Indigenous water frogs can be crowded out by immigrant or imported species. This is the finding of a Franco-German study. The scientists investigated water frog populations in France and Northern Spain and noticed that the marsh frog (Rana ridibunda), which normally occurs only in Eastern Europe, has the potential to crowd out indigenous species like Graf's hybrid frog (Rana grafi) and the Iberian water frog (Rana perezi).The scientists believe this ability is related to the fact that the marsh frog lives longer and grows faster than the indigenous species. In addition, the female marsh frogs produce more progeny than their competitors, the researchers write in the scientific journal Comptes Rendus Biologies.
The marsh frog has now spread from Central Asia as far as France and Spain. This latest spread is attributed to imports of live animals for culinary purposes. As the foreign species mix with the indigenous species, the indigenous water frogs are being pushed back into just a few areas.
Imports of invasive species by humans are regarded as one of the main threats to species diversity on earth, alongside climate change.
Most stable marsh frog populations in France and Switzerland originated from frogs bred for gourmet restaurants or imported directly from various source countries. The frog fauna along larger river flood plains in both countries has now altered significantly in favour of the new frog species. Until now though, it was not clear why the new species had been able to establish themselves in the face of the indigenous species. The team of scientists investigated over 700 water frogs from 22 locations in the Rhone drainage basin in France and four locations in the Ebro drainage basin in Spain.
"We noticed that the introduced marsh frog has a high competitive potential, particularly in high-oxygen, low-salinity freshwater. In these conditions the indigenous frogs hardly stand a chance," declares Dr Dirk Schmeller of the Helmholtz Centre for Environmental Research (UFZ). The displacement process is linked to the fact that the marsh frog grows faster than the indigenous frogs, and is in direct competition for food.
In addition, female marsh frogs live longer and are more fertile. This means that over the course of their lives they produce significantly more progeny, thereby crowding out the other frog species. The number of progeny is increased still further because when the marsh frog mates with Graf's hybrid frog (Rana grafi) or the edible frog (Rana esculenta), it produces marsh frog progeny.
This may sound astounding, but can be attributed to a special type of reproduction called hybridogenesis, which is a special feature of water frog reproduction. All these factors may lead to the extinction of the indigenous water frog species, the researchers believe, and therefore recommend that the spread of the marsh frog be closely monitored.
Physically fit frogs have faster-changing genomes, says a new study of poison frogs from Central and South America. Stretches of DNA accumulate changes over time, but the rate at which those changes build up varies considerably between species, said author Juan C. Santos of the National Evolutionary Synthesis Center in Durham, North Carolina.In the past, biologists trying to explain why some species have faster-changing genomes than others have focused on features such as body size, generation time, fecundity and lifespan. According to one theory, first proposed in the 1990s, species with higher resting metabolic rates are likely to accumulate DNA changes at a faster rate, especially among cold-blooded animals such as frogs, snakes, lizards and fishes. But subsequent studies failed to find support for the idea.
The problem with previous tests is that they based their measurements of metabolism on animals at rest, rather than during normal physical activity, Santos said.
"Animals rarely just sit there," Santos said. "If you go to the wild, you'll see animals hunting, reproducing, and running to avoid being eaten. The energetic cost of these activities is far beyond the minimum amount of energy an animal needs to function."
To test the idea, Santos scoured forests in Colombia, Ecuador, Venezuela, and Panama in search of poison frogs, subjecting nearly 500 frogs -- representing more than 50 species -- to a frog fitness test.
He had the frogs run in a rotating plastic tube resembling a hamster wheel, and measured their oxygen uptake after four minutes of exercise.
The friskiest frogs had aerobic capacities that were five times higher than the most sluggish species, and were able to run longer before they got tired.
"Physically fit species are more efficient at extracting oxygen from each breath and delivering it to working muscles," Santos said.
To estimate the rate at which each species' genome changed over time, he also reconstructed the poison frog family tree, using DNA sequences from fifteen frog genes.
When he estimated the number of mutations, or changes in the DNA, for each species over time, a clear pattern emerged -- athletic frogs tended to have faster-changing genomes.
Santos tested for other factors as well, such as body and clutch sizes, but athletic prowess was the only factor that was consistently correlated with the pace of evolution.
Why fit frogs have faster-changing genomes remains a mystery. One possibility has to do with harmful molecules called free radicals, which increase in the body as a byproduct of exercise.
During exercise, the circulatory system provides blood and oxygen to the tissues that are needed most -- the muscles -- at the expense of less active tissues, Santos explained.
When physical activity has stopped, the rush of blood and oxygen when circulation is restored to those tissues produces a burst of free radicals that can cause wear and tear on DNA, eventually causing genetic changes that -- if they affect the DNA of cells that make eggs or sperm -- can be passed to future generations.
Before you ditch your exercise routine, Santos offers some words of caution. The results don't debunk the benefits of regular physical exercise, which is known to reduce the risk of cancer, heart disease, and diabetes.
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