Husbandry and Captive Propagation of the
Crocodile monitor, Varanus salvadorii,
At the Houston Zoo
Stan Mays
Abstract:
Due in part to its large size and colorful appearance, the Crocodile monitor is a popular display animal in many zoos. Currently there are 63 animals in 30 zoological institutions worldwide. However, the Crocodile monitor has proven to be a difficult species to reproduce in captivity. In North America, only five zoos have successfully bred this species. The Houston Zoo first began working with Crocodile monitors in 1992. From 1992 through 1994, five animals were acquired and kept in indoor facilities. Over the next several years three animals were lost from conspecific aggression or medical conditions. The last pair was sent to the Detroit Zoo in 1997. Two new pairs of Crocodile monitors were received when a new annex was added to the Herpetology building which contained outdoor exhibits for both Crocodile monitors and Komodo dragons. Because of the hot, humid sub-tropical climate of south Texas, which is similar to their native habitat, these two species are able to be kept outdoors almost year-round. This paper deals with the successful reproduction of Crocodile monitors since being housed in their new exhibit. Enclosure and reserve facilities are described. Observations of the general behavior of adults and juvenile individuals are mentioned, as is mating behavior and egg incubation data.
Key words: Crocodile monitor, Varanus salvadorii, husbandry, breeding, aggression, incubation.
Introduction and Natural History
The Crocodile monitor, Varanus salvadorii, is a popular display animal in many zoos world-wide. It is reputed to be the longest extant lizard species with total lengths reported to be up to 4.5 meters; however, this measurement has never been authenticated. Data compiled by Horn (1997) indicates that most adults range between 2.0-2.65 m, although one male has been documented as having reached 3.53 m (Wood, 1977). There is currently an adult male at the Dallas Zoo (Dallas, TX) that exceeds 3 m in length (Watson, pers. comm.) and another male at the St. Louis Zoo (St. Louis, MO) that appears to be close to 3 m in length (pers. obs.).
In its coloration and pattern, the Crocodile monitor is a particularly attractive animal, with a dark brown to blackish ground color with oval or round yellow spots, often arranged in transverse rows along the dorsum. There are yellow rings around the tail, and there are often yellow scales irregularly scattered along the body. Color variations may be present, depending upon the area of the range where the animal was collected (Horn, 2004). The tail is long, whip-like, and is usually 2.3 to 2.6 times as long as the snout-vent length (Adams, 1995), but can be up to 2.7 times snout-vent length (Horn, 2004). The tail also differs from other monitor species by being weakly compressed, keeled above distally, and rounded at the base. Because of this and several other anatomical features, Mertens (1962) placed the Crocodile monitor in its own subgenus, Papusaurus. More recent studies have placed Varanus salvadorii, V. varius, and V. komodoensis in a sub-clade of the Indo-Australian lineage (see Horn, 2004, for a review). Other morphological characteristics include a long, flat, blunt head with long, slightly curved backward, pointed teeth. There are many folds of skin along the neck region; recent studies indicate that these folds may be used as a sort of air injection mechanism to force air into the lungs which may help partially to account for the high metabolic rate of this species (Horn, 2004).
The range of the Crocodile monitor is restricted primarily to the lowland rainforests and adjacent savannahs of New Guinea. Most confirmed sightings and collection data place this species along the southern coast, although there are confirmed specimens found in a small area on the northern coast; individuals have also been sighted in the far western part of the island (O¡¯Shea, 1991; Horn, 2004).
Due in part to its secretive nature and the inaccessibility of its native habitat, there is very little information available concerning the natural history and ecology of the Crocodile monitor. All behavioral observations have been conducted on captive individuals and are anecdotal in nature. No data exists for behavior in the wild. Natural reproductive behavior is also unknown and all reproductive data has been collected from captive situations. Field studies on Crocodile monitors are definitely needed and would be much appreciated by the holders of this species in captivity. Data from these studies could aid immensely in improved husbandry and successful reproduction in the captive population.
In terms of natural diet and prey items, little is known concerning the wild population. The shape of the teeth have led to the belief that birds compose a significant part of the natural diet (Auffenberg, 1981). The fact that the Crocodile monitor is primarily arboreal and diurnal in its habits lend some credence to this idea. Speculation exists that bird eggs and naked-tail rats also form part of the natural diet (Adams, 1995). Anecdotal evidence gathered at the Houston Zoo does support a part of this hypothesis. On 30 June 1994, an adult (¡Ý2 m) wild-caught female Crocodile monitor escaped from its outdoor reserve enclosure. It was recaptured after extensive searching on 5 July 1994. The animal was found in broad daylight in a tree located about 15m from the original point of escape. It was approximately 7 m above the ground and when first found was raiding the nests of Great-tailed grackles (Quiscalus mexicanus) and eating the eggs and nestlings. She was very well camouflaged, and would not have been noticed had it not been for the increased activity of the adult grackles defending their nests (to which the lizard appeared to pay little attention).
