The Green Poison Arrow Frog
A Comparison of Tadpole Rearing Techniques

Tim Skelton
Reptile Keeper, Edinburgh Zoo
Last update: 08 April, 2002

Introduction
The Green and Black Poison Arrow Frog, Dendrobates auratus, is a member of the neotropical frog family Dendrobatidae ranging across Costa Rica, Colombia and Panama. As its name suggests, this is a green and black frog approximately 3cm in length. Like other members of this family, D. auratus produces toxic skin secretions, and its bright coloration is a warning to other animals that this frog is not good to eat. Typically, this species inhabits the leaf litter on the forest floor, feeding on small invertebrates such as ants and small beetles. It is thought that much of the frog’s toxicity is derived from this type of diet. In captivity, a diet of fruit fly, greenfly and hatchling crickets probably contribute to a marked reduction in the frog’s toxicity. However, this has not been proved and the frogs must still be handled with care.

Courtship and egglaying also take place in the leaf litter with between six and twelve eggs being deposited at any one time. The eggs take about two weeks to hatch, during which time the male frog cares for and keeps them moist. This he achieves by regularly filling his cloaca with water from a convenient source and depositing it over the eggs. When the eggs are due to hatch, the male sits amongst them, wriggling his hind legs to encourage the tadpoles to break free. One at a time they crawl up onto his back and he carries them to a nearby pool of water, often in a bromeliador tree hollow. Here, they feed on algae and any other aquatic organisms that happen to be there.

Aims
In captivity, we aim to breed and produce strong, healthy frogs in order to maintain a viable captive breeding population. It is, therefore, essential to rear strong healthy tadpoles. This project aims to test four tadpole-rearing techniques to determine which produces the largest, strongest and healthiest frogs.

Husbandry
The breeding group: the breeding group of four (1.3) D. auratus is housed in the reptile house at Edinburgh Zoo in a unit measuring 175x90x90cm. This unit incorporates a pool with a waterfall and stream created by a small pump hidden in the pool. Decor includes planted areas and rocks for the frogs to hide amongst with branching and cork bark to improve the aesthetic appeal as well as providing more cover for the frogs. The front of the unit is extensively covered with sphagnum moss to aid moisture retention and preserve high humidity. Lighting is by a 120cm fluorescent striplight on a timer, giving a twelve-hour day/night cycle. Ambient air temperatures of 22°C are maintained by under floor heat pipes running the entire length of the reptile house controlled by a thermostatically-controlled boiler. Additional heat is provided by a 150 W ceramic heat bulb controlled by a ‘HABISTAT’ pulse-proportional thermostat giving a hot spot of 28°C. This provides a range of temperatures and allows the frogs to choose the optimum temperature for them. All cage maintenance i.e. pool cleaning, spraying and feeding is carried out in the morning on a daily basis.

Because of the huge appetites of these frogs, it is important that we culture a range of invertebrates on the premises. European black crickets, Gryllus bimaculatus, fruit flies, Drosophila spp., and Waxmoth larvae, Galleria mellonella are all bred as food for the frogs. During the summer months, aphids are collected from nearby plants e.g. Sycamore trees, Acer pseudoplatanus and nettles, Urtica vulgarii.

All the insects are liberally dusted with a multi-vitamin powder such as SA37 before being fed to the frogs. Additionally, the fruit flies are provided with a cotton-wool pad soaked in freshly squeezed orange juice with a few drops of Abidec multi-vitamin liquid prior to being used as food. They are left to feed on this for about 30 minutes. These steps ensure that the frogs are obtaining a well-balanced and nutritious diet.

Courtship and egg laying
The male initiates the courtship by producing a two or three second trill which may be repeated at regular intervals of ten seconds or more. The female approaches the male and begins stroking him. If more than one female approaches, they may wrestle with each other to determine which will gain the attentions of the male. The male then leads the victorious female to his chosen nest site. This is invariably one provided by us and consists of a 10-cm diameter plastic lid (with drainage holes) containing a section of plastic leaf. The whole thing is covered with a half coconut shell with an entrance to one side. Assuming the site to be acceptable to the female and that she is ready to lay, the eggs are deposited on the plastic leaf and the male fertilises them immediately unless disturbed for some reason. If the eggs are found during routine maintenance, they are left in situ for 24 hours to ensure that the male has definitely fertilised them. The lid containing the eggs is then removed for off show incubation and a replacement lid is carefully put onto the same site for the next clutch of eggs.

