Islands serve as ideal natural laboratories for conducting studies on evolution. It is no coincidence that the first hypotheses about 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 the adaptations of species in these isolated territories. Since then, the study of evolution in conditions of insularity has been the objective of many research teams in order to find an explanation for the evolutionary regularities of these places.

In the case of island giants, that is, those species that are significantly larger on islands compared to their continental ancestors, it has been observed that the pace of life history slows down with increasing body size. Larger species mature later, have fewer offspring, and live longer than smaller ones. 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, as in most species, larger ones tend to have a slower life history than smaller ones. In an attempt to clarify the causes of this slowdown, ICREA professors at the Institut Català de Paleontologia Miquel Crusafont have 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 kg, about 5 times more than a current rabbit, making it 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. It had a rather rigid spine and reduced lung capacity, but perhaps the most characteristic feature is that it had short limbs and moved slowly, supporting the entire palm of its feet on the ground. All these characteristics are common in environments where there are no predators or very little pressure from them, as is also the case with the Myotragus balearicus dwarf goat from Mallorca.

Comparison between the estimated body size of Nuralagus rex and a current rabbit.

In recent years, many pieces of evidence have been found indicating that island dwarfs mature later and live longer than their corresponding continental relatives, as seen in the case of the dwarf elephant of Sicily, Palaeoloxodon falconeri. This observed trend aligns with the predictions of the "island syndrome" model and life history regarding age, size, and sexual maturity. However, it contradicts the predictions of allometric scaling models (which suggest that the smaller the size, the faster they reach sexual maturity). In island dwarfs, therefore, the evolution of life history is uncoupled from size reduction, indicating that a delay in reproductive timing and longer lifespan are adaptive phenomena.

But what about in the case of island giants like the rabbit Nuralagus rex? A delay in reproductive timing and longer lifespan could simply be the product of size increase and thus not be related to adaptation to the ecological conditions of the islands. To address this question, the research team used bone histology of this rabbit to reconstruct key features of its life history.

Following Ludwig von Bertalanffy's size-age growth models, the giant rabbit of Menorca shows a marked deceleration in growth rate, which is five or six times slower than that of the current hare. It also shows a significant delay in the age of sexual maturity. This considerable change far exceeds the implicit change in size increase. Therefore, the study's results demonstrate that the life history pattern of the island syndrome model affects all mammals evolving on islands, regardless of whether they have evolved into giant or dwarf forms. These studies contribute to understanding the significant implications behind changes in body size in island mammals, which remain a mystery.

Original research article:

• Köhler, M., Nacarino-Meneses, C., Quintana Cardona, J., Arnold, W., Stalder, G., Suchentrunk, F. & Moyà-Solà, S. (2023). Insular giant leporid matured later than predicted by scaling. iScience, 26, 107654. https://doi.org/10.1016/j.isci.2023.107654