Montane amphibians: Modifications of ecology and morphology in an extreme environment
Alpine habitats must be considered as extreme to amphibians due to the short period available for breeding and the permanent possibility of sudden and strong temperature decreases. Increased ultraviolet radiation at high altitudes may also be problematic for amphibians - a large part of recent unexplained amphibian declines took part in mountain areas of North and South America, and the sensitivity of the larvae of certain species to ultraviolet radiation has been demonstrated experimentally. Assessment of the natural adaptations of amphibians to the extreme conditions at high altitudes are therefore also crucial to understand the causes of possible future declines.
Between
1997 and 2003 we have carried out intensive studies on populations of the
common frog, Rana temporaria, in the Spanish Pyrenees. The main
focus was a large population at ca. 2150 m altitude in the Circo de Piedrafita
area, Aragon. The fieldwork is carried out in cooperation with David
Vieites and Sandra Nieto. Results show a shift towards more aquatic
and diurnal habits in montane frogs (as compared to lowland populations),
a high importance of thermoregulation for activity and microhabitat
choice, and a large morphological plasticity of larvae depending on ecological
conditions. Montane R. temporaria (especially males) develop ontogenetically
black dorsal spots, which may be a response to UV radiation or have thermoregulatory
function. A study of morphological variables over a wide sample of populations
showed that certain morphological characteristics (black spots, short legs)
are widespread in montane populations but lacking in lowlands. The encountered
morphological differentiation would, in other cases, be clearly indicative
of separation at the species level. In R. temporaria, however,
morphological differentiation is not correlated with genetic differentiation
as assessed by DNA sequencing and allozyme studies. Selective pressures
may have led independently to the same morphological adaptations in very
short time spans (probably only posterior to the last glaciation).
The diurnal habits of this population also allowed a detailed observation of the reproductive behaviour of these frogs revealed the existence of clutch piracy: males follow amplecting pairs and attempt to fertilize eggs in recently deposited clutches that remained unfertilized by the amplecting male. This strategy has advantages not only for the pirate males but also for the females, because a higher percentage of eggs are fertilized.
We are currently planning new studies that will focus on the metapopulation network in the Circo de Piedrafita area and its amount of genetic exchange with lowland populations. Extension of morphological studies to other montane regions (e.g. the Alps) will further help to better understand - eventually also in a quantitative approach - the relevant factors. We are also studying montane herpetofaunas in Madagascar.
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