Laurent SCHLEY

Research associate of the

Research > Mallorcan Midwife Toads

My research centered around the activity patterns and anti-predator behaviour of the Mallorcan midwife toad Alytes muletensis. This small amphibian is endemic to the island of Mallorca and threatened with extinction. In fact the species was thought to be extinct until its rediscovery in 1980. Before that time, the species was known only from semi-fossils, and was thought to have disappeared from Mallorca following the colonisation by man and the accompanying introduction of non-native predators. Foremost amongst the latter is the amphibian-eating viperine snake Natrix maura, which was probably introduced by the Romans for religious purposes about 2000 years ago. The continuous presence of the viperine snakes in the present geographic range of the toad raises the question whether the Mallorcan midwife toad has evolved defence strategies to minimise the risk of predation by snakes during the 2000 years or so since the snakes were introduced to the island.

Activity and Microhabitat Selection

Tadpoles of the Mallorcan midwife toad displayed a clear diel pattern of activity and microhabitat selection in natural torrent pools in Mallorca. Tadpoles spent much of the day foraging in a non-aggregated fashion in warm shallow areas of the pool, and activity peaked at around 17.00-19.00h. However, in mid-afternoon when much of the water surface was in direct sunlight, tadpoles moved away from the shallow areas and formed dense aggregations under rock overhangs at the sides of the pools. After dark both activity and tadpole density in the shallows dropped dramatically, and there was a net movement of the population back into deep water.

Anti-Predator Behaviour

I compared the activity levels of Mallorcan midwife toad tadpoles in two natural torrent pools which differed in their use by predatory viperine snakes: one pool was regularly used by these predators, whereas in the other no snake had been observed previously. Activity levels were lower in the pool regularly used by snakes than they were in the snake-free pool. The presence of snakes, experimentally introduced into the pools in nylon bags, reduced activity levels and changed distribution dramatically in the pool that was usually snake-free. The effect was much less conspicuous in the pool regularly used by snakes. In the laboratory, corresponding reductions were observed when tadpoles were treated with chemical cues from Mallorcan viperine snakes in a gravitational flow-through system. These tadpoles had been bred as part of the recovery programme for the species. However, the tadpoles failed to respond to chemical cues from other amphibian-eating snakes, or even to those from viperine snakes collected from a different population in mainland Spain. As none of the snakes had previously eaten midwife toads, the responses cannot be related to previous diet, and seem to be specific to those N. maura from the island of Mallorca. Similarly, in a substrate choice experiment carried out in the laboratory, postmetamorphic Mallorcan midwife toads also exhibited an active avoidance of substrate impregnated with chemical cues of viperine snakes. As viperine snakes were probably introduced to Mallorca about 2000 years ago, the evolution of anti-predator behaviour in midwife toads must have occurred relatively recently.

Conservation

From a conservation point of view, it is reassuring that, through many generations of captive breeding, Mallorcan midwife toads have retained the natural anti-predator behaviour exhibited by the wild tadpoles in their natural torrent pools. Captive-bred toads that are reintroduced to the wild should therefore be able to minimise predation risk from snakes by continuing to express such behaviours. However, in view of its precarious status, removal of viperine snakes from breeding pools should continue to be part of the population management strategy for this endangered amphibian.

Acknowledgements

The research was co-funded by AAT- Garten- und Teichfreunde Luxemburgs (Luxembourg) and by the Durrell Institute of Conservation and Ecology (U.K.), and supported by the Consellaría de Medi Ambient (Mallorca, Spain). Dr Richard Griffiths supervised the research. P. Sharp and J. Dennis carried out some of the laboratory work. The following people (in alphabetical order) provided logistical support: Dr A. Alcover, Dr S. Bush, A. Forteza, L. Gillett, Dr J. Mayol, E. Mendoza, A. Román, J.-M. Román and P. Vicenç. We thank landowners on Mallorca for allowing us access to the study sites.