gen and carbon dioxide exchange for developing Pipping 69.4_+4.1 61.2+3.5 0.882 ball python eggs, isolated from the mother at ovi- Pipped, ventilating 91.2 _ 6.2 87.5 + 2.5 0.959 position and incubated separately, are similar to Newly emerged 85.2_+2.6 75.5_+4.8 0.886 those reported for the eggs of other reptiles (Dmi'el 30 Days old, anorexic 61.9_+2.6 47.2_+4.0 0.763 1970; Ackerman 1980). However, the ontogeny of gas exchange in P. regius eggs more closely resem- bles the sigmoid pattern reported during the em- 4-fold to 207.4_+29.8 g (n=15) while length in- bryological development of precocial birds (Vleck creased less dramatically (snout-vent length: et al. 1980) and sea turtles (Ackerman 1980) than 57.2_+2.3 cm, total length: 61.6+2.6 cm). We the exponential patterns reported for the eggs of measured metabolism several times during the first other types of snakes (Dmi'el 1970). Presumably, year of growth. The 12o~ of hatchlings declined embryonic growth proceeds most rapidly early in steadily during the initial 20-40 day anorexic peri- the incubation period and abates after days 50-55 od (Table 1) and then stabilized when the animals (Fig. 4). Comparable data over the entire incuba- began to feed. When combined with data from tion period are lacking for other python eggs. non-gravid, non-incubating adults, the 12o2 data However, Van Mierop and Barnard (1978) mea- from snakes older than 40 days show a linear rela- sured the 12o2 of P. molurus eggs during the nine tionship between log I)o 2 and log body mass. For days that immediately preceded pipping; these eggs data taken at Ta = 20 ~ and converted to the allo- hatched synchronously with those in the mater- metric form y = aM b: nally incubated clutch, but over this period no in- ml O2/h=0.149 M ~ (1) crease in 17o2 occurred. These data suggest that a sigmoid pattern of metabolism with a plateau where: M=mass in g; n=23, mass range phase late in development may be the general case 43-1531 g; r2=0.83; 95% confidence interval for for python eggs. One striking (and unexplained) b=0.460-0.691. For Ta=30 ~ observation was the substantially shorter incuba- ml O2/h = 0.264 M ~176 (2) tion period of P. regius eggs attended by female snakes (58-59 days) compared to that for unat- where: n=29, mass range 43-1738 g; r2=0.92; tended eggs (71-76 days). 95% confidence interval for b =0.567-0.712. Gravid ball pythons were behaviorally indistin- guishable from other females, except that they did not eat. Nevertheless, they had rates of oxygen Discussion consumption that were disproportionately high In many respects the thermal physiology and me- when compared to non-gravid snakes, or to their tabolism of ball pythons resembles that of other own Vo2'S measured after their eggs had hatched reptiles. Both adult and young ball pythons show (Fig. 2b). The I?o2 appears to remain elevated over regular increases of 12o2 with increasing tempera- the entire five to six month course of egg produc- ture, with Qlo values of 2.2-2.3. The 12o2 increases tion. During the ca. 150 day gestation period a as a function of mass ~ at 20~ and as fasting 1.85 kg gravid female resting quietly at mass ~176 at 30 ~ (Eqs. i and 2). These values 30 ~ needs to metabolize about 91 g of fat. This are not significantly different from each other or represents roughly a 33% increase in fat utilization from 0.67 (P >0.2, t-test). They are significantly above that of a non-gravid snake of similar mass. different (P <0.003; t-test) from the scaling to The eggs themselves consist mainly of stored yolk mass ~ predicted by McMahon's (1973) model and albumin and are presumably metabolically in- of elastic similarity, but are consistent with active throughout ovigenesis, but their mass con- Heusner's (1982) prediction, based on dimensional tributes substantially to the total mass of the analysis, of scaling to mass ~ Bennett and Daw- mother. This would be expected to depress esti- son (1976) summarized the available data for sev- mates of maternal mass-specific l?o~, instead