Amami rabbits show exceptionally delayed maturity in island environment

Key Findings
•    Analysis of bone tissues revealed for the first time that the Amami rabbit likely takes about five years to reach maturity—much longer than closely related rabbit species, which mature within a year.
•    Such extreme growth delay has previously been known only in extinct fossil mammals, making this the first discovery in a living mammal species worldwide.
•    The bones of the Amami rabbit are exceptionally dense and robust, suggesting a unique skeletal adaptation to life in the steep, rugged island terrain.

Summary
A research team led by Associate Professor Shoji Hayashi of the Department of Dinosaur Paleontology, Okayama University of Science, and Associate Professor Mugino Kubo of the Graduate School of Frontier Sciences, The University of Tokyo, has analyzed bone tissues of the Japanese endemic Amami rabbit, a nationally designated Special Natural Monument. Their study revealed for the first time that this species grows at an exceptionally slow pace, taking about five times longer to mature compared to closely related rabbit species.

Such remarkable growth delay has previously been documented only in extinct island-dwelling mammals, making this the first confirmed case in any living mammal species worldwide. The findings provide important evidence that small mammals can evolve extremely slow life histories under unique island environments—characterized by limited food resources and few predators.

The results of this study were published online on April 2, 2025, in Mammal Study (Vol. 50, No. 2), the international journal of the Mammal Society of Japan, and appeared in the April 30, 2025, printed issue. This issue featured a special collection on the mammals of the Ryukyu Islands, co-edited by Associate Professors Hayashi and Kubo. 

Research Findings
The research team conducted bone histological analysis (bone histology)—a method originally developed in dinosaur studies (Note 1)—on limb bones of the Amami rabbit preserved in museum collections. The results revealed, through the observation of growth arrest lines in the bone tissues, that while closely related rabbit species inhabiting mainland Japan and continental regions typically reach maturity within about one year, