iHerp Australia Issue 12 | Page 43

Dietary specialisation in turn is linked to heightened aerobic capacity, probably because ants have relatively poor nutritional value, so the frogs have adapted by becoming active predators, with large home ranges. It is not known when humans first learnt of the deadly nature of dendrobatids, but Amerindian hunters have been using the poison of three species of frogs (notably P. terribilis) to coat the tips of their blow darts and arrows for centuries. Using dendrobatid poison for hunting is by no means a benign activity for the frogs themselves; P. aurotaenia and P. bicolor are impaled on sticks or spears, and may be heated as well, which supposedly maximises the amount of toxic secretions. According to an account published in 1978, the method in which the local tribespeo- ple obtain poison from P. terribilis is completely inhumane. The hapless frog is imprisoned in a hollow cane, and when the poison is required, they ‘pass a pointed piece of wood down his throat, and out at one of his legs’, causing the frog to become agitated and secrete toxins. Although the poisonous secretion can remain potent for a year, the indigenous people apparently have nothing to fear from consuming game felled with poison darts or arrows, possibly because the toxins are denatured by cooking. There has been much research into the medical potential of dendrobatid poisons. Epibatidine, a compound isolated from the Ecuadorian dendrobatid Epipedobates tricolor, is a powerful painkiller, 200 times more potent than morphine. However, further work is required before it hits the market, as the therapeutic dose is very close to the fatal dose! There is additional potential for drugs to assist smokers and drug users to quit their habits. The dendrobatid frogs have escaped predation by evolving toxicity and advertising it. However, evolution is often a constant arms race, and the colubrid Erythrolamprus epinephelus is able to predate upon P. terribilis due to an acquired immunity to its toxins. Although most available information about dendrobatids focusses on their unsavoury attributes, they are also highly unusual for being the only large family of frogs in which all species perform some aspects of parental care. In fact, this is obligatory, because dendrobatids lay their eggs on land, on moist substrates on the forest floor. Unlike some other terrestrial-laying frogs, in which the larvae undergo metamor- phosis in the egg and emerge as froglets, dendrobatid eggs hatch into tadpoles which require an aquatic habitat. The parents may guard the eggs and gather moisture to prevent them from dessicating; once the eggs hatch, they will ‘piggyback’ the larvae to aquatic nurseries, located in epiphytic plants such as bromeliads. In these little pools, the tadpoles are not subject to dessication, are at lower risk of predation, and can feed on microbes, aquatic plants and invertebrates. Often the male is responsible for moving the tadpoles, which adhere to him thanks to the mucus coating his body. In some instances, the male will attend several clutches of eggs, and may carry the tadpoles on his back for up to nine days. In species where the male is mostly responsible for parental care, the traditional sex roles are reversed, and the female will often guard the male against rivals. For example, in Dendrobates tinctorius and D. auratus, it is the male that both attends the eggs and transports the tadpoles to water. In these species, the females are highly territorial and aggressive towards other females, and compete for access to males. Unlike most other male frogs, which have loud, complex calls to attract females, in these species the males are not capable of long-distance calls, and instead produce only weak vocalisa- tions when in the presence of a female. Females of some species take parental investment to the extreme, sacrificing eggs for the good of their tadpoles; many species in the genera Oophaga and Ranitomeya supple- ment the diet of their offspring by returning regularly to the nurseries and depositing unfertilised eggs into the water. In the Oophaga pumilio clade (O. pumilio, O. speciosus, O. histrionicus, and O. lehmanni) oophagy is necessary, as the larvae have reduced denticle rows, making them incapable of predating upon other food sources. Each larva will receive between 20-40 unfertilised eggs during its development. In the genus Ranitomeya both parents work to ensure the best chance of survival for their off- spring. These species are phytotelm specialists, only breeding in Heliconia, Dieffenbachia, and Xanthosoma plants. The eggs are laid in pairs among the bracts of the plant, and once the eggs hatch, the father then transports the tadpoles. The mum feeds the tadpoles with unfertilised eggs, with the dad accompanying her to help her locate the various plant nurseries where their tadpoles reside. The investment of a male in his off- spring can take on a rather darker side. The male Allobates femoralis both attends the eggs and transports the tadpoles to nurseries (often outside the male’s territory; he may distribute the clutch in several locations as a bet-hedging strategy). In this species, males are highly territorial and exhibit strong fidelity to their territory throughout the breeding season. Their prominent