Why do female ball pythons (Python regius)
coil so tightly around their eggs?
Fabien Aubret,1,3 Xavier Bonnet,1,2* Richard Shine2
and Stéphanie Maumelat1
1Centre d’Etudes Biologiques de Chizé – CNRS, 79360 Villiers en Bois, France,
2Biological Sciences, A08, University of Sydney, Sydney, NSW 2006, Australia and
3School of Animal Biology, M092, University of Western Australia, Crawley, WA 6009, Australia
Question: What benefits does brooding confer to offspring viability that outweigh its costs to
the nest-attending female?
Organisms: Thirty captive Python regius females and their clutches.
Site: Vicinity of Lomé, Togo.
Background: It has previously been shown that brooding enhances ball python hatching
success by reducing desiccation of eggs.
Methods: We captured wild, gravid females just before the time of egg-laying. Then we varied
maternal attendance, allowing it to last 0, 15 or 60 days.
Conclusions: Brooding weakly influenced incubation temperature but markedly decreased egg
mass loss owing to water loss and associated yolk coagulation. Brooded eggs produced larger,
more active, faster swimming and more rapidly developing neonates than did non-brooded
Keywords: brooding, incubation, parental care, phenotypic plasticity, Python regius.
Mathematical models suggest that whether or not a given life-history ‘tactic’ will evolve and
continue to be expressed is dependent on the relative magnitude of two opposing forces: the
costs and benefits that accrue from expression of that characteristic (e.g. Charnov and Krebs, 1974).
One example of a complex behavioural trait with high costs is parental care (Alckock, 1993;
Andersson, 1994). In many organisms, reproductive individuals experience substantial risks, and
spend considerable energy in the process of raising their offspring (Clutton-Brock, 1988, 1991).
Presumably, the compensating benefit in this case accrues to the offspring: higher parental
investment may increase the offspring’s probability of survival or its subsequent
reproductive success (Townsend, 1986; Clutton-Brock, 1988; Clutton-Brock and Godfray, 1991). In cases where
* Address all correspondence to Xavier Bonnet, Centre d’Etudes Biologiques de Chizé – CNRS, 79360 Villiers en
Bois, France. e-mail: email@example.com
Consult the copyright statement on the inside front cover for non-commercial copying policies.
Evolutionary Ecology Research, 2005, 7: 743–758
© 2005 Xavier Bonnet
the parent provides direct protection against predators or nutritional input, the benefits to
offspring fitness are well known (Woodruff, 1977; Forester, 1979; Woodside et al., 1981; Gross and Sargent, 1985;
Wolf et al., 1988). However, the benefits of energetically expensive forms of parental care that do
not involve protection or nutrient transfer to the neonates are less obvious.
Although parental care of eggs is relatively rare among squamate reptiles, a distinctive
form of this behaviour appears to be ubiquitous in one lineage of snakes. Female pythons
remain tightly coiled around their eggs throughout incubation (Noble, 1935; Cogger and Holmes, 1960;
Hutchinson et al., 1966; Shine, 1985; Somma, 1990). In cool climates (where almost all studies have been
conducted), brooding females maintain high and constant temperatures within the clutch
by shivering thermogenesis (Vinegar et al., 1970), a behaviour that entails high energetic costs
(Vinegar et al., 1970; Harlow and Grigg, 1984; Slip and Shine, 1988). In addition, brooding females do not
feed during incubation (Ellis and Chappell, 1987). Given these high costs, what are the benefits?
The most likely benefits involve the effects of maternally controlled incubation regimes
on embryogenesis. Developmental trajectories in reptile embryos are highly sensitive to the
physical conditions encountered during incubation (Andrews, 2004; Deeming, 2004; Shine, 2004). Eggs
that experience conditions that are too dry or too wet, or too hot or too cold, either may die
before hatching or may hatch but produce inferior hatchlings with a lowered probability of
subsequent survival and growth (Fox, 1948; Taning, 1952; Licht and Moberly, 1965; Osgood, 1978; Muth, 1980;
Burger et al., 1987; Webb, 1987; Packard and Packard, 1988). This sensitivity has acted as a strong selective
force on maternal behaviour in reptiles from a diverse array of phylogenetic lineages, and
has favoured the evolution of careful nest-site selection in egg-laying species, and careful
thermoregulation by pregnant females of viviparous taxa (Beuchat, 1988; Shine, 2004). Plausibly,
the same kinds of selective forces have acted on maternal nest-attending behaviour (Shine et al.,
1997). That is, the benefit of maternal attendance might involve control over the physical
conditions experienced by incubating eggs, in ways that enhance egg survival and/or
hatchling phenotypes. The only obvious alternative hypothesis is that maternal attendance
functions to reduce predation on the eggs, but this could be achieved by the mother simply
remaining near the eggs rather than coiling around them and twitching. Indeed, this simpler
form of parental care is seen in many other squamate species (Shine, 1985; York and Burghardt, 1988;
Somma, 1990). Thus, attention focuses on two additional pathways by which maternal egg-
brooding might enhance offspring viability: through changes to the thermal and/or hydric
regimes experienced by the developing eggs.
