The animals are shrinking, two-by-two

From mice and whales to birds and salamanders, many species are decreasing in size and experts are unsure as to the cause

Tamaño animales
A genetic variation that allows salmon to spend four years at sea and reach 20 kilograms in weight is disappearing among Atlantic salmon in Finland.Mikko Kytökorpi
Miguel Ángel Criado

Field mice (Apodemus sylvaticus) in southern Spain’s Doñana National Park weigh up to a third less than they did 40 years ago. Northern gray-cheeked salamander (Plethodon montanus) in the Appalachian Mountains have shrunk by 8% since 1960. Atlantic Salmon (Salmo salar) in northern Finland are smaller and reach sexual maturity quicker than they did four decades ago. And one of the giants of the oceans, the North Atlantic right whale (Eubalaena glacialis), has on average reduced in size by a meter since the 1980s. All of this has led scientists to ask: what is happening to the animals?

In recent years evidence that the so-called sixth great extinction event is already underway has been piling up. Since the beginning of the 20th century, the rate of extinction among animal species has increased a hundredfold. The causes or this are diverse and some are global, such as climate change or overfishing, while others are more confined to certain regions, as is the case with deforestation of tropical jungles. What all have in common is that they are directly or indirectly provoked by the activities of mankind. That said, not all species suffer to the same extent. Recent research has demonstrated that the bigger the animal, the more at risk the species is. And scientists have observed another phenomenon among many animal populations: it is no longer the case that they are ever-fewer in number, but that they are getting progressively smaller. And this shrinking is happening among all families of the animal kingdom.

In Spain’s Doñana National Park, field mice have gone from weighing an average of 30 grams to barely registering 20 grams

The case of the Doñana field mice is one of the most pronounced. Their population, like those of other small rodents in the national park, has been declining over the past few years. An article published in the environmental journal Quercus outlines the data. In 1978, when field researchers set traps to count populations, 300 garden dormice and 200 field mice were caught. Using a similar number of traps set at night in the same area, the process was repeated every few years and, on every occasion, a downward trend was observed. Forty years later, not one dormouse was found and only 27 field mice were captured. Equally noteworthy is the fact that of the tiny sample that was studied, the mice have gone from weighing an average of 30 grams to barely registering 20 grams.

Miguel Delibes de Castro, honorary professor at the Doñana Biological Station, lays out several possible causes for the mice becoming smaller. “The first is climatic, related to temperature,” he says, pointing to Bergmann’s rule, an ecogeographical model that suggests animals belonging to the same species tend to be larger in colder climates, compared to their brethren in warmer zones. Doñana, like the rest of southern Europe, has been getting hotter over the past few decades as a result of climate change. “The mice are now finding their optimum at smaller sizes,” Delibes de Castro notes.

Another possibility is more specific to the park: the absence of rabbits, which have been ravaged by disease, has led predators to seek out the larger rodents for food. This has caused a process of selective pressure in favor of smaller animals. However, in order to verify that this pressure has been incorporated into the genetic structure of the mice it would first be necessary to determine if variations have been produced in the part of the genome that governs size. And this has been observed in Atlantic salmon at the other extreme of Europe, in northern Finland.

University of Helsinki biologist Craig Primmer has spent years studying genetic changes in salmon. In particular, he has focused on the variations in one genotype, the vgll3 locus, which is related to the “age at which salmon return to their marine migration routes, mature and reproduce.” This genotype also influences the start of puberty in humans. Its role in the maturation of fish is key to the size they can potentially reach.

Primmer has studied two populations of salmon that return to two different rivers to spawn and die. One, in the Inarojoki River, has not experienced alterations in its average size for 40 years. The other, in the Tenojoki River, has shed half its body mass among males and around 10% in the female population. There have been no significant changes in the genetics of the first population, but in the second the frequency of the genetic variant that favors growth has decreased by 18% over the same timeframe.

Primmer offers two possible explanations for this: “One is the lower survival rate of salmon during their marine migration, which would have a greater [negative] effect on those individuals that spend more years at sea, that is to say, the late-maturing ones, which are also the largest. The other is that selective fishing of larger individuals could result in a reduction in overall average size. In both cases, it would be adaptive [producing a greater chance of survival to reproduce] for the salmon to return to the sea earlier, and therefore they would be smaller on average.”

This bringing forward of migration patterns is one of the most significant impacts that climate change is producing in many species, particularly among birds. Ornithologists at Chicago’s Field Museum of Natural History have been studying these changes. For decades, they have had the macabre task of recovering birds that crash into nearby skyscrapers. These buildings are in the middle of the southern migration route. And the researchers have found that these birds are hitting the skyscrapers earlier and earlier each year. But upon measuring the unfortunate travelers, they have found that their body weight is consistently less while their wings are becoming ever-larger.

University of Michigan ornithologist Brian Weeks explains there are two hypotheses for these changes: “A smaller body size indicates that there is a greater ratio of superficial area by volume. This could help to dissipate heat and confer a selective advantage in warmer temperatures, which is related to the classical explanation of the spatial relation between size and temperature.” As such, Bergmann’s rule comes into play again.

The other explanation has to do with plasticity – the ability to adapt to the most rapid changes. “In this case, individuals that grow in places with warmer temperatures develop more quickly, but are smaller in size overall: this does not require any type of selection or genetic change. This mechanism is extremely common in ectotherms [cold-blooded animals] and could also be what is driving the relationship between temperature and size in endotherms [warm-blooded animals], but we do not know this for sure,” Weeks says.

The process of shrinking does not appear to be universal and we are still trying to learn more about general patterns” Jennifer Sheridan, Carnegie Museum of Natural History

These doubts outweigh the certainties regarding the universality of the shrinkage of animal life and its causes. Jennifer Sheridan of the Carnegie Museum of Natural History in Pittsburgh published a review of what the scientific world knew about what was happening to the animals a decade ago. Now she is revising the latest studies, which have still not provided a clear answer as to the whys and the hows.

“Curiously, the process of shrinking does not appear to be universal and we are still trying to learn more about general patterns,” Sheridan says via email. Among the doubts researchers harbor are “that we still do not know if the reduction in size is more common among ectotherms or endotherms [which have both displayed changes in size but also some species have shown no alterations] or if tropical organisms or those in temperate climates are more likely to suffer this impact.” In her new study, which has yet to be published, Sheridan again finds there is no unanimity over the universality of the shrinkage phenomenon.

There is more certainty over the causes, although Sheridan notes that research must be carried out on a case-by-case basis: “In those cases where an alteration in temperature is responsible it is reasonably safe to say they are anthropogenic, because we know that climate change is in large part anthropogenic. There are other examples of shrinkage that are not related to climate change, such as those due to fishing pressures, which are also caused by human. Beyond that, until the specific mechanisms are identified, we won’t know how much of this is down to anthropogenic causes.”

English version by Rob Train.

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