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More than 10 million years ago, a group of three-toed equids called hyparions, which were similar in size to a current large pony (about 150 kilos in weight) and fed on a greater variety of resources, such as leaves, bark and fruits, than today's horses, donkeys and zebras, which mainly graze.
These The first hyparyons to reach Europe rapidly diversified, and in some cases they reduced their size considerably.
In present-day Greece, large species coexisted with a small hyparion, which was decreasing in size until weighing only about 70 kilos, nine million years ago.
In the Iberian Peninsula, on the other hand, the decrease in the size of hyparions occurred later, at the end of Miocene higher, about six million years ago.
But why and how these size changes occur throughout evolution? Researchers affirm that changes in height may be an indirect consequence of the action of natural selection on life cycles.
Analyzing the inside long bones Much information about these cycles can be gleaned from animals.
"In the fossils we observe certain marks that - in a similar way to what we see in the rings of tree trunks - inform us of the stops in the annual growth of the animal", explains Guillem Orlandi, researcher at the Institute's Evolutionary Paleobiology Group Català de Paleontologia Miquel Crusafont (ICP) and lead author of the study.
Comparison of the bone growth rates and the relative maturity ages of hyparia allows us to affirm that the Balkan and Iberian horses followed two different strategies but that led them in parallel to a decrease in body size.
"On the one hand, we see that Iberian dwarf hyparions grew more slowly and matured later, while their Greek analogues stopped growing early and reached maturity earlier," says Meike Köhler, ICREA research professor and head of the Paleobiology Research Group Evolutionary of the ICP that has also participated in the research.
“We think that these different strategies respond to different environmental situations. They would probably be related to the different habitats and, with it, different selection pressures, which existed in the past on both sides of the Mediterranean ”, explains the researcher.
Dwarf horses in different habitats
The habitats of present-day Greece in the late Miocene were open, with little tree density. In this environment, hypariones would be more exposed to predation by carnivores, such as hyenas and saber teeth.
When a population is subjected to a high predation –And, consequently, to a high mortality rate in adult individuals– population losses are minimized as sexual maturation advances.
Thanks to this system, the time between one generation and the next is shortened and the population can remain stable despite predators.
Instead, in those times, the Iberian Peninsula was dominated by more closed and wooded environments. In these circumstances, small herbivores are less prone to attack by predators.
At the same time, however, these ecosystems tend to be poorer in food for equines, a situation that would be aggravated in periods of drought such as those in the area where the remains come from.
Under these conditions, hyparions would have had less food available, an eventuality that mainly affects the mortality of juvenile individuals. The optimal strategy in this scenario is to breed late, as delayed maturity leads to an increase in the juvenile survival rate as a consequence of more experienced parents.
At the same time, low levels of resources cause a decrease in growth rates (as occurs in human populations) and favor a decrease in body size. In this context, the animals grow slowly and for a longer time, as has been seen in the population of small Iberian hyparions.
The study results published in Scientific Reports, beyond providing new information on the complex evolutionary history of equidae, provide new evidence that indicates how similar changes in size can be caused by adaptations of life cycles to different selection pressures.
The evolution of horses
The lineage of horses has been a classic example of evolution in natural history books for over 100 years. At the end of the 19th century, some American authors proposed a linear evolution that would end with the appearance of today's horses and that would go through a decrease in digits in the extremities and an increase in tooth height and body size.
This excessively simplistic conception has long been discarded and the idea of a complex tree of lineages of which only the species of the genus Equus, like horses or zebras.
The hyparion branch, although totally extinct, was very prolific and widely diversified in shape and size, as evidenced by the presence in the Iberian Peninsula of forms of more than 300 kilos and others of little more than 30. Lineages confirm the idea of a branched and complex evolution of equidae.
Orlandi-Oliveras, G., Nacarino-Meneses, C., Koufos, G. D., Köhler, M., 2018. «Bone histology provides insights into the life history mechanisms underlying dwarfing in hipparionins«. Scientific Reports. DOI: 10.1038 / s41598-018-35347-x
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