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IGUANA  •  VOLUME 13, NUMBER 4  •  DECEMBER 2006 283VARANUS PRASINUS

Natural History
Varanus prasinus, commonly known as the Green Tree Monitor,
is a gracile lizard reaching a total length of less than 1 m. Ground
color is light to emerald green with some black markings from
nape to tail base, usually as lines or chevrons across the back.
First described by Schlegel in 1839, this member of the family
Varanidae is native to the Cape York Peninsula of Australia, most
of New Guinea, and some surrounding islands. Green Tree
Monitors lay 2–5 eggs that hatch after 150–190 days. They are
notorious for consuming their own eggs in captivity. In the wild,
they inhabit the canopies of remaining lowland forests at eleva-
tions to 500 m. Little is known of their habits in the wild.
Although highly arboreal, these animals are excellent swimmers
with keen eyesight. Varanus prasinus is reported to lay its eggs in
arboreal termite nests. Wild hatchlings probably eat termites or
termite eggs. Green Tree Monitors are very wary, nervous ani-
mals, fleeing for cover at the slightest disturbance. The prehen-
sile tail is nearly twice the snout to vent length and is used for
balance and as a stabilizer or fifth limb when moving through
trees, on the ground, or in water. They will curl their tail tightly
and flatten their necks dorsally and ventrally to appear larger
when in defense mode. The tongue is long and forked, and is
used to gather scent molecules, which are interpreted by using
the Jacobson’s organ in the upper palate. Varanus prasinus has
very sharp teeth that are used for subduing prey such as inverte-
brates, mice, lizards, frogs, birds, and bird eggs. Teeth also are
used for defense, as is the expulsion of fecal material.

Captive Husbandry
In 1998, the San Diego Zoo acquired a pair of unrelated V. pras-
inus. Following two parasite treatments and two clean fecal
checks in quarantine, the pair was brought into the zoo’s main
reptile building. Lizards are maintained at an ambient tempera-
ture of 24–29 °C (75–84 °F), with basking site temperatures
reaching 35–38 °C (95–100 °F), using 250-w spotlights
(PHILIPS‰ 250w REFLECTOR) above their enclosures. UV-
fluorescent (PHILIPS‰ F40T12/C50 COLORTONE) and
black fluorescent lights (SYLVANIA‰ BLACKLIGHT
F40/350BL) also are used above the enclosures. The ambient
lights are on from 0600–1800 h and basking lights from
0700–1700 h. The relative humidity averages 50–75%, and
misters above the cages are on 3–4 times per week for 1–2 hours.

The monitors are offered 5–10 gut-loaded and vitamin
dusted crickets 2–3 times each week, 3–4 neonatal (pink) mice
are offered 2–3 times per week, and ground turkey with supple-
mental vitamins mixed occasionally with a crushed, boiled egg
also is provided 3–4 times a week. The San Diego Zoo has just
recently switched from turkey to Dick Van Patten’s Natural
Balance Zoo Carnivore Diet™ (Pacoima, California). Ovulating
females are offered food daily. Males feed very readily and can
become obese if their diets are not carefully regulated.

H U S B A N D R Y

Successful Care and Reproduction of 
Green Tree Monitors (Varanus prasinus) 

at the San Diego Zoo
Brett Baldwin

Herpetology Department
San Diego Zoo

Photographs by the author except where noted.

Rows of cages in the section of Klauber-Shaw Reptile House where
individual Green Tree Monitors (Varanus prasinus) are housed.



Enclosures are sink cages (plastic sinks without the detach-
able legs are designed and built by Ed Snow, Naturally Exotic,
Valley Center, California). Our versions are custom designed by
the author. They have thin plastic walls with screen fronts and
tops. The top half opens independently of the bottom half.
Dimensions are 220 x 57.6 x 63.5 cm in height, width and
depth. The height allows for appropriate heat gradients and
security. The cages have nest boxes attached to the sides that are
71.1 x 12.7 x 64.8 cm in size and 105.4 cm above the ground.
Access to the box is through a hole 8.9 cm in diameter.
Supersoil™ brand potting soil filled to a depth of 30.5–40.6 cm

is used as nest box substrate. The door for outside access to the
nest box is hinged and opens down with two latches on each top
corner of the door, which is situated near the top and measures
25.4 x 64.8 cm. Long branches are placed at angles from bot-
tom to top of the cage, and another branch is screwed into the
sides near the nest box opening. Basking on this perch, the
female can attain temperatures of up to 33 °C. During the day,
temperatures at the bottom of the cage are around 24–26 °C
(75–78 °F). At 61 cm above the bottom, temperatures climb to
26–29 °C (78–81 °F), and at 122 cm, temperatures range from
27–29 °C (81–84 °F). The highest temperatures (46–47 °C
(114–117 °F)) are attained in the top corner under spotlights.

