Tribolonotus gracilis

Overview

Tribolonotus gracilis de Rooij, 1909, commonly known as the Red-eyed Crocodile Skink, is a distinctive terrestrial skink from New Guinea. Its heavily keeled scales, robust head and characteristic orange-red coloration around the eyes give the species its unmistakable appearance.

Although T. gracilis is well established in herpetoculture, surprisingly little detailed information is available on its ecology in the wild. Much of what is known about its social behaviour, vocalization, reproduction and parental care comes from observations of captive animals.

This species account therefore distinguishes between published information and observations from more than ten years of personal experience keeping and breeding T. gracilis.

Natural History

Distribution & Habitat

Tribolonotus gracilis occurs in New Guinea and is documented from Papua New Guinea. Historical records have also listed the Admiralty Islands, including Manus, although the accuracy of this locality has been questioned.

Members of the genus Tribolonotus are generally described as secretive, predominantly terrestrial or semi-fossorial skinks occurring beneath vegetation and other cover in humid environments, frequently in the immediate vicinity of water.

Detailed field data for T. gracilis remain limited. This is important when interpreting husbandry literature: observations from captive animals can provide valuable information about the behavioural repertoire of the species, but should not automatically be treated as descriptions of its behaviour in the wild.

Appearance

Tribolonotus gracilis has a heavily armoured appearance created by strongly keeled scales and prominent longitudinal rows of enlarged scales along the back. The robust, angular head and dorsal scalation are responsible for the crocodile-like appearance reflected in its common name.

The characteristic orange-red coloration surrounding the eyes develops as the animals mature. Juveniles initially lack the fully developed coloration of sexually mature animals.

Activity & Behaviour

Although often described primarily as a secretive ground-dwelling species, long-term captive observations show a considerably broader behavioural repertoire when the animals are provided with a structurally complex environment.

In my animals, activity is predominantly diurnal, with some activity extending into twilight. Juveniles have shown more nocturnal activity than adults, while adult animals encountered at night were generally resting.

The species is naturally cautious and spends considerable time under cover. However, the degree of shyness varies substantially between individuals.

Digging has been observed only occasionally, somewhat more frequently in juveniles than adults. Climbing, however, is common when suitable structures are available. My animals regularly use textured backgrounds, bark and branches and spend substantial periods at elevated positions.

These elevated areas are used not only while exploring, but also for resting, sleeping and feeding. Eggs have occasionally even been deposited above ground where sufficiently humid sites were available.

This behaviour suggests that describing T. gracilis simply as a ground-dwelling skink can underestimate the importance of vertical structure in captivity.

Association with Water

Water appears to be an important component of the behavioural environment of T. gracilis. Published descriptions of the genus associate these skinks with humid habitats close to water, and my captive animals make extensive voluntary use of aquatic areas.

In my permanent enclosures, adults frequently enter the water and particularly favour shallow cascades associated with a waterfall. They often rest partially submerged or sit directly in flowing water, sometimes for extended periods.

Animals also frequently retreat towards or into the water when disturbed. Complete submersion occurs occasionally, although they usually remain positioned with the head above the surface.

Juveniles use water as well, although in my experience less extensively than adults.

Shed skin is frequently found in the waterfall area, suggesting that the animals commonly use these wet areas during ecdysis.

Diet

Detailed field data on prey selection in T. gracilis are scarce. The species is an invertebrate predator and readily consumes a range of appropriately sized invertebrates in captivity.

Because the natural diet has not been comprehensively quantified, lists of feeder insects used successfully in captivity should not be interpreted as a complete representation of prey consumed in the wild.

Communication & Defensive Behaviour

Vocalization is one of the more unusual documented characteristics of Tribolonotus gracilis. Published observations have demonstrated defensive vocalizations and vocal behaviour associated with parental care.

I have likewise observed vocalization primarily in defensive contexts. The calls can be described as short, frog-like or “quacking” sounds and may occur when an animal is handled or otherwise feels threatened.

Juveniles show particularly strong individual differences. Some respond to a perceived threat by opening the mouth, vocalizing and attempting to bite, whereas others show little or none of this behaviour.

Defensive vocalization has also occurred during interactions with other animals. During a period in which the enclosure was shared with a frog, a skink vocalized defensively when the frog moved directly over it.

This former mixed-species housing is reported here as a behavioural observation, not as a recommendation for keeping the species together.

Social Behaviour & Parental Care

The social behaviour of T. gracilis is more complex than its secretive appearance might suggest. Published observations provide evidence of maternal egg guarding and continued association between females and newly hatched offspring.

My observations support parental care but also suggest that it is not necessarily restricted to the female.