Conservation and Captive Status
There are no population estimates on the abundance of the Crocodile monitor in the wild, although professional collectors consider it rare and difficult to acquire (Yuwano, 1998). Consequently, nothing is known about its status in the wild (Adams, 1995; Horn, 2004). It is not protected as an Endangered or Threatened species by the United States Fish and Wildlife Service but is listed under Appendix II of CITES.
A living Crocodile monitor was first displayed at the Wilhelma Zoo in Germany in 1960 (Mertens, 1960). In North America, the Dallas Zoo was the first institution to exhibit this species with the import of two specimens in 1970 and 1972 (Hartdegen, 2002). There were no further imports into North America until Herpetofauna began importations in 1989. In fact, few animals were exported from New Guinea until the 1990s, when trade in this species began to increase. Although still somewhat expensive, wild-caught individuals are not difficult to acquire and are often seen for sale on dealer price lists and in classified ads.
According to the International Species Information System (ISIS) there are currently 63 Crocodile monitors (27 male, 23 female, 13 unsexed) residing at 30 zoological institutions worldwide. In addition, there are a number owned by private individuals. There are some animals that are collected for their skins only. A studbook for the Crocodile monitor exists in North America (Hartdegen, 2002), but no such data set exists for Europe or Australia. Historically, there have been 129 animals in North American institutions; most have been wild-caught.
Despite the number of Crocodile monitors in captivity, both current and historic, this species has proven to be difficult to reproduce successfully. In North America, only five zoos and one private individual have bred Crocodile monitors; only one zoo (Honolulu, HI) breeds them regularly. Consequently, the captive population is not self-sustaining. The Lizard Advisory Group (LAG) of the American Zoo and Aquarium Association (AZA) has deemed the establishment of a stable, self-sustaining captive population of this species to be a high priority (Card and Hudson, 2002).
Historical Background
The Houston Zoo received its first pair of Crocodile monitors in October 1992 from an animal dealer. From 1992 through 1994, a total of five animals were acquired. Unfortunately, these animals proved to be difficult to maintain in captivity and suffered from numerous problems, the most prevalent being metabolic bone disease, bacterial infections, and injuries from con-specific aggression. Of the animals acquired, three eventually died. Interestingly, two of these died of cardiac failure; one from cardiovascular failure and the other from an aneurysm of the descending aorta. A recent survey indicated that 25% of Crocodile monitor deaths in North American zoos were related to cardiac or circulatory problems (Hartdegen, 2002). The third animal died from a chronic bacterial septicemia while still housed in quarantine facilities. The remaining pair was sold to the Detroit Zoo in November, 1997.
Lack of general knowledge about the natural history and ecology of this species played a significant role in the lack of captive husbandry success during these initial attempts, although much was learned. For example, it was noticed that animals kept outside in large off-exhibit enclosures during summer months displayed markedly better health and fewer medical problems than those housed indoors. Heat, humidity, and enclosure size also appeared to be important factors.
In the year 2000, funds were raised by the Zoo Friends of Houston for the construction of a new annex and a cosmetic renovation for the existing Herpetology building. Included in the annex were plans for two outdoor enclosures.
Several factors were considered in determining the species composition for the new outdoor facilities. The sub-tropical climate of South Texas played a major role in the final decisions. Houston is characterized by a hot, humid climate for much of the year with mild winters (Figure 1). Rainfall is abundant, with much of the annual amount coming during the late spring and summer months in the form of afternoon thunderstorms (Figure 2). During this time period humidity ranges between 90% in the morning to 60-70% in the late afternoon. This information, coupled with previous experience, led to the final decision to use the outdoor enclosures to display two large monitor species. The species chosen were the largest (Komodo dragon, Varanus komodoensis) and the longest (V. salvadorii) extant lizards. The Komodo dragon was for display only, while the Crocodile monitor was intended for both display and attempted reproduction.