Incubation
The lid containing the eggs is placed in a plastic box containing just enough tepid water to touch the edge of the egg mass and a tight-fitting lid is added. They are incubated at room temperature of 24°C. The eggs are checked daily and lightly sprayed to mimic the attentions of the male frog. Infertile eggs soon deteriorate and may be removed or left since they rarely affect healthy eggs. After about two weeks, the tadpoles begin hatching, usually taking two days for the whole clutch to emerge. When the first hatching is noticed, the other tadpoles may be assisted by gently rupturing the egg mass with a blunt instrument. This is only simulating what the male frog would be doing using his hind legs.

Having hatched, the tadpoles are assigned to one of four rearing techniques and kept until metamorphosis when they will be measured to assess the most successful method.

Tadpole rearing
Four consecutive clutches of eggs, each containing at least eight eggs were removed from the display unit and incubated in the way already described. The four experimental techniques for rearing the tadpoles are described below.

Experiment 1
The first clutch of tadpoles was reared individually in plastic beakers, each half full with tepid tap water. They were fed daily and had their water changed daily. Initially, they were all fed on a paste of ‘Tetramin Flake Fish Food’. As soon as hind limb development was evident, the diet was changed to eight parts powdered Tetramin, two parts multi-vitamin powder (SA37) and one part powdered cuttlebone. Once a week, additional protein was provided in the form of tinned cat food or chopped mussel. Also at this stage, the tadpoles were given ten minutes of ultra-violet light exposure every two weeks using a ‘Wotan’ Ultra-vitalux 300W spotlight.

It should be pointed out that E. BLAKE and D. Sherriff (1993) of the Reptile Department at Edinburgh Zoo pioneered this rearing technique.

Experiment 2
The second group of tadpoles was reared in an almost identical manner except that feeding only occurred twice weekly. This was an attempt to see if metamorphosis could be delayed without affecting the size and strength of the emerging froglet.

Experiment 3
The third clutch was maintained communally in a container measuring 45x25x15cm. It is said that this species’ tadpoles are cannibalistic and must be kept separately [Schafer, 1981]. This experiment is designed to test this theory. The container was set up to be ‘maintenance-free’ and contained 2cm of gravel on the base, pieces of slate for cover and plenty of aquatic vegetation (e.g. Elodea sp.) and invertebrate life. Food was the same as in Expts. 1 and 2 but was only presented once a week in order to minimise water fouling. It was thought that the tadpoles would be able to feed ‘naturally’ on the algae and invertebrates present. The container was set up one week before it was required, to allow the plants and invertebrates to establish themselves.

The water was never changed but did need to be topped up occasionally with tepid tap water. Ultra-violet treatment was the same as for the other experiments.

Experiment 4
The fourth group of tadpoles was split and reared singly in ‘maintenance-free’, miniature versions of Expt. 3. This consisted of coffee jars containing 500ml of water. Each jar also contained 2cm of gravel and a similar amount of pondweed (Elodea sp.). Food presentation and UV treatment remained the same as for the previous experiment. The water was topped up from time to time to counteract evaporation.

All the systems were maintained at room temperature of about 24°C and in close proximity to a fluorescent strip light for the benefit of the aquatic plants.

Metamorphosis
In all cases, the tadpoles were removed from their experimental containers once all four limbs were evident and the tail was beginning to be re-absorbed. They were placed in a small plastic tank containing a little water. The tank was tilted by placing a coffee jar lid under one end creating a ‘sloping beach’ effect. A plastic leaf was also put into the tank to provide some cover for the emerging froglet as well as an additional escape route from the water. No feeding was attempted at this time as it is thought that sufficient nutrients are obtained from the re-absorption of the tail. Once the tail is completely re-absorbed, the froglet is placed into a glass aquarium containing peat, moss, rocks and a small shallow pool: a miniature version of the adults’ enclosure. In a day or two, once the froglet has settled, feeding will commence.

Results
The time taken from the tadpole hatching to metamorphosis and the length of the emerging froglet were both recorded.