Multiple juveniles of different ages have been tolerated alongside their parents for extended periods. In one case, offspring remained within the family group for approximately 1.5 years without obvious problems.

This tolerance should not be interpreted as permanent social compatibility. Aggression becomes increasingly important as juveniles mature. In one group, the oldest juvenile, at approximately two years of age, began attacking three younger juveniles that had previously coexisted without problems.

Based on this experience, I now separate juveniles proactively at around one year of age, or earlier if the first signs of aggression appear.

Husbandry

Experience note: The husbandry information below includes observations from more than ten years of keeping and breeding Tribolonotus gracilis. Numerical values describe conditions that have worked successfully in these enclosures rather than experimentally determined physiological optima for the species.

Enclosure

I have maintained adult T. gracilis either individually or as male-female pairs. Permanent adult enclosures have ranged from approximately 80 × 50 × 50 cm to 100 × 50 × 80 cm (L × W × H).

Given the behaviour observed over the years, I now favour larger enclosures, particularly those providing sufficient depth and height for a substantial water area, a three-dimensional background and multiple elevated retreats.

A good enclosure should offer extensive cover rather than large open areas. Bark slabs and tubes are particularly useful, and both cork and beech bark have been readily accepted. Artificial shelters such as clay caves are also used.

Dense planting is highly beneficial. Plants I have successfully used include Ficus pumila, Epipremnum spp., Rhaphidophora tetrasperma, Monstera deliciosa, Monstera adansonii, spider plants, various ferns and mosses.

A setup using predominantly plants selected to be regionally appropriate has also worked well.

Fast-growing climbing plants require regular pruning. More delicate plants may occasionally be flattened as the skinks move across them, but I have not observed the animals feeding on living plants.

Vertical Structure

Vertical space should not be considered wasted space for this species.

Textured branches, long pieces of bark and structured backgrounds are used regularly for climbing. In my enclosures, backgrounds coated with coconut fibre substrate have been particularly well used.

Animals routinely select elevated positions for resting, sleeping, feeding and general activity. Providing both humid and relatively dry elevated locations increases the range of microhabitats available to them.

For a new enclosure, I would therefore favour more height and depth than I did when I first began keeping the species.

Substrate & Furnishing

A mixture of ordinary pesticide-free topsoil and coconut-based substrate has worked reliably over many years.

The substrate is kept slightly moist rather than permanently saturated. Different moisture levels should remain available, including comparatively dry areas, which are also used by the animals.

Leaf litter, moss and pieces of bark provide additional cover and create a more heterogeneous surface structure.

Live plants are also useful practical indicators of substrate conditions: conditions that maintain healthy tropical vegetation without waterlogging the entire substrate generally help create a range of humid microhabitats.

Digging occurs, but in my animals it has not been a dominant behaviour. Juveniles appear somewhat more inclined to dig than adults.

Temperature & Humidity

My enclosures provide a thermal gradient rather than one uniform temperature. Daytime temperatures range approximately from 20 to 30 °C across different parts of the enclosure.

A 75 W reflector lamp positioned above the enclosure provides both light and heat. This naturally produces warmer elevated areas while lower, shaded areas remain cooler.

At night, temperatures fall to approximately 20 °C, corresponding roughly to room temperature. No additional night heating is used.

Relative humidity remains generally above 70%, depending on the location within the enclosure. I do not routinely mist the permanent adult enclosures: the integrated water area, waterfall, moist substrate and dense vegetation maintain a humid environment.

More important than maintaining one humidity reading is providing different microclimates. The animals can then choose between warmer, cooler, wetter and drier locations.

Water

In my experience, a substantial water area is one of the most valuable features of a permanent T. gracilis enclosure.

Adult enclosures have therefore included a permanent filtered aquatic section, often combined with a waterfall and shallow cascades.

Water depth has generally been limited to approximately 10–15 cm. I have not experienced drowning incidents, including with juveniles, but deliberately avoid unnecessarily deep water and ensure that animals can leave the water easily.

The cascades are used more intensively than open water. Animals frequently sit in shallow flowing sections and allow water to pass around their bodies.

The water also functions as a retreat and is regularly used during shedding.

Filtration has been provided using pumps or external filters together with extensive plant-root growth. Dense roots, particularly from Ficus pumila, have contributed to maintaining clear water.

Under these conditions, complete water changes have rarely been necessary. Water is primarily topped up and the system maintained as required rather than routinely emptied.

Interestingly, my animals do not normally use the water as a toilet. Faeces are often deposited repeatedly at one or two preferred terrestrial locations, which can make spot cleaning straightforward.

