Monday, January 24

Size: Animals are shrinking | Science


Field mice (Apodemus sylvaticus) of the Doñana National Park today weigh a third of what they weighed 40 years ago. Northern gray-cheeked salamanders (Plethodon montanus) of the Appalachian Mountains (USA) have shrunk 8% since 1960. Salmon from northern Finland (Salmo salar) are smaller and reach sexual maturity earlier than four decades ago. And one of the giants of the sea, the whale of the Basques (Eubalaena glacialis), has decreased by more than a meter since the 1980s. What is happening with the animals?

In recent years, evidence has accumulated that the so-called sixth great extinction is underway. The rate of disappearance of species is 100 times higher since the 20th century. The causes are diverse, some global, such as climate change or fishing, and others more regional, such as deforestation of tropical forests. But all or almost all are directly or indirectly caused by humans. However, not all species suffer from it equally: recent work has shown that the larger the animal, the more endangered the species is. And there is another phenomenon that is occurring within many populations: it is no longer that they are less and less in number, but that they are smaller and smaller. This shrinking phenomenon occurs in all families of the animal kingdom.

The case of the Doñana mice is one of the most extreme. Its population, like that of other small rodents in the park, has been declining in recent years. A scientific article in the environmental magazine Quercus gives some data. In 1978, when scientists began setting traps to calculate populations, they captured 300 dormouse and 200 field mice. Using a similar number of traps per night in the same area, they repeated the sampling every few years and always observed a downward trend. Forty years later, they no longer found a dormouse, and only 27 mice. Equally striking is the fact that the few that remain have gone from having an average of 30 grams to adding just 20 grams.

In the Doñana park, in addition to having fewer field mice, they weigh a third less

Miguel Delibes de Castro, honorary professor at the Doñana Biological Station, points out several possible causes of the dwarfing of mice. “The first is climatic, it has to do with temperature,” he says. And remember what it stipulates Bergmann’s rule, a kind of biological law according to which within the same species animals that live in colder places tend to be larger than their congeners in warmer zones. Doñana, like the entire region, has been heating up in recent decades due to climate change. “The mice would now find the optimum with smaller sizes,” adds Delibes de Castro.

Another possibility is more park-specific: that the absence of rabbits is causing predators to prey on larger rodents. This would cause selective pressure in favor of the little ones. However, to know if such pressure has been incorporated into the genetics of the mice, it would be necessary to find out if there have been variations in the part of the genome that has to do with size. And this they have observed with Atlantic salmon at the other end of Europe, in northern Finland.

The biologist at the University of Helsinki (Finland) Craig Primmer has been studying genetic changes in salmon. In particular, he has looked at variations in a gene, vgll3, which appears to be related to “the age at which salmon return from their marine migration, mature and reproduce.” This gene is also involved in the onset of puberty in humans. The role in the maturation of the fish is key in the size they can reach.

The genetic variation that allowed a salmon like the one in the image to spend four years at sea reaching 20 kilos in weight is disappearing among Atlantic salmon.
The genetic variation that allowed a salmon like the one in the image to spend four years at sea reaching 20 kilos in weight is disappearing among Atlantic salmon.Mikko Kytökorpi

Primmer has been investigating two salmon populations that return to two different rivers to spawn and die. Some, those of the Inarojoki River, have not changed their average size in almost 40 years. The latter, that of the Tenojoki river, have lost half their average weight in males and 10% in females. By studying their genes, they did not detect major changes in salmon from the first river, but in the second the frequency of the genetic variant that favors a larger size has been reduced by 18% in four decades.

Primmer has two possible explanations for this reduction: “One is the lower survival of salmon during their marine migration, which would have a greater (negative) effect on those individuals that spend more years at sea, that is, those of maturation late, which are also the largest. The other is that targeting larger individuals could result in a reduction in average size. In both cases, it would be adaptive (greater probability of surviving to reproduce) if the salmon returned from the sea earlier and, therefore, it would be smaller on average ”.

Advance migration is one of the biggest impacts that climate change is having on many species, particularly birds. This is what ornithologists from the Field Museum of Natural History (Chicago, USA) have been checking. For decades they have had the macabre habit of picking up the birds that collide with the nearby skyscrapers. The buildings are in the middle of the migration to the south. And they have verified that the birds each time collide before. But by measuring and weighing them they have also seen that they weigh less and less even though their wings are getting bigger.

“A smaller body size means there is a higher ratio of surface area to volume. This can help dissipate heat and confer a selective advantage in warmer temperatures. “

Brian Weeks, ornithologist at the University of Michigan (USA)

The Ornithologist from the University of Michigan (USA) Brian Weeks He exposes the two hypotheses that exist about these changes: “A smaller body size means that there is a greater ratio of surface area per volume. This can help dissipate heat and provide a selective advantage in warmer temperatures. This is related to the classical explanation of the spatial relationship between size and temperature ”, he says. Therefore, Bergmann’s rule appears again, seen in the Doñana mice. The other explanation has to do with plasticity, with the ability to adapt to faster changes: “In this case, individuals that grow in environments with warmer temperatures develop faster, but they are smaller; this does not require any selection or genetic change. This mechanism is very widespread among ectotherms [animales de sangre fría] and it may also be what’s driving the temperature-size relationship in endotherms, but we don’t know, ”Weeks concludes.

Those doubts dominate the certainties about the universality of the shrinking of animal life and its causes. Jennifer Sheridan of the Carnegie Museum of Natural History (Pittsburgh, USA) published a decade ago a review of what science knew about what was happening to animals. You are now reviewing the latest studies again and there is still no clear answer as to the what and why.

“Interestingly, the contraction process does not appear to be universal and we are still trying to learn more about general patterns,” Sheridan replies in an email. Among the doubts, “we still do not know if the reduction in size is more common among ectotherms or in endotherms (in both there has been shrinkage, but also absence of changes), or if tropical organisms or those of temperate climates have more probabilities of suffering this impact ”, he adds. In their new study, yet to be published, they again observe that there is no unanimity on the universal character of belittling.

There are more certainties about the causes, although Sheridan reminds us that we must go case by case: “For cases in which the alteration of temperature is responsible, it is quite safe to say that they are anthropogenic, because we know that climate change is in largely anthropogenic. There are other examples of shrinkage not related to climate change, such as those due to fishing pressures, which are also due to humans. Beyond that, until the specific mechanisms are identified, we won’t know how much of all this is due to anthropogenic causes. “

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