The most detailed analysis of this question has come from research on water pythons
(Liasis fuscus) from tropical Australia (Shine et al., 1997; Madsen and Shine, 1999). Reproducing
females display facultative nest attendance, depending upon thermal regimes inside the
burrows where they lay their eggs (Madsen and Shine, 1999). Experimental incubation of eggs at a
variety of thermal regimes (but without maternal attendance) suggested that maternal
thermogenesis might substantially enhance offspring fitness, in some but not all natural nest
sites (Shine et al., 1997). This thermal effect was manifested both via increased hatching success
and via modifications to phenotypic traits of hatchlings (Shine et al., 1997). However, this study
was conducted on artificially incubated clutches only and was restricted to temperature
variations. Thus, Shine and colleagues’ (1997) study provided no information on the
determinants or consequences of hydric conditions during incubation, nor did it include
maternally brooded clutches.
We have addressed some of these missing elements with a study on ball pythons (Python
regius) at a field site in equatorial Africa (Aubret et al., 2003, 2005). This study is the third part
of an experiment that investigated: (a) the effects of brooding on the female’s energy
Aubret et al.744
expenditure (Aubret et al., 2005), (b) the influence of clutch size manipulation on hatching
success and hatchling traits (Aubret et al., 2003), and (c) the effects of brooding duration on
the phenotype of neonates (this study). The questions addressed in each of the three
constituents of the study diverge clearly: they focus respectively on the ‘relationships
between parental care and costs of reproduction’ versus ‘clutch size and reproductive
success’ versus ‘brooding duration and offspring phenotype’. Although complementary,
these issues were consequently considered separately. One part of the study showed that
brooding females spent very little energy over the 2 month incubation period, and that such
expenditure was independent of fecundity, challenging the notion that intensive parental
care necessarily entails major energy costs (Aubret et al., 2005). Experimental manipulation of
clutch size in fully attended clutches (i.e. ignoring brooding duration) strongly influenced
hatching success because females were unable to physically cover enlarged clutches, and
thus some eggs desiccated and died (Aubret et al., 2003). The current study focuses specifically
on the effects of the duration of maternal brooding on phenotypic traits of hatchling
snakes. Our current data thus bear directly on the effects of parental care duration on the
phenotypic traits of offspring.
Ball pythons (Python regius; Pythonidae) are small (up to 170 cm snout–vent length and
4 kg in weight) nocturnally active, non-venomous constricting snakes. The species extends
over a vast area of Africa in terms of longitude, latitude and habitat types (Luiselli and Angelici,
1998; Chippaux, 2001). High-density populations occur from South Ghana to South Benin,
especially in disturbed areas where rodents [the main prey of these snakes (Luiselli and Angelici,
1998)] are abundant. Our study was conducted in the extreme south of Togo (Lomé; 6⬚7⬘N,
1⬚13⬘E), an equatorial area characterized by high and relatively stable temperatures all year
round (from 25 to 35⬚C).
In the study area, females lay 3–14 eggs in tortoise or rodent burrows or abandoned
termite mounds, usually in early February (Aubret et al., 2003). Females coil tightly around their
clutches and adopt a defensive posture when disturbed (Fig. 1). Based on observations by
professional snake hunters (and personal observations), almost all clutches are attended by
females. However, some clutches are found without a female in attendance, suggesting
that brooding may be interrupted (perhaps for short periods) or may be facultative in this
species as it is in the water python (Shine et al., 1997).