of ele- 400 T.M. Ellis and M.A. Chappell: Ball python reproduction vating it as we observed. The high 1)o2 of gravid be noted that l?o2max during short bouts of intense ball pythons may reflect not only the maintenance exercise may be higher than the l?o2max sustain- metabolism of the mother but also the physiologi- able over long periods of thermogenlc activity. cal events associated with egg production, includ- The production of temperature gradients be- ing the conversion, mobilization and transfer of tween body and environment depends on thermal maternal resources into storage within nascent conductance as well as thermogenic capability. We eggs. computed conductances of P. molurus and M. spi- The behavior of female ball pythons during in- lotes from I7o~ and steady-state Tb-T a gradients cubation is quite similar to that seen in Python reported by Van Mierop and Barnard (1978) and molurus and Morelia spilotes. All three species re- Harlow and Grigg (1984). In this respect, ball py- main in nearly constant attendance on their thons are at a severe disadvantage compared to clutches for two months or more, departing only P. molurus and M. spilotes. Because of their small when the hatchlings emerge. However, the meta- size (1-1.2 kg), incubating ball pythons have rela- bolic responses to cold and the amount of thermo- tively high rates of thermal conductance: about genesis in ball pythons are much less impressive 0.65 W/[kg'~ as compared to 0.27 W/[kg.~ than reported for the two larger species. Ball py- for the 3.8 kg M. spilotes and 0.114 W/[kg. ~ for thons were apparently unwilling or unable to pro- the 21 kg P. molurus. Even assuming it could sus- duce enough metabolic heat to elevate body tem- tain its proportionally high ~7o2max for long peri- perature more than a few tenths of a degree above ods, the ball python can maintain a Tb-Ta gradient ambient temperature. Several physiological, eco- of no more than 2.1 ~ at Ta's near 30 ~ Because logical, and evolutionary factors could account for of Qlo effects the maximal gradient would be even the relatively poor performance of ball pythons, smaller at lower Ta, where thermogenesis would including small mass-specific 17o2 or aerobic scope, presumably be most important for embryonic de- high heat loss rates due to small size, inadequate velopment. At the maximum Vo2 we observed dur- energy stores due to small size, and lack of selec- ing metabolic responses to sudden drops in T, tion pressure for endothermic incubation in the (Fig. 3), brooding ball pythons produced about natural habitat. 0.18 W/kg of metabolic heat, which is sufficient Our data suggest that ball pythons are capable to support a Tb-T~ gradient of only 0.28 ~ Thus, of utilizing oxygen at mass-specific rates that com- surface-volume relationships, rather than metabol- pare favorably to those reported for incubating ic scope, may be the principal physiological factor P. molurus and M. spilotes, at least for short peri- limiting the evolution of endothermic brooding in ods. The comparison with M. spilotes is most ger- pythons. mane, because that species is much closer in size Energy and material storage are also poten- to the ball python than is P. molurus. After adjust- tially limiting to the evolution of endothermic ing for the metabolic contribution of the egg mass, brooding in ball pythons. If the animals were capa- the maximum I/o2 observed during peak periods ble of sustaining a high enough 1)o2 to maintain of thermogenesis was 1.47 ml/[kg, rain] at 27 ~ for a modest Tb-T a gradient of 3 ~ a typical 1.1 kg a 3.8 kg M. spilotes, or about 22 times the resting post-oviposition female would need to metabolize level at that Ta (Harlow and Grigg 1984). When about 4.7 g of fat per day to support this level computed on the basis of body temperature of thermogenesis. The fat requirement over a long (29.2 ~ instead of Ta, the factorial increase above period of thermogenesis would be prohibitive - ap- resting. 