Very young juveniles are initially provided with extremely shallow dishes, approximately 5 cm in diameter. As they grow, larger containers of roughly 10 × 10 × 5 cm can be used, with stones or other structures providing easy exit points.

Lighting

The animals have been maintained successfully without dedicated UVB lighting for more than ten years. This observation should not be interpreted as evidence that UVB exposure has no potential benefit.

Lighting is provided for approximately ten hours per day. A 75 W reflector lamp supplies visible light and heat, supplemented by indirect natural daylight entering the room.

No dedicated plant lighting has been required in these enclosures.

Where UVB lighting is provided, animals should retain access to shaded retreats and be able to regulate their own exposure.

Feeding

A varied invertebrate diet has worked well.

Regular foods include earthworms, crickets, isopods, mealworms and Zophobas larvae. Occasionally available invertebrates such as moths and spiders have also been consumed.

Live prey can simply be released into the enclosure to encourage hunting. Animals accustomed to their keeper may also accept prey directly from forceps or the hand.

Juveniles are normally allowed to hunt independently, particularly while they remain shy.

Adult animals are generally fed once or twice per week, while juveniles are offered food approximately every two to three days.

Crickets and other feeders are gut-loaded with vegetables and fruit and are regularly supplemented. Standard calcium and vitamin supplementation, including vitamin D-containing products, has been used.

The animals are active predators and individual food preferences can occur.

On one occasion an animal consumed a thawed fish, but this was not repeated despite subsequent offerings and has not formed part of the normal diet.

Social Housing

Adult animals have been maintained individually or as male-female pairs.

Same-sex adults should not be assumed to be compatible, as aggression can occur. Even a male and female may require time to become accustomed to one another.

A lack of immediate fighting is not sufficient evidence of long-term compatibility. Animals should be monitored for avoidance, restricted access to preferred areas, changes in feeding behaviour and other subtle signs of social pressure.

Parents can tolerate their offspring for considerable periods, but juveniles should eventually be separated before sexual maturity or sooner if aggression develops.

Based on my experience, I now regard approximately one year of age as a reasonable point at which to consider proactive separation of juveniles.

Long-term Husbandry

I have not experienced significant recurring health problems in more than ten years of keeping and breeding the species.

The most important change in my approach has been towards larger, more structurally complex enclosures.

If designing a new enclosure today, I would prioritize generous floor space but also sufficient height and depth to construct a substantial background, waterfall and larger aquatic section. Several elevated resting sites should be available at different temperatures and moisture levels.

Dense vegetation and numerous retreats are particularly important. Although this may appear counterproductive when keeping a naturally shy animal, my experience has been the opposite: animals with abundant nearby cover are more confident and consequently observed more frequently.

Three commonly repeated assumptions deserve particular caution based on my experience:

• T. gracilis will climb extensively when appropriate structures are available.
• Water can be much more than a drinking source; my animals voluntarily spend substantial amounts of time in shallow and flowing water.
• The species does not necessarily remain extremely shy. Individual differences are considerable, and some animals can become remarkably confident through slow, food-based conditioning.

Breeding

Tribolonotus gracilis produces one egg at a time and can reproduce repeatedly under stable captive conditions.

My principal breeding pair reproduced throughout the year rather than showing a strict breeding season. In a documented series of six consecutive eggs, the intervals between egg laying ranged from 44 to 78 days, with an average of 63 days.

Renewed mating behaviour was commonly observed within the weeks following egg laying.

Sexing

Sexually mature males and females can be distinguished by several external characteristics, although not all of them are equally reliable.

The most useful characteristic in my experience is found on the hind feet. Mature males develop conspicuous enlarged pore-bearing scales on the underside of the toes and foot, whereas these structures are absent or much less developed in females. This has been the primary characteristic I use for reliable sex determination.

Other characteristics can provide additional indications. Differences in head shape may be visible between mature males and females, and throat coloration or markings can also differ. However, I consider these features less unambiguous than the structures on the hind feet and would not rely on them alone for sex determination.

TODO — SEXING AGE

• approximate age or developmental stage at which the male foot structures become reliably distinguishable

TODO — SEXING IMAGE Insert comparison photograph here.

Hind feet of adult Tribolonotus gracilis. Female on the left and male on the right. The enlarged pale pore-bearing scales on the male foot provide a particularly useful characteristic for sex determination.

Courtship & Mating

Courtship includes conspicuous head movements. I have repeatedly observed head nodding in association with mating behaviour.

During copulation attempts, the male bites and holds the female around the neck and posterior head or “helmet” region.

A receptive female permits the interaction, whereas an unreceptive female actively turns and manoeuvres until she can free herself. Defensive vocalizations can occur during these rejected mating attempts.