As this study is the last part of a larger experiment, material (e.g. animals) and methods
overlap partly. However, all the current statistics are original. There is also a weak overlap
between previous mean values [e.g. maternal and offspring characteristics (size and mass) of
the control group, initial clutch sizes and initial maternal characteristics of the manipulated
groups remain unchanged] and those presented here. Such overlap was inevitable,
and necessary to provide links between the three studies. However, the current study
focuses on offspring phenotype, behaviour and growth rate as a result of incubation
regime; maternal and initial clutch characteristics are only provided to illustrate the
homogeneity of the procedure during the allocation of the mothers in the three treatment
Benefits of brooding to offspring viability 745
Snake hunters employed by the registered farm TOGANIM (SARL) captured 30 gravid
female pythons from the wild in the vicinity (<50 km) of Lomé at the beginning of the
laying season in January 2000. Each female was initially measured for total length (±0.5 cm)
and snout–vent length (±0.5 cm) with a flexible ruler, and body mass with an electronic scale
(resolution 1 g, precision ±0.2%). The snakes were maintained in small wooden cages
(50 ×50 ×30 cm) in a quiet, dark room. Water and food (pre-killed mice) were provided to
the snakes once a week. Although the females drank regularly, they refused to eat [as in
reproductive females of many snake species (Lourdais et al., 2002a)]. The 30 females produced
their clutches 15–45 days after capture. The clutch was weighed less than 6 h after
oviposition. Abnormal eggs (i.e. undersized or with incomplete shell) were discarded to
avoid possible mould contamination to the entire clutch. The mass, maximum length and
width of the eggs were recorded at the beginning of the experimental period, and then every
15 days until hatching. Because python eggs are strongly adherent, it was not always pos-
sible to separate them without damaging the shell. In such cases, the mass of each egg was
inferred from the mass of the clutch divided by egg number instead of weighing the eggs
individually. An estimate of the volume of the eggs was obtained using the equation to
calculate the volume of an ellipsoid: 4/3πab2, where a=1/2 the length of the egg and b=1/2
the width of the egg (Mayhew, 1963). As soon as the females began to lay their eggs, they were
randomly allocated to one of the three treatment groups:
Fig. 1. Female ball python coiled around her clutch. The defensive posture adopted by the female
allows observation of the eggs that are normally completely hidden.
Aubret et al.746
1. Ten ‘maternally brooded’ clutches were left with their mothers until hatching (i.e. control
2. Ten ‘partly brooded’ clutches were left with the mother for the first 15 days after laying;
then the female was removed and the eggs left without maternal attendance.
3. Ten ‘artificially incubated’ clutches were separated from the mother immediately after
The clutches left without maternal attendance were placed in boxes (50 ×50 ×20 cm)
filled with wood shavings. The eggs were placed in the middle of each box, close to the
surface, and were covered by a thin layer of shavings. Similar artificial incubators are used at
TOGANIM. However, the room we used was large and well ventilated, while local farmers
incubate the eggs in small and closed rooms. Despite the fact that the boxes we used were
watered once a week to keep the uppermost shavings damp, the humidity (not measured)
may have fallen below 100% at times. The high ambient temperatures in Lomé were buffered
in the incubators in a similar way as occurs in natural nests inside the burrows of tortoises
(see below). The clutches were inspected several times a week and any eggs affected by
mould were removed. Eggs that died during development were dissected, and we recorded
the body mass and body length of the embryo and the residual egg mass.
At the end of the experiment, all the females were apparently healthy and in good body
condition (Aubret et al., 2005). The females were released along with 10% of the neonates,
under regulations set down by local wildlife authorities. The rest of the neonates were
legally exported to the USA, Japan or Europe. None of the animals involved in our study
were mistreated, sick or injured. Our study was carried out under the ongoing legal activity
of TOGANIM. The IUCN recently undertook a survey on ball python populations of
Togo suggesting that this species adapts well to this legal trade (further information
is provided at the following IUCN site: http://www.iucn.org/themes/ssc/programs/
Incubation temperatures were recorded using two data loggers per treatment (Tinytag Ultra
−40 to 85⬚C; 1929 data for each recorder; delay between each record of 16 min and 30 s). We
attached the loggers (n=6) to the clutch. We placed two other temperature recorders in
potential natural nest sites: one in a termite mound and the other in a tortoise burrow. This
allowed us to compare potential natural incubation temperatures without maternal attend-
ance to those we monitored in our experiment. We also recorded the ambient temperature
of the room that housed the three sets of clutches. For analysis, we focused on the first
2 weeks of incubation, because early development is the period of highest sensitivity of the
embryos in squamates as well as many other vertebrates (Gerhart and Kirschner, 1997; Shine, 1999; Shine
and Elphick, 2001; Andrews, 2004). All our clutch data loggers recorded temperatures within a
narrow range [extremes were recorded in the ‘natural nest site’ (25.9⬚C) and ambient room
temperature (33.0⬚C)], and with similar means (28.8–30.5⬚C). Because the conditions were
further buffered in the cages and boxes, all the embryos experienced similar temperatures;
there were subtle differences among treatment groups, however. On average, mean temper-
atures of the maternally brooded or partly brooded clutches were slightly higher (1⬚C) than
those of the artificially incubated clutches (30.5⬚C in maternally or partly brooded clutches
versus 29.5⬚C in artificial incubators; the extremes ranged from 26.3 to 32.2⬚C). As expected
Benefits of brooding to offspring viability 747
with such a narrow range of temperatures, the variances were low (all standard deviations
lower than 1.3).