Exhibit Design (Figure 3)
The renovation and annex were completed in February 2002. Both exhibits are constructed of brick with artificial rockwork. Non-reflective glass is used for public viewing and invisible wire mesh covers the tops. The Crocodile monitor display measures 5.18 x 4.57 x 3-3.6 m high. The substrate is a one meter deep compacted soil/sand mixture of 1/3 sharp sand and 2/3 topsoil covered with pea gravel. Two layers of plastic sheeting and a layer of galvanized chain link fabric were placed on the bottom of the exhibit before the soil mixture was added to a depth of one meter. French drains are employed for drainage. A 1.3 x 1.0 x .67 m deep pool with a 2.75 m high waterfall is located at the south side of the exhibit. There is a large artificial tree limb stretching across the exhibit and a palm tree planted in the center. Although seldom used by the animals, additional heat sources are provided by a series of heat lamps located across the front of the exhibit. In addition, the artificial tree limb and the waterfall are heated. The animals rarely use these supplemental heat sources and usually are seen basking on the artificial rockwork. Original exhibit plans called for forced air heating and a retractable roof; however, these were eventually determined to be too costly.
There are two indoor reserve enclosures attached to the exhibit. Each measures 2.4 x 1.6 x 1.55 m and connect to each other and to the outside exhibit through a series of lexan guillotine doors. The reserve enclosures are too small to keep adult Crocodile monitors for long periods and are intended only for temporary housing during periods of cold weather.
Husbandry
Two young adult pairs of Crocodile monitors were acquired in October and November of 2000 and raised to maturity in off-exhibit quarantine facilities. While there they received treatments for both endo- and ectoparasites. Cestodes were the major parasitic problem encountered. The animals were transferred to the exhibit upon its completion. In general, they are kept outdoors constantly from April through October. During the remaining months, animals are brought in at night and during colder daytime periods (anytime temperatures fall below 15¡ã Celsius). Anecdotal observations indicate that this species appears to be particularly susceptible to cooler temperatures and their activity levels will decrease dramatically when even slightly chilled.
Freshly killed juvenile rats are used most often for feeding purposes. Monitors are fed 2-3 times weekly. Quail or domestic chicks are also used as prey items when available. Quail appears to be the most favored food item. The amount given per feeding is increased during May and June when breeding activity has been noted, and is decreased during colder months. No problems have been encountered during feedings even though all adults are fed at the same time in the exhibit. However, care is taken to separate the individuals into different exhibit corners before any food items are offered.
Adult health problems have been the result primarily of injuries received from con-specific aggression. In general, wounds tend to not be serious and heal with few problems, although one major incident resulted in the later death of one of the female animals. Other medical problems have not been encountered.
Courtship
What was thought to be the beginnings of courtship behavior was observed in early June 2002. Males were seen following females and initiating mounting behavior. Several small bite wounds were seen on one of the females around the head region, but these healed quickly. No ritual combat was ever observed, but such behavior has been noted in the literature (Horn, 2004). A nesting are of moist sphagnum and mulch was prepared in an isolated shaded corner of the exhibit and was checked daily.
In early August one of the females became increasingly aggressive, especially towards the males. Aggression between cage mates is widespread in captive situations (Adams, 1995), but this behavior was primarily directed towards the males. It was hypothesized that this may have been some sort of misdirected nest protection or maternal behavior but there has been no quantifiable evidence to support this idea. No aggression was observed directed towards the other female but this may have been due to their being able to avoid contact with each other. The males were subsequently transferred into reserve facilities.
Egg Incubation and Hatchling Care
Five eggs were found in the nesting area on 18 August 2002. They were weighed, measured, and set-up in a covered plastic container with a substrate of 2:1 vermiculite/deionized water. Eggs were 2/3 covered by the substrate. Incubation temperature ranged between 27 and 29¡ã C. A data logger was kept in the incubator to monitor temperature variations. Eggs were checked daily and the substrate was misted with deionized water as needed to maintain high humidity.
One egg was found to be infertile and was discarded. Three of the remaining four hatched on 7 March 2003 after an incubation period of 200 days (Figure 4). The remaining egg was opened ten days later and found to be developed but dead; histopathological analysis revealed an acute bacterial septicemia due to yolk sac infection.
The three hatchlings averaged 457 mm total length; average weight was 60 grams. Hatchlings were maintained in a well planted display with several basking sites. A temperature gradient of 40¡ã to 30¡ã C. was established. Westron Active UV heat lamps were used for heat and as a source for UV light. Daily artificial rains kept humidity above 90%.
Hatchlings readily accepted newborn and small furred mice, both alive and freshly killed; they showed little interest in crickets or other invertebrates. Growth was rapid. However, it was noted that one individual did not appear as robust as the other two although it appeared healthy, had a normal appetite, and exhibited normal behavior. Nevertheless, it did not grow or gain weight as quickly as its siblings. After several months it was transferred to a reserve enclosure. It remained much smaller than the others and eventually died on 4 July 2004. A necropsy revealed a greatly enlarged heart while histological analysis discovered cardiac function impairment, an ante-mortem blood clot, and overall vascular congestion.
