Journal of the Royal Society of Western Australia, 86:83-84, 2003 
Notes 
on .... 
Clutch hydration following 
oviposition by urination 
may reduce desiccation in 
thorny devil (Moloch 
horridus) eggs 
M T Ladyman 1 & S A Thompson 2 
‘School of Animal Biology, University of Western Australia, 
Crawley WA 6009 E3 mitchl@cyllene.uwa.edu.au 
2 Centre for Ecosystem Management, Edith Cowan 
University, Joondalup WA 6027 E s.thompson@ecu.edu.au 
(Manuscript received August 2002; accep)ted April 2003) 
Abstract. During fieldwork in the eastern goldfields, a 
gravid female thorny devil (Moloch horridus) was 
collected. Whilst held in captivity the female deposited 
eight eggs in a calico bag and apparently urinated over 
the clutch. We propose that thorny devils may hydrate 
their eggs after oviposition to reduce desiccation during 
development in the nesting burrow. This may lead to a 
fitness increase for the offspring at hatching in the form 
of increased hatchling size 
Key words: clutch, hydration, Moloch horridus, dragon 
lizard 
Introduction 
Among oviparous species, optimal nest site selection, 
nest construction and maintenance of the egg clutch ( e.g . 
brooding) can contribute to a reduction in offspring 
mortality (Covacevich & Limpus 1972; Cooper-Preston 
1992; Shine 1998) and increase offspring fitness (Shine 
1988). 
Desiccation of reptilian egg clutches occurs most 
rapidly in environments with low water vapour pressure 
and high thermal conductivity (Ackerman 1994), and in 
situations where a large proportion of the total surface 
area of the clutch is exposed to soil or open air 
(Ackerman & Dmi'el 1985). Consequently, reptiles often 
deposit their clutch in a burrow that provides thermal 
stability, higher humidity than ambient or surface 
conditions, and reduced exposure (Baudinette 1973; 
Bennett 1988; Roper 2001). 
Like other agamids, thorny devils (Moloch horridus) lay 
eggs and excavate their own nesting burrow (Pianka et 
al. 1996; Thompson in press). Eggs are incubated in the 
burrow for approximately 115 days (range 90-132 days; 
Sporn 1965; Pianka et al. 1996), and during this time there 
is great potential for egg desiccation. We describe a 
fortuitous observation of clutch hydration that suggests 
thorny devils urinate on their egg mass as a mode of 
parental care to increase offspring fitness. Observations 
of this nature have previously been described for 
oviparous reptiles such as skinks (Fitch 1954; Somma & 
Fawcett 1989), crocodiles (Whitaker & Whitaker 1977) 
© Royal Society of Western Australia 2003 
and snakes (Bell & Van den Sande 1986; York & 
Burghardt 1988; Ross & Marzec 1990; Shine 1998). 
Observations 
During fieldwork in the eastern goldfields of Western 
Australia (Koolyanobbing Ranges; 30° 29’ S; 119° 30’ E) a 
gravid female thorny devil was collected in open sand 
plain country. We placed the thorny devil in a calico bag 
(21 cm x 30 cm) and transferred it to our field station 
where it was held in a dark enclosure and maintained at 
temperatures between 18-22 °C. The lizard was 
monitored daily, and on the fourth day egg deposition 
had occurred in the calico bag. Eight eggs were deposited 
in total. In addition, we noticed that the calico bag was 
saturated with fluid and contained evidence of faeces 
(ant remnants). We concluded that the female thorny 
devil had urinated and defaecated over the clutch, 
resulting in saturation of the eggs and at least half of the 
calico bag. 
Discussion 
We suggest that this thorny devil, once confined but 
left undisturbed, had deposited its clutch and hydrated 
the eggs as it might do in a nesting burrow in the field. 
Egg deposition could have been a coincidental 
observation, or oviposition and defecation could have 
been a stress induced response to captivity. However, 
this is unlikely for two reasons. Firstly, reptilian excreta 
consists mainly of insoluble uric acid and undigested 
food material and contains very little fluid (Bradshaw 
1997). A considerable amount of fluid must have been 
liberated from the cloaca to saturate the bag to the extent 
that we observed. Secondly, egg deposition was not 
likely to be a stress response as the normal response of 
gravid individuals to stress is egg retention (Hughes et 
al. 1986; Mills et al 1991). 
We therefore suggest that urination/defaecation was 
the action of an individual not unduly stressed by 
captivity and that clutch hydration in this fashion may be 
a tactic used by female thorny devils in the field to 
increase the number of eggs that hatch successfully and 
increase offspring size (Shine 1988; Shine & Brown in 
press). Thorny devils burrow in relatively compact 
substrates, such as red lateritic clays (Thompson in 
press), and these substrates should be favourable for 
incubation (i.e. humid with stable temperature). 
However, hydration of the clutch by the adult female 
during or after oviposition should substantially increase 
the relative humidity of the burrow and extend the 
period over which the burrow remains humid. Moreover, 
most reptilian eggs are permeable to water and 
potentially hygroscopic, drawing in water if the 
surrounding medium is sufficiently moist (Seymour et al. 
1997; Quintana 2001). Urination over the clutch by the 
adult female may provide water for uptake and increase 
the total water content of the eggs, and increase offspring 
size at hatching. 
Ackerman & Dmi'el (1985) reported that egg 
desiccation is accelerated by exposure to open air or dry 
soil. Thorny devils lay eggs in a chamber, leaving the 
eggs sitting on (rather than buried in) loose soil (Pianka 
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