Incubation and hatchling characteristics
As soon as the neonatal snakes began to slit their eggshells, the eggs were removed from
the females or from the incubator and placed in individual containers. The time elapsing
between oviposition and egg-slitting was recorded (incubation time). The delay between
the first shell-slitting and the full emergence of the hatchling was also recorded. Most
hatchlings required more than 24 h to fully emerge from the egg (see Results), perhaps
because the snakes slit the shell before absorbing their residual yolk completely. On a few
occasions with unusually dry eggshells, the young snakes were unable to slit an opening
large enough to escape from the egg. After 1 day of unsuccessful attempts, we opened a
window (1 cm incision) with a scalpel. This reduced the total time necessary for emergence,
but prevented unnecessary mortality. Data on ‘delay of emergence’ for these animals were
not used in our analyses.
After full emergence, hatchlings were measured for body length, snout–vent length and
body mass (±0.1 g with an electronic scale). We counted the number of ventral scales,
recorded scale abnormalities, and determined sex by eversion of hemipenes. The size and
the shape of the head were measured with callipers as follows: (1) jaw length (from the tip of
the snout to the quadrato-articular projection); (2) skull length (from the tip of the snout to
the base of the skull); and (3) head width (maximal width above the eyes, from the external
margins of the supraoculars). The remaining egg mass was weighed [shell plus remaining
yolk (Deeming 1989)]. Water was provided to the neonates immediately after completion of the
Locomotor performance and behaviour of hatchlings
Locomotor performances of 1-week-old neonates were assessed by several tests, similar to
those previously used to quantify phenotypic quality in neonate reptiles (Van Damme et al., 1992;
Shine et al., 1997; Aubret et al., 2003). Swimming ability was recorded in a circular pool (1 m in
external diameter; 0.9 m in internal diameter; water temperature 28⬚C). Dropped from 5 cm
above the water, the hatchlings usually started swimming after a few seconds. During a
3 min trial, we recorded the total number of laps swum and the total time spent swimming
(disregarding the time during which the hatchling was immobile, or was trying to escape).
Hatchling swimming speed (distance covered in centimetres per minute) and the percentage
of time spent swimming per trial were calculated.
The crawling aptitude of the hatchlings was assessed in an open area of sand, a common
natural substrate for ball pythons in South Togo. The experimenter sat 3 m from the snake
to minimize disturbance. Over 2 min, the distance travelled from the departure point to the
final position was recorded, as well as the total number of tongue flicks (using a manual
counter). Scores on this test may reflect a combination of variables such as the vigour with
which the animal attempted to sample cues (tongue licking) and to escape (crawling speed)
from a potentially dangerous open area.
The defensive behaviour of the hatchlings was also recorded. Their propensity to strike
defensively at a small object (a pen moved at 10 cm from the snout) was assessed. The first
strike started the test, and then we counted the total number of strikes during the next 30 s.
Aubret et al.748
If the snake refused to strike after 3 min of harassment, or had adopted a passive defensive
position such as curling itself into a compact ball by that time, we scored the trial as null.
We also measured growth rates of the hatchling snakes from birth to 10 days after
emergence. Water was available, but the snakes were not fed during that period, so
variations in body mass or body size must reflect utilization of energy stores originally
present in the egg, including yolk conversion into new tissues and the associated water
intake. Notably, residual yolk can provide enough materials to sustain growth in body size
(Congdon et al., 1982; Ji and Sun, 2000). Finally, we recorded the age when the snakes first shed their
It was not possible to allocate the eggs randomly among treatments because they were often
strongly adherent to each other. Consequently, to control for the variance due to a possible
maternal effect, we used mixed-model analyses of variance (or analyses of covariance) with
maternal identity as a random factor, experimental treatment as a fixed factor, and morpho-
logical traits of the eggs or of the hatchlings as the dependent variables. We also used
mixed-model analyses of variance to assess physiological performances post hatching.
Snake body condition (mass relative to length) was analysed using analyses of covariance
with body mass as the dependent variable and snout–vent length as the covariate
(Garcia-Berthou, 2001). Because the neonates used in the behavioural tests were chosen randomly
within different clutches and equally distributed among groups, it was not necessary to
control for maternal (i.e. clutch) identity. Indeed, the mean number of neonates per mother
included in the behavioural analyses was 1.14 (range 1–2), removing a potential maternal
(pseudo-replication) effect. Growth trajectories were analysed using multivariate analysis of
variance with repeated measures of snout–vent length and body mass over time; maternal
identity was included as a random factor [multivariate analysis of variance (O’Brien and Kaiser,
1985)]. For all behavioural tests, several variances were not homogeneous (even after log
transformation), so we used non-parametric Kruskal-Wallis analyses of variance for these
tests. Null scores (i.e. no strike during the defensive behaviour test) were taken into account
in the analysis to avoid comparing behavioural measurements that may emerge from
different ‘decisions’ taken by the snakes (facing versus escaping the danger). All statistical
tests were performed with Statistica 6.1.