The section of the building in which the monitors are
housed has built-in skylights situated directly above the enclo-
sure where the two males are on exhibit; the females have no
direct access to the skylights. Three of the skylights are opened
during cool, rainy months and intermittantly throughout the
year. The heating in the section is set at 29 °C (84 °F) at 0600 h
and 27 °C (80 °F) at 2200 h. Cooling is set at 30 °C (86 °F) at
0500 h, 31 °C (88 °F) at 0900 h, and 30 °C (86 °F) at 2200 h.

Reproductive History
On 6 February 2003, a captive-bred male was introduced to the
female and copulation was observed on the same day. Four des-
iccated eggs were found in the enclosure on 10 March 2003.
Two were inside the nest box on top of soil and two were on top
of the nest box. These eggs were incubated for two weeks but
did not rehydrate. No further eggs were found and the female
showed no obvious weight gain until 11 January 2004, when

284 IGUANA  •  VOLUME 13, NUMBER 4  •  DECEMBER 2006 BALDWIN

A juvenile Green Tree Monitor showing defensive posturing.

A subadult male Green Tree Monitor (Varanus prasinus) signals defensive behavior by inflating its throat.



IGUANA  •  VOLUME 13, NUMBER 4  •  DECEMBER 2006 285VARANUS PRASINUS

one egg was found in the nest box; it went bad early in the incu-
bation. Another two desiccated eggs were removed from the top
of the soil in the nest box on 25 April 2004; these did not rehy-
drate. Four eggs were removed from the bottom of the soil in
the new nest box on 24 October 2004. A tunnel was dug down
one side to the bottom and across to the other side of the nest
box. These eggs went bad 2–3 weeks into incubation. On 5 July
2004, this female was moved into a new cage with the dimen-
sions listed above. In 2005, she laid four eggs on 30 January,
four eggs on 11 May, five eggs on 15 August, and five eggs on
11 November. On 27 February 2006, she laid two eggs in the
water bowl. More soil was added to the nest box, increasing the
depth from 10 in to 16 in. She laid two more eggs in the nest
box the next day. On 17 May 2006, this female laid another
clutch of five eggs that quickly went bad and displayed no signs
of fertility. She had been with the male for only three days (23,
24, and 25 March) between 17 May 2006 and the last clutch.
In August 2006, she laid one egg on the top of the soil in the
nest box. The following day, she laid four more eggs in the bot-
tom of the soil of the nest box. These eggs also were infertile. She
had been with a male seven days in total since the last clutch on
17 May (11 and 12 June, and 16, 17, 25, and 26 July).
Copulation was observed on 25 July 2006. The male was intro-
duced to her again on 22 and 23 August 2006. He still showed
interest in her, chasing her around the cage. She laid the last

clutch on 25–26 August 2006. The days between laying from
March 2003 and May 2006 were 251 days, 105, 182, 98, 101,
96, 92, 104, 79, and 100 days.

San Diego Zoo captive-hatched adult male Green Tree Monitor feed-
ing on Carnivore Diet.

Captive-hatched male Green Tree Monitor (Varanus prasinus) copulat-
ing with a wild caught female.

When the time is right, male Green Tree Monitors aggressively chase
females, which quickly submit.



All Varanus prasinus are kept separately. Other institutions
report housing groups of two or more animals together, includ-
ing more than one male with no problems. During 2005, a cap-
tive-bred male was introduced to the female at certain times dur-
ing the year for no more than a week at a time, sometimes for
only 1–2 days: 15 and 28 February, 15 March, 23 May, 26 July,
10, 17, and 23 August, 25 September, 23 November 2005, and
14 February 2006. On several occasions, the female ate 2–4 of
her eggs.