Reproduction has occurred throughout the year in my animals. Following egg laying, mating commonly resumes within the following few weeks.

Egg Laying

Females produce a single egg per reproductive cycle. Reproduction has occurred throughout the year in my breeding animals without a clearly defined breeding season.

For one documented sequence of six consecutive eggs between February 2016 and January 2017, the intervals between egg laying were 78, 65, 44, 57 and 71 days. This corresponds to a range of 44–78 days and an average interval of 63 days.

Eggs are deposited in humid, protected locations, but there is no single obligatory nest type.

I have found eggs directly within substrate at ground level, beneath pieces of bark and inside artificial shelters such as clay caves. Eggs have also been deposited at elevated positions when a sufficiently humid and protected site was available.

An egg may be partly exposed, buried or covered with pieces of bark. Changes in the material covering the egg have repeatedly been observed after laying, consistent with continued parental interaction with the nest site.

Eggs can be left in the parental enclosure when the environmental conditions are suitable, and this has been my usual approach.

Incubation

Incubation duration in my breeding records varied considerably depending on the conditions and location in which an egg developed.

For six consecutively documented eggs, incubation lasted 65, 71, 77, 78, 92 and 115 days. Eggs were successfully incubated both artificially and within the animals’ enclosures, including at different vertical positions.

One egg incubated artificially at approximately 26 °C hatched after 71 days. An egg deposited in an elevated position within the parental enclosure hatched after 65 days. At the other end of the recorded range, an egg incubated in the lower, cooler part of the parental enclosure required 115 days.

These observations illustrate that incubation time can vary substantially under different thermal conditions. They should not, however, be interpreted as a controlled experiment, since eggs developing within the enclosures were exposed to naturally varying temperatures and other environmental conditions.

For artificial incubation, I use moist vermiculite under high-humidity conditions. The incubation container remains ventilated to reduce the risk of mould and is checked regularly.

Only a small number of apparently infertile eggs have occurred over more than ten years of breeding.

Hatching & Early Development

Hatchlings from my breeding animals weighed between 3.49 and 4.23 g immediately after hatching.

They normally begin feeding within the first few days after emergence. Small crickets and appropriately sized worms have been accepted as early foods.

The conspicuous coloration around the eyes is not fully developed in young juveniles. In my animals, the characteristic coloration becomes prominent as sexual maturity approaches.

Growth is relatively slow, with animals taking several years to reach full adult size.

TODO — HATCHLING LENGTH

Raising Juveniles

Juveniles can be raised under the same general environmental principles as adults: access to humid and drier microhabitats, secure retreats, appropriate temperatures, shallow water and a varied diet.

I have raised juveniles both in small terraria and in fauna boxes. A 60 × 30 × 30 cm enclosure has also been used for groups of juveniles.

Very young animals receive only shallow water dishes. As they grow, the water area can be enlarged, provided that safe and easy exit routes are available.

Juveniles are generally fed every two to three days with appropriately sized live prey and receive regular calcium and vitamin supplementation.

Several juveniles can coexist successfully for extended periods, but this should not be assumed to remain stable indefinitely. Age and sexual development appear to change social compatibility.

Following an incident in which an approximately two-year-old juvenile began attacking three younger animals, I now separate growing juveniles at around one year of age as a precaution, or immediately if aggression becomes apparent.

Apart from this social issue, I have not encountered recurring problems during juvenile rearing.

Media

Over the years, I have documented the husbandry, behaviour and breeding of Tribolonotus gracilis in a series of videos. Selected videos are embedded throughout this species account where they provide additional context to the observations described.

The complete video collection includes enclosure setups, feeding behaviour, breeding, hatching, juvenile development and general observations of the species.

References

Hartdegen, R. W., Russell, M. J. & Young, B. (2001). Vocalization of the crocodile skink, Tribolonotus gracilis (De Rooy, 1909), and evidence of parental care. Contemporary Herpetology, 2001, 1–6. https://doi.org/10.17161/ch.vi1.11956

Miralles, A. (2004). Biologie, ecologie en verzorging van de Krokodilskink van Nieuw-Guinea, Tribolonotus gracilis. Lacerta, 62(4), 166–173.

Mys, B. (1988). The zoogeography of the scincid lizards from North Papua New Guinea (Reptilia: Scincidae). I. The distribution of the species. Bulletin de l’Institut Royal des Sciences Naturelles de Belgique, Biologie, 58, 127–183.

Uetz, P., Freed, P., Aguilar, R., Reyes, F., Kudera, J. & Hošek, J. (eds.). The Reptile Database. Tribolonotus gracilis species account.