1~o2 was about 16. The maximum thermo- proximately 280 g over a 60-day incubation cycle. genic Vo2 of a 21 kg P. molurus was 2.82 ml/[kg. Based on the above analyses, we would not min], and the factorial scope (computed as de- expect to find significant endothermic brooding scribed above) was 11.2 (Van Mierop and Barnard ability in pythons smaller than 2.5-3 kg. Excep- 1978). These values are similar to the factorial tions to this rule may occur in hole-nesting species, scope of 12 we observed in ball pythons exercised which could benefit from the increased insulation at T, = 30 ~ Moreover, mass-specific l/o max in provided by the walls of a wooden cavity. This ball pythons (3.5-4 ml/[kg-mini) was considerably may be the case for Chondropython viridis, a tree- greater than the mass-specific I)o max in the other cavity nester which has been reported to shiver two species. From these data, and because the IZo while attending eggs despite a mass of less than of incubating ball pythons never exceeded 25% o~" I kg (Kratzer 1962; Van Mierop et al. 1983). Bask- Po2max, we conclude that the lack of significant ing behavior may also be used to augment meta- thermogenesis in this species is not directly attrib- bolic heat production. Early-morning solar bask- utable to low aerobic scope. However, it should ing was employed by an incubating 3.8 kg M. spi- T.M. Ellis and M.A. Chappell: Ball python reproduction 401

lores to rapidly raise Tb. The warmed snake then optimal thermal environment for development. returned to her clutch and the absorbed heat was Ball python incubation confers no thermal advan- transferred to the eggs. This behavior reduced the tage, but may help protect the vulnerable and valu- animal's need to produce heat endothermically able clutch against attacks by egg predators (e.g., (Harlow and Grigg 1984). Similar basking behav- small mammals or birds). Furthermore, our data ior at artificial light sources has been reported for suggest that parental attendance shortens the incu- incubating P. regius, but these small snakes (and bation period by 15-20% (compared to unat- their clutches) cooled rapidly when the heat source tended eggs kept at identical temperatures), which was removed (Van Mierop and Bessette 1981). would also help reduce vulnerability to predation. Even though brooding female ball pythons are In an environment with substantial predator pres- unable to conserve body heat, they increase l?o~ sure, the additional investment in protective paren- considerably and rapidly when challenged with a tal care might greatly benefit the female's fitness decrease in Ta (Fig. 3). We have no satisfactory by increasing the probability the clutch will survive explanation for this response. Similarly, we cannot until hatching. adequately explain the apparent temperature inde- pendence of metabolic rate in incubating females Acknowledgements. We thank J. Wasilewski for help in obtain- ing animals, D. Rios and E. Hice for assistance in building (Fig. 2c). One possible interpretation is that high equipment, T. Greek for helping with measurements, and T. l?o~ at low Ta or following a sudden decrease in Moisi for providing incubation and growth rate data and loan- Ta simply reflect increased agitation or discomfort ing us several ball python eggs. Funds were provided by an of the females at temperatures that may be too intramural grant from the University of California, Riverside, low for optimal egg development. Long-term incu- and by NSF Grants BSR-8104699 and BSR-8202708 to M.A.C. bation at Ta below 27.5 ~ prevents development in the eggs of P. molurus and M. spilotes (Vinegar 1973; Harlow and Grigg 1984); presumably the References development of P. regius eggs also requires warm conditions. However, we observed few if any be- Ackerman RA (/980) Physiological and ecological aspects of havioral manifestations of agitation when incubat- gas exchange by sea turtle eggs. Am Zool 20:575 583 ing females were held at low Ta. Alternately, the Bartholomew GA, Vleck D, Vleck CM (/981) Instantaneous brooding ball python's