The remaining juveniles were housed together until August 2005 when aggressive behavior forced their separation. Recently it was determined that they were of different sexes. The male is still kept alone on display, while the female has been successfully introduced to the adults.
The rearing of the two juveniles has been routine. It definitely does appear that heat, sunlight, and high humidity are all key parameters in the successful maintenance and reproduction of Crocodile monitors in captivity. Institutions and private holders in sub-tropical and tropical climates thus have a natural advantage in keeping this species. In fact, of the five North American zoos that have bred Crocodile monitors, only one (Denver, CO) is located in a temperate climate and housed exclusively indoors.
At Houston, no breeding of Crocodile monitors was attempted during 2003 and the sexes were kept separate. Unfortunately, one adult female received massive trauma from the other female and died from her injuries in August 2003. The timing of this aggressive behavior coincided with the timing of clutch deposition the previous year. Reproduction attempts were made during 2004 but were also unsuccessful. There was some evidence that the remaining adult female laid eggs but then consumed them. No breeding activity was noted during 2005 although an extended drought and dry conditions during the summer may have been a contributing factor to the lack of reproductive activity.
Discussion
Despite its large size and striking appearance, there is very little known about the Crocodile monitor. Given current trends, it is likely that this species may soon experience the same sort of habitat destruction and exploitation as has befallen many other varanid species. In terms of captive management, several factors should be of concern. For example, only one institution (Honolulu, HI) has been able to consistently breed Crocodile monitors; all other reproduction has been sporadic at best. Another factor of concern at the Houston zoo relates to the five Crocodile monitor deaths that have occurred; three were caused by cardiac malfunction or impairment, and two of these deaths were in wild-caught individuals. Whether this condition is a result of captive husbandry or is also found in the wild is unknown.
Clearly, more research is needed on this species, both under natural conditions and in captivity before a sound conservation management plan can be enacted. Additionally, captive husbandry and breeding techniques need to be improved dramatically and standardized before a stable, self-sustaining captive population can be achieved.
Acknowledgements
I would like to thank Judith Bryja, Charles Mann, William Farr, Paul Crump, Andrew Godambe, and Tiffany Kosch for their efforts and diligence in keeping Crocodile monitors at the Houston Zoo. Zoo Friends of Houston provided funding for the construction of the large monitor facilities. Thanks also to Sharon Joseph, Director of Animal Programs, for allowing me to attend this symposium. Finally, special thanks to Dr. Hans-Georg Horn for inviting me and for the overwhelming generosity and hospitality that he displayed to me and other visitors. His dedication and enthusiasm for varanid lizards in general, and Crocodile monitors in particular, should serve as an inspiration to all of us.
References
Adams, C. (1995): Taxon management account: Crocodile or Papuan monitor, Varanus salvadorii. In: Hartdegen, R. (2002): Asian Forest Monitor North American Regional Studbook, 3rd edition. Dallas Zoo. Dallas, TX..
Auffenberg, W. (1981): The behavioral ecology of the Komodo monitor. University of Florida Press. Gainesville, FL.
Card, W. & R. Hudson. (2002): Lizard Advisory Group: Regional collection plan. American Zoo and Aquarium Association. Silver Springs, MD.
Hartdegen, R. (2002): Asian Forest Monitor North American Regional Studbook, 3rd, edition. Dallas Zoo. Dallas, TX..
Horn, H.-G. & G.J. Visser (1997): Review of reproduction of Monior lizards (Varanus spp.) in captivity II. Int. Zoo Yb., 35: 227-246.
Horn, H.-G. (2004): Varanus salvadorii. In: Varanoid Lizards of the World. Pianka, E.R., D.R. King & R.A. King (eds.). Indiana University Press, Bloomington, IN. 234-243.
Mertens, R. (1960): Seltene gaste in Zoo: Der Papus-waran. KOSMOS (Stuttgart) 56: 547-549.
Mertens, R. (1962): Papusaurus, eine neue untergattung von Varanus. Senckenbergiana Biologica 43: 331-333.
O¡¯Shea, M. (1991): The Reptiles of Papua New Guinea. British Herp. Society Bulletin 37: 19-29.
Wood, G.L. (1977): Animal Facts and Feats. Bantam Books, NY.
Yuwano, F.B. (1998): The trade of live reptiles in Indonesia. In: Conservation, Trade and Sustainable Use of Lizards and Snakes in Indonesia. Erdelen, W. (ed.). Mertensiella 9: 9-15.
Author: Stan Mays, Department of Herpetology, Houston Zoo, Inc., 1513 North MacGregor, Houston, TX 77030 USA.