Maternal and clutch characteristics
The mean characteristics of the females and of their clutches allocated to the three
treatment groups are given in Table 1. We did not find any significant differences among
the three batches in the mean body lengths of the mothers (ANOVA with treatment as the
factor: F2,27 =0.47, P=0.63), their body masses (F2,27 =0.71, P=0.50), their body condition
(same design ANCOVA with maternal size as a covariate: F2,26 =0.31, P=0.74), their clutch
sizes (same design ANOVA: F2,27 =0.68, P=0.51), clutch masses (same design ANOVA:
F2,28 =0.19, P=0.82), or laying dates (same design ANOVA: F2,27 =0.59, P=0.56; P>0.50
in all post-hoc tests for these five analyses of variance). Similarly, egg mass at oviposition
(mixed-model ANOVA with maternal identity as a random factor and treatment as the
Benefits of brooding to offspring viability 749
main factor: F2,9 =0.02, P=0.98) and egg volume at laying (same design mixed-model
ANOVA: F2,26 =1.36, P=0.28) did not differ among the three groups. Therefore, any
difference in relevant traits among our treatment groups should reflect the influence of
incubation regimes on incubation periods and on the phenotypes of hatchlings.
Incubation regime, incubation period and morphology of the hatchlings
Incubation periods averaged 2 months and did not differ significantly among the three
treatment groups (Table 2). However, many traits we measured were affected by the
incubation regime (Table 2). Maternally incubated neonates were larger, heavier and had
longer jaws than neonates in the other two groups (all P<0.001). The artificially incubated
hatchlings were small and in poor body condition. However, our experimental treatments
did not induce any significant difference in many other traits, such as length or width of the
heads, or scale counts (total number of ventral scales, number of abnormal scales). Import-
antly, the mass of the material remaining after hatching (yolk +shell) was significantly
lower in the maternally brooded group, intermediate in the partly brooded group and higher
in the ‘artificially’ incubated group. This pattern suggests that maternal brooding allowed
hatchlings to incorporate their available yolk material into the body cavity before leaving
the egg. As expected, the mass of the remaining material was negatively correlated with the
body mass of the hatchlings (n=79, r=−0.40, P=0.0002).
Due to a net loss of water (Wangensteen et al., 1970; Rahn and Ar, 1974; Packard, 1991), the eggs lost mass
from laying to hatching in all three groups. However, the amount of water lost differed
significantly among the treatments (repeated measures of mass over time: Wilks’ λ=0.53,
F2,19 =6.34, P<0.008). Maternally brooded eggs lost, on average, 16% of their initial mass,
partly brooded eggs lost 37% of their initial mass, while the artificially incubated eggs lost
51% of their initial mass. This result clearly suggests that the presence of the mother limited
desiccation of the eggs.
Considering only maternally brooded hatchlings, greater hatchling mass was associated
with longer incubation (n=21, r=0.54, P<0.01) [Fig. 2; mean values per clutch plotted –
in these analyses we also used supplementary data from a parallel experiment in which
females were allowed to completely brood their clutches (Aubret et al., 2003)]. Also, larger eggs
gave rise to larger hatchlings (n=18, r=0.47, P<0.04).
Table 1. Maternal and clutch characteristics of the three brooding treatment groups of ball pythons
(n=10 females in each group)
Variable Maternally brooded Partially brooded Not brooded
Snout–vent length (cm) 112.5 ±2.2 114.5 ±1.6 115.3 ±2.6
Body mass (g) 1844.2 ±98.5 1933.3 ±91.2 2032.3 ±139.2
Pre-laying body condition (g) 1910.2 ±71.3 1917.7 ±70.6 1981.9 ±71.0
Clutch size 7.3 ±1.2 7.4 ±1.1 8.0 ±1.9
Egg masses at laying (g) 86.9 ±1.9 88.7 ±1.1 87.9 ±2.8
Egg volume at laying (cm3) 80.0 ±1.0 86.0 ±1.2 84.8 ±1.7
Note: Pre-laying maternal body condition represents maternal body mass adjusted by size. Sample sizes for egg
mass (n=12, 4, 12) and individual egg size at laying (n=62, 64, 65) for the maternally brooded, partially
brooded and the ‘artificially’ incubated group, respectively. Egg volume =volume of an ellipsoid (4/3πab2, where
a=1/2 the length of the egg and b=1/2 the width of the egg). Mean values are given with standard errors.