elevated r2o2 at low T~ may measurements of oxygen consumption during pre-flight be an evolutionary relic from larger ancestral warm-up and post-flight cooling in sphingid and saturniid moths. J Exp BioI 90:17-32 forms which were capable of effective endothermic Bennett AF, Dawson WR (1976) Metabolism. In: Gans C, brooding. Adequate evidence for evaluating these Dawson WR (eds) Biology of the Reptilia, vol 5. Academic hypotheses is currently unavailable. Press, New York, pp 127-223 An obvious question concerns the evolutionary Boos HEA (1979) Some breeding records of Australian py- persistence of incubation behavior in the ball py- thons. International Zoo Yearb 19:87-91 Dmi'el R (1970) Growth and metabolism in snake embryos. thon despite the absence of any thermal benefit. J Embryol Exp Morphol 23:761-772 A clear answer to this question requires a thorough Harlow P, Grigg G (1984) Shivering thermogenesis in a brood- knowledge of natural history, ecology, and evolu- ing diamond python, Python spitotes spilotes. Copeia tion, which is unfortunately lacking for ball py- 1984:959-965 Heusner AA (1982) Energy metabolism and body size. II. Di- thons. Nevertheless, it is unclear why females mensional analysis and energetic non-similarity. Respir should undertake a 50-70 day incubation cycle, Physiol 48 : 13-25 which is probably dangerous and is certainly ener- Hill RW (1972) Determination of oxygen consumption by use getically costly because of the concomitant fast, of the paramagnetic oxygen analyzer. J Appl Physiol unless they achieve some selective advantage. The 33:26/-263 Hutchison VH, Dowling HG, Vinegar A (1966) Thermoregula- eggs comprise nearly one-third (32%) of the fe- tion in a brooding female Indian python, Python molurus male's pre-oviposition mass and clearly represent bivittatus. Science / 51 : 694-696 a large fraction of the female's energy and material Kratzer H (/962) Uberraschende Nachzucht yon Chondro- stores. Similar relative clutch masses have been re- python viridis. Aquar Terrar 15 : 11 2119 McMahon T (1973) Size and shape in biology. Science ported for P. regius (31.1%; Van Mierop and Bes- 179:1201 1204 sette 1981), P. molurus (24.7%; Van Mierop and Murphy J, Barker D, Tryon B (1978) Miscellaneous notes on Barnard 1978) and M. spilotes (30.5%; Harlow the reproductive biology of reptiles. 2. Eleven species of and Grigg 1984). For a large snake, the additional the family Boidae, Genera Candoia, Corallus, Epicrates and cost of incubation and thermogenesis is propor- Python. J Herpetol 9:385-390 Pitman CRS (1974) A guide to the snakes of Uganda. Revised tionally small and presumably helps safeguard the edition. Wheldon and Wesley Ltd., Great Britain large investment in the clutch by providing a more Sclater PL (/862) Notes on the incubation of Python sebae, 402 T.M. Ellis and M.A. Chappell : Ball python reproduction

as observed in the Society's Gardens. Proc Zool Soc London Vinegar A (1973) The effects of temperature on the growth 1862:365-368 and development of embryos of the Indian python: Python Van Mierop LHS, Barnard SM (1978) Further observations rnolurus (Reptilia: Serpentes : Boidae). Copeia on thermoregulation in the brooding female Python molurus 1973:171 173 bivittatus (Serpentes : Boidae). Copeia 1978 : 615-621 Vinegar A, Hutchison VH, Dowling HG (1970) Metabolism, Van Mierop LHS, Bessette EL (1981) Reproduction of the ball energetics and thermoregulation during brooding of snakes python, Python regius in captivity. Herpetol Rev 12:20-21 of the genus Python (Reptilia, Boidae). Zoologica 55: Van Mierop LHS, Walsh T, Marcellini DL (1983) Reproduc- 19-48 tion of Chondropython viridis (Reptilia, Serpentes, Boidae). Vleck CM, Vleck D, Hoyt DF (1980) Patterns of metabolism In: Marcellini DL (ed) The sixth annual reptile symposium and growth in avian embryos. Am Zool 20:405-416 on captive propagation and husbandry. Thurmont, Mary- Wall F (1926) The reticulate python Python reticulatus land, pp 265-274 (Schneider). J Bombay Nat Hist Soc 31:24-90