From the results, it was apparent that Expt. 1 produced the largest frogs (17.3mm) in the shortest time (47 to 50 days). These frogs were also much stronger and more able to feed on the standard diet of fruit fly and hatchling crickets. The tadpoles in Expt. 2 did take longer to metamorphose (61 to 66 days) but were also much smaller (12mm) and weaker than the previous group. However, after a month of intensive feeding on very small invertebrates such as springtails (Family: Collembola) they did eventually catch up. Expt. 3 gave some interesting results with some tadpoles taking almost twice as long to metamorphose as others (85 to 161 days). This may have been as a result of competition for food or perhaps the production of some inhibitory compound by a dominant tadpole. The lengths of the froglets were intermediate between those of Expt.1 and Expt. 2 at 14.5mm. This experiment gave the tadpoles completely unnatural conditions, since in the wild, they would each normally occupy their own pool or bromeliad. However, Expt. 4 tadpoles took almost as long to metamorphose (64 to 123 days) and were a little smaller (13mm). This tends to suggest that competition for food was the main factor. The larger tank set-up of Expt. 3 supported a larger community of plants and animals, which the tadpoles could graze on. Also, the volume of water per tadpole was much greater in Expt. 3 (especially after two of the tadpoles died). 

Other authors e.g. [Esposito, 1995] have also noted that communally-raised tadpoles take much longer to metamorphose than those kept singly, perhaps due to metabolic by-products impeding development [Bajcar, 1995]. However, it is also said is that these communally reared animals metamorphose at a larger size [Bajcar, 1995]. My results do not agree with their theories and perhaps more work needs to be done in this area. Within the communal tank it was observed that each tadpole would remain in its own little territory. Any disturbance such as adding more water or removing some of the weed would precipitate aggression towards one another. They would attempt to bite each other’s tails and ‘dance’ around for a while until calm was restored several minutes later.

All the froglets continued to receive ultra violet light treatment at the same dosage as for the tadpoles. All remained healthy and showed no signs of growth disorders such as metabolic bone disease.

Conclusion
Given the limited numbers of tadpoles reared it will be necessary to repeat these experiments several times in order to obtain conclusive results. However, in this case, it seems reasonable to conclude that rearing tadpoles singly in small beakers is the easiest and quickest way to obtain strong, healthy, large frogs. This statement assumes that the husbandry techniques set out above (in Expt. 1) are strictly adhered to.

Reducing food intake does increase the tadpole’s time in the water, but, in these experiments, does not give rise to a larger froglet.

It may also be of interest that according to an unpublished survey by D. Callaway [Callaway, 1995] of Omaha’s Henry Doorly Zoo, rearing tadpoles singly is the method favoured by the majority of zoos and private individuals (97% of 54 institutions/individuals).

Acknowledgements
I would like to thank my colleagues, Mr. E. Blake and Mr. D. Sherriff for giving me advice and assistance throughout this project.

Illustrations
Top left: typical pattern.  Photo by Mick Bajcar
Bottom right: similar to Carara form.  Photo by John Skillcorn

Products mentioned in text
Abidec Multi-vitamin Drops: Warner Lambert Health Care, Lambert Court, Chestnut Avenue, Eastleigh, Hampshire, SO5 3ZQ.

Ceramic Heat Bulb: Philips Lighting, PO Box 299, City House, 420-430 London Road, Croydon, CR9 5QR.

HABISTAT Pulse Proportional Thermostat: Living Earth Electronics, The Cottage In The Wall, Daily Road, Hayes, Middlesex, UB3 1EF.

SA-37 Vitamin Powder. Intervet UK Ltd., Science Park, Milton Road, Cambridge.

Tetramin Flake Food: Tetrawerke, Dr. Rer. Nat. Ulrich Baensch GmBM. D4520 Melle 1, Germany.

Wotan U.V.B. 300W spot bulb: Osram Ltd., Lea Green Road, St. Helens, WA9 4QQ.

References
[Bajcar, 1995] Bajcar M. 1995
British Dendrobatid Group Newsletter, No.24, pp. 3-4.

[Blake, Sherriff, 1993] Blake E. & Sherriff D. 1993
Maintenance of the Blue Poison-Arrow Frog, Dendrobates azureus, at Edinburgh Zoo.

[Callaway, 1995] Callaway D. (1995).
DENDROBATIDAE REARING TECHNIQUES IN CAPTIVITY

[Esposito, 1995] Esposito M. 1995
British Dendrobatid Group Newsletter, No.24, p. 4.

R.Z.S.S. Annual Report, 1993, pp. 58-63.

[Schafer, 1981] Schafer S. 1981
Don’t Handle The Merchandise.

Zoonooz 54 (2), pp. 11-13.
SURVEY RESULTS (Unpublished)

Omaha’s Henry Doorly Zoo, July 1995.

This page last updated: Monday, 08 April 2002