Aubret et al.750
Locomotor performance and behaviour of hatchlings
Incubation regimes affected several aspects of hatchling performance. First, maternally
brooded hatchlings emerged more rapidly from their eggs than did the partly brooded
snakes, which in turn emerged more rapidly than did the artificially incubated hatchlings
(Table 2). Larger neonates emerged from the egg more rapidly than smaller snakes, as
indicated by a negative correlation between body size and the duration to escape from
the egg (snout–vent length: n=66, r=−0.39, P<0.001; body mass: n=66, r=−0.47,
P<0.001). Thus, maternally brooded snakes were not only larger, but they also emerged
Table 2. Effects of incubation regime on incubation period, remaining egg mass and hatchling traits
(mean ±standard error) in the ball python, Python regius
brooded Not brooded d.f. F/H P
Incubation period (days) * 60.73 ±0.15 60.55 ±0.35 61.83 ±0.83 2,19 0.74 0.49
Emergence duration (days) 1.51 ±0.08 2.07 ±0.15 2.60 ±0.60 2,16 5.43 <0.01
Remaining egg mass (g) 5.50 ±0.43 13.14 ±2.00 17.18 ±3.58 2,19 11.31 <0.001
Morphology at hatching
Body mass (g) 55.05 ±0.91 44.31 ±3.23 38.35 ±2.96 2,19 9.00 0.001
Snout–vent length (cm) 39.26 ±0.28 35.01 ±0.81 35.83 ±0.95 2,19 15.66 <0.001
Body condition index 48.38 ±1.22 47.65 ±1.54 40.68 ±2.80 2,19 1.51 0.24
Jaw length (mm) ** 26.95 ±0.14 25.25 ±0.19 25.67 ±0.37 2,19 21.09 <0.001
Skull length (mm) *** 9.65 ±0.07 9.51 ±0.09 9.45 ±1.16 2,19 0.75 0.48
Head width (mm) ** 4.76 ±0.04 4.61 ±0.06 4.80 ±0.11 2,19 2.23 0.13
Number of ventral scales 207.04 ±0.48 205.82 ±0.65 205.33 ±1.09 2,19 0.82 0.45
Abnormal ventral scales (n) 2.37 ±0.26 3.55 ±1.10 3.00 ±0.68 2,19 1.32 0.28
Distance swum (m) 6.62 ±0.56 3.52 ±0.74 3.31 ±0.14 2,63 11.04 <0.005
Swimming speed (m ·min−1) 3.07 ±0.27 2.45 ±0.34 1.49 ±0.64 2,63 8.17 <0.02
Percentage of activity 67.95 ±4.34 47.62 ±5.69 56.18 ±10.64 2,63 7.86 <0.02
Distance covered on ground (m) 1.12 ±0.14 0.70 ±0.18 0.62 ±0.34 2,63 3.32 0.19
Number of strikes elicited 5.57 ±0.98 6.08 ±1.18 9.00 ±1.91 2,37 0.99 0.61
Number of strikes elicited †2.94 ±0.73 3.76 ±0.96 7.50 ±1.79 2,63 3.18 <0.02
Number of tongue flicks 112.11 ±7.67 95.05 ±10.03 103.83 ±18.76 2,63 2.73 0.26
Delay to first slough (days) 10.47 ±0.16 12.25 ±0.62 12.50 ±0.96 2,10 9.21 <0.01
BM (g) (10 days old) 58.18 ±1.13 52.31 ±2.64 40.50 ±3.28 2,12 8.12 <0.01
SVL (g) (10 days old) 44.31 ±0.30 42.44 ±0.73 40.10 ±1.05 2,12 5.43 <0.02
Increase in BM (g) **** 1.90 ±0.72 3.62 ±0.78 0.59 ±1.51 2,12 0.96 0.41
Increase in SVL (cm) *** 5.83 ±0.29 6.22 ±0.34 4.08 ±0.62 2,12 2.05 0.17
Note: All comparisons among the three treatments were performed using mixed-model analyses of variance with
maternal identity as a random factor, except for behavioural traits where Kruskal-Wallis analyses of variance were
used (see text for details). BM =body mass; SVL =snout–vent length.
Maternally brooded =clutch left with their mother until hatching (n=51 neonates); partially brooded =clutch
left with their mother during the first 2 weeks, then placed into an artificial incubator (n=22 hatchlings); artificially
incubated =clutch placed into an artificial incubator throughout incubation (n=6 hatchlings); * mean value per
clutch; ** relative to skull length; *** relative to initial snout–vent length; **** relative to initial body mass;
† number of strikes including null scores.
Benefits of brooding to offspring viability 751
from the egg more quickly. However, this latter effect may have been a consequence of the
former: when the effect of neonate size was taken into account, statistical significance was
lost using an analysis of covariance with clutch identity nested within incubation treatment
as factors, hatchling mass as covariate and escape time as the dependent variable
(F2,47 =2.96, P<0.061), but not with snout–vent length as covariate (F2,47 =4.95, P<0.011).
