2023-10-11 19:15:47
Islands are ideal natural laboratories for carrying out studies on evolution. It is no coincidence that the first hypotheses on the theory of evolution by natural selection, formulated in the mid-19th century by naturalists Charles Darwin and Alfred Russel Wallace, were based on observations made on islands around the world to study their biodiversity and adaptations of species in these isolated territories. Since then, the study of evolution in insular conditions has been the objective of many research teams in order to find an explanation for the evolutionary regularities of these places.
The interest aroused by the study of evolution on islands is due to a series of characteristics, such as geographic isolation or scarcity of resources in island ecosystems, which trigger common evolutionary patterns in the faunas that live there. This phenomenon is known as island syndrome and is known to result in a slow pace in the life history of island species. This often translates into changes in body size (gigantism or dwarfism phenomena often occur), in longevity and in the age at which they reach sexual maturity, among other parameters. These differences from their ancestral species are adaptations to selective pressures arising from the particular ecological conditions operating on the islands.
In the case of island giants, that is, those species that are significantly larger bodied on islands compared to their continental ancestors, it has been observed that the pace of life history slows down with increasing body size. Species with larger body sizes mature later, have fewer offspring, and live longer than those with smaller bodies. What is not so clear is whether this slowdown is an adaptation to the ecological conditions of the islands or simply a consequence of their size, since in most species, the largest ones tend to have a slower life history than the larger ones. smaller. To try to clarify the causes of this slowdown, the team led by Meike Köhler, from the Miquel Crusafont Catalan Institute of Paleontology, has taken the giant rabbit of Menorca, Nuralagus rex, as a model.
This species, which became extinct about 5 million years ago, had quite particular morphological characteristics. It weighed around 8 kilograms, about 5 times more than a modern rabbit, thus becoming the largest leporid (the taxonomic group that includes rabbits and hares) known to date. It had a relatively small brain, the size of its eye sockets and the characteristics of its auditory system suggest that it did not have a highly developed sense of sight or smell. He had a fairly rigid spine and reduced lung capacity. But perhaps the most characteristic thing is that it had short limbs and moved slowly, resting the entire palm of its paws on the ground. All of these characteristics are common in environments where there are no predators or very little pressure from them, as is also the case of the Mallorcan pygmy goat Myotragus balearicus.
Comparison between the estimated body size of Nuralagus rex and that of a much smaller modern rabbit. (Image: Meike Köhler, Miquel Crusafont Catalan Institute of Paleontology)
In recent years, much evidence has been found that island dwarfs mature later and live longer than their corresponding continental relatives, as is the case with the Sicilian dwarf elephant, Palaeoloxodon falconeri. This observed trend is consistent with the predictions of the “island syndrome” and life history models for age, size, and sexual maturity. However, it contradicts the predictions of allometric scaling models (which indicate that the smaller they are, the faster they reach sexual maturity). In island dwarfs, therefore, life history evolution is uncoupled from size reduction, indicating that a delay in reproductive timing and longer lifespan are adaptive phenomena.
But what happens in the case of island giants like the rabbit Nuralagus rex? A delay in reproductive time and longer lifespan could simply be the product of increasing size and therefore unrelated to adaptation to the ecological conditions of the islands. To clarify this question, the research team has used the bone histology of this rabbit to reconstruct key characteristics of its life history.
Following Ludwig von Bertalanffy’s size-age growth models, the Menorca giant rabbit shows a marked slowdown in growth rate, which is five or six times less than that of today’s hare. It also shows a significant delay in the age of sexual maturity. This considerable change far exceeds the change implied by the increase in size. Therefore, the results of the study show that the life history pattern of the island syndrome model affects all mammals that evolve on islands, regardless of whether they have evolved into giant or dwarf forms. Studies like this contribute to a better understanding of the important implications of changes in body size in island mammals, which largely remain a mystery.
The study is titled “Insular giant leporid matured later than predicted by scaling. And it has been published in the academic journal iScience. (Source: Catalan Institute of Paleontology Miquel Crusafont)
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