On 28 February 2005, the male showed interest, tongue-
flicking with jerky motions over her back and along her side,
and pushing his vent area close to hers. On 23 May, he displayed
similar behavior. On 26 July, the male chased the female around
the cage while tongue-flicking and trying to copulate. On 27
July, the pair was observed in copula during both morning and
afternoon. On 30 August, he also chased her, but both were
observed later in the day basking on separate branches. On 23
November, the female was put on exhibit with a wild-caught
male and removed the next day. On 11 January 2006, she was
placed with a wild-caught male on exhibit, and they were
observed copulating in the afternoon and on the following day.
She was removed 16 January and placed back into her enclosure.
This female has bred with a male and had clutches of eggs peri-
odically beginning in 2000. Successful hatches did not occur
with this female until 2004, when two of four eggs laid 24
October 2004 hatched on 1 April 2005. The incubation period
for this clutch was 151 days. The second successful clutch was
five eggs laid 15 August 2005. These eggs hatched 14–17

286 IGUANA  •  VOLUME 13, NUMBER 4  •  DECEMBER 2006 BALDWIN

All Varanus prasinus successfully hatched at the San Diego Zoo have slit and fully emerged from their eggs within 24 hours.

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Green Tree Monitor (Varanus prasinus) egg #1 slit just a few hours
before fully emerging.

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IGUANA  •  VOLUME 13, NUMBER 4  •  DECEMBER 2006 287VARANUS PRASINUS

January 2006 with an incubation period of 153–156 days. The
third successful clutch consisted of four eggs laid 27 February
2006. Two eggs arrested 2–3 weeks into incubation, but the
other two hatched on 28 and 29 July 2006 with incubation peri-
ods of 152 and 153 days.

A female V. prasinus no longer in the zoo’s collection laid
three eggs in a log in her enclosure on 8 April 2000. One egg
went bad, one went full-term but died, and one hatched on 10
September 2000 with a 153-day incubation period. This animal
is the father of two successful clutches. He is an aggressive feeder
of Natural Balance Zoo Carnivore Diet™ and crickets, but
refuses pink mice.

Eggs are incubated at 30 °C (86 °F) in a substrate with a
1:1 ratio of water to vermiculite. Average mean egg weight is 9.8
g. Average weight of the female over the last year was 238 g and
average weight for the male was 414 g.

Husbandry of Juveniles
Average mean weight for hatchlings is 8.5 g. About one week
after hatching, juveniles can be enticed to eat two-week-old
crickets by either releasing 2–3 into the cage or by tong-feeding.
Some individuals are more difficult to feed than others during
the first 2–3 weeks. During the third week, they are offered
Natural Balance Zoo Carnivore Diet™ with vitamin supple-
ments. They also may need to be tong-fed turkey or the carni-
vore diet if they are not taking it from a plate. In the third or

fourth week, a pinky mouse chopped into four pieces is added
to the turkey plate. This is offered about 2–3 times a week.
Three or four crickets are offered twice a week. Once the hatch-
lings start eating regularly, they can gain from 0.25–0.50 times
their body weight each month.

All juveniles are maintained separately. The substrate in
juvenile enclosures is wet sphagnum moss. Branches are angled
from top to bottom of the enclosure, a piece of bark is provided
for cover, and a bowl of water, which is just large enough to
accommodate the animal, is used. A 50-w spotlight
(PHILIPS™ Indoor Flood), and fluorescent UV- and black
lights (SYLVANIA™ BLACKLIGHT F40/350BL) are situated
above each cage. Cages are custom glass tanks that are 45.7 x
25.4 x 50.8 cm in size and have sliding screen tops and fronts.

References
Barker, D. 1984. Maintenance and reproduction of Green Tree Monitors,

Varanus prasinus, at the Dallas Zoo. Eighth Annual Reptile Symposium on
Captive Propagation and Husbandry. Columbus Zoo, Columbus, Ohio.

Bennett, D. 1998. Monitor Lizards Natural History, Biology & Husbandry.
Edition Chimaira, Frankfurt am Main.

Eidenmuller, B. 2003. Warane: Lebensweise, Pflege, Zucht. Herpeton, Offenbach.

Greene, H.W. 1986. Diet and arboreality in the Emerald Monitor, Varanus
prasinus, with comments on the study of adapation. Fieldiana, Zoology
(New Series) 31:1–12.

Pianka, E.R., D.R. King, and R.A. King. 2004. Varanoid Lizards of the World.
Indiana University Press, Bloomington.

Sprackland, R.G. 1992. Giant Lizards. T.F.H. Publications, Neptune City, New
Jersey.

Newly hatched Green Tree Monitor (Varanus prasinus).

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Neonate Green Tree Monitors at the San Diego Zoo have had well-
absorbed yolk sacs and few remnants of an umbilicus.

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Day-old Green Tree Monitors are very quick and ready to disperse.

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