Maternally brooded hatchlings swam faster and over a longer distance than did young
snakes from the other treatments. We found a positive influence of snout–vent length on
locomotor ability. However, when significant, this relationship was always very weak
(n=63; 0.02 <r2<0.08, 0.25 <P<0.03 in all correlations between locomotor perform-
ances and snout–vent length), and including snout–vent length as a covariate in the analyses
did not affect the results. Consequently, the better swimming performance of maternally
incubated hatchlings was mostly attributable to the effect of maternal attendance per se
rather than a by-product of size. The maternally incubated hatchlings were also more active
(see Table 2). Our results suggest that on average maternally brooded hatchlings tended to
travel greater distances over the ground and explored their environment more intensively by
tongue flicking. However, the results for tongue-flicking rates and distance travelled over the
ground did not reach statistical significance, perhaps due to high variability for these traits.
The maternally incubated hatchlings did not display more intense defensive behaviour when
harassed during experimentation. In fact, the reverse trend was observed, and this effect was
significant when null scores were included. The proportion of snakes that struck the pen or
adopted a defensive passive posture differed, although not significantly so (due to the small
sample size of several cells in the contingency table), between treatments: 53% of the mater-
nally brooded snakes decided to strike, versus 62% for the partially brooded and 83% for the
non-brooded neonates (χ2=2.11, d.f. =2, P=0.34).
Post-hatching growth rates
Despite the absence of food, all young pythons increased their snout–vent length and body
mass from hatching to the age of 10 days (Table 2). Importantly, water was available ad
libitum over this period. This early growth undoubtedly was sustained by abdominal yolk or
other body reserves (Ji et al., 1997; Ji and Sun, 2000). Hatchlings from all groups showed similar
increases in size and mass (Table 2), but maternally brooded snakes maintained a significant
Fig. 2. Relationship between the duration of incubation and the body mass of hatchlings in the ball
python. These two parameters are plotted as means per clutch.
Aubret et al.752
advantage in terms of body size and body mass after 10 days (Table 2). Partially brooded
snakes were also larger and heavier than the ‘artificially’ brooded offspring. Snakes with the
highest body condition index at the time of hatching, and hence with greater body reserves,
exhibited a higher post-natal growth rate (growth rate after hatching was positively
correlated with initial body condition: n=54, r=0.64, P<0.0001) (Fig. 3).
Maternally brooded hatchlings also sloughed their skins earlier than did other hatchlings.
The delay from birth to the first slough was significantly correlated with the initial body
mass of hatchlings (n=46, r=0.52, P<0.0002). When this effect of body size was controlled
through an analysis of covariance (with body mass as the covariate), the influence of
treatment on sloughing delay was not statistically significant (F2,34 =1.25, P=0.30). Thus,
incubation treatment influenced sloughing mainly via its effect on the size of neonates;
small neonates tended to delay their first slough until they had attained a larger size.
Overall, our data suggest that maternally brooded hatchlings were in ‘better’ condition
(larger, faster, more active, faster-developing) than those that were partly brooded. The
hatchlings that were ‘artificially’ incubated were in the poorest condition.
Our data show that for ball pythons, parental care over a prolonged period strongly affected
not only hatching success of the eggs (Aubret et al., 2005), but also the phenotypic traits
of hatchlings that emerged from the viable eggs (this study). The variety of traits that
we measured support the inference that maternally incubated eggs gave rise to ‘better’
hatchlings. That is, not only was hatching success much higher from maternally incubated
clutches, but the hatchlings that emerged were larger, more active, swam faster and
for longer, and developed more rapidly post hatching than did offspring from artificially
Why did maternal brooding enhance hatching success and generate ‘superior’ hatchling
phenotypes in our experiment? Previous discussions of the benefits of maternal brooding
for offspring fitness have generally focused on thermal regimes (Vinegar et al., 1970; Vinegar, 1973;
Harlow and Grigg, 1984; Shine et al., 1997). Notably, in pythons, enhanced embryonic survival and
development has been attributed to the maintenance of high stable temperatures via
shivering thermogenesis. More generally, many studies on squamate reptiles have concluded
Fig. 3. Relationship between body condition index at hatching and growth in snout–vent length until
the age of 10 days in neonate ball pythons.
Benefits of brooding to offspring viability 753
that hatchling phenotype temperatures are more sensitive to thermal than to hydric con-
ditions during incubation (Van Damme et al., 1992; Shine et al., 1997; Flatt et al., 2001; Shine and Elphick, 2001).
However, several studies have reported strong hydric effects (e.g. Warner and Andrews, 2002; Brown
and Shine, 2004). Our results support this latter conclusion. Although we acknowledge that our
experimental design did not separate out thermal and hydric effects, the consequences of
maternal brooding for hatching success and offspring phenotypes are more likely to reflect
hydric than thermal factors. We base this conclusion on four observations:
1. Maternally brooded eggs lost less water than eggs in the other treatments, and the partly
brooded clutches lost less water than the non-attended ones [see also Aubret et al. (2003,
2005) for the difficulties faced by the mother in covering an artificially enlarged clutch
2. The low hatching success of ‘artificially’ incubated eggs was due to yolk desiccation:
many of these hatchlings left substantial solidified yolk behind in the egg, rather than
incorporating it into their bodies before hatching (note the negative correlation between
hatchling mass and residual egg mass; and the heavier residual mass of non-brooded
eggs). Yolk in ‘artificially’ incubated eggs began to solidify, especially on the desiccation-
prone upper surface. The solid mass of yolk may have directly impaired the hatching
3. Mean hatching dates did not differ among treatments. In reptiles, incubation period and
gestation length are strongly influenced by mean temperatures during development
(Blanchard and Blanchard, 1941; Naulleau, 1986; Lourdais et al., 2002b), so the similarity in incubation
periods infers a similarity in mean temperature.
4. We did not find any difference in the number of ventral scales or in the occurrence of
scale anomalies among the three groups. These traits are also sensitive to incubation
temperatures (Fox, 1948; Osgood, 1978; Lourdais et al., 2004). Overall, the differences in temperature
experienced by the embryos generated by our experiment were probably too small
(≈ 1⬚C) to induce any major effects.
How can the mother’s presence modify the hydric balance of her clutch? Female ball
pythons coil so tightly around the clutch that the eggs are completely hidden (Fig. 1;
personal observation). This ‘bell’ surrounding the clutch could create a saturated micro-
climate around the eggs, substantially reducing evaporation (O. Lourdais and D. DeNardo,
personal communication). Thus, our results differ from those of most previous research by
advocating an important hydric rather than thermal benefit for parental care in pythons. If
optimal incubation conditions generate optimal hatchling phenotypes, there will be strong
selection for maternal behaviours that expose embryos to such conditions [for example,
temperature-dependent sex determination processes (Shine, 1999); nest-site selection, shivering
thermogenesis (Shine et al., 1997)]. In such cases, natural selection can act at two levels: on genes
that code for maternal influence on the clutch (e.g. thermal criteria for nest-site selection or
the intensity of shivering thermogenesis), and on genes involved in norms of reaction
The mechanism that generated phenotypic variation among our hatchling pythons is one
that has not attracted previous interest – yolk coagulation due to egg desiccation that
reduces the amount of resources available to the embryos (Sinervo, 1990). We note that the
relative importance of hydric control on fitness versus that of thermic control remains an
open question in wild-brooding pythons. However, the eggs of many species that are not
Aubret et al.754
maternally incubated (reptiles, insects) often suffer from desiccation; hydric control may
allow pythons to incubate their eggs in areas where the clutch would potentially desiccate if
left alone (Snell and Tracy, 1986; Tracy and Snell, 1986).
A plausible scenario for the evolution of parental care in pythons involves an initial step
whereby the female’s presence benefited offspring survival by discouraging egg predators
(Shine, 1985). The subsequent change to the female’s posture (coiling tightly around her eggs)
may have been favoured because of the resultant substantial reduction in water loss from
the clutch at a very low additional cost (Aubret et al., 2005). Lastly, advantages associated
with high and more stable incubation temperatures may have resulted in the evolution of
shivering thermogenesis. Our results suggest that even a brief initial period (2 weeks) of
maternal brooding enhances hatching success and improves phenotypes compared with
artificially incubated eggs. If offspring fitness is enhanced even by brief parental attendance,
it is easy to imagine the evolution of obligate brooding through a series of intermediate
stages that involved a gradual increase in the duration of this behaviour (Mell, 1929; Shine, 1985;
Farmer, 2000, 2003). Any such adaptationist scenario is speculative, and relies upon data from
present-day taxa to extrapolate back to the ancestral condition. We may be misled by
adaptive changes that have occurred subsequent to the evolution of brooding. For example,
if maternal brooding maintains high constant humidity around the clutch, eggs may evolve
a lowered resistance to desiccation (because they are never exposed to this problem):
the sensitivity to desiccation might be secondary, notably to allow embryonic respiration
(Wangensteen et al., 1970). In keeping with this possibility, the eggs of ball pythons are more
vulnerable to desiccation than are the eggs of most other squamate reptiles (Alberts et al.,
1997). Comparative data on desiccation rates of eggs from brooding species and their
non-brooding relatives could clarify the validity of this hypothesis.
We thank TOGANIM SARL, and the snake hunters who helped us in Togo: Kodjo, Edmond,
Bélémé, Tamer, Enchorte and Apouale. Rex Cambag built the boxes and analysed the temperature
data. Radikale Menvotre helped with the English. Manuscript preparation was supported by
the Australian Research Council and the Conseil Général des Deux-Sèvres, Niort 79, France. All
experiments complied with the current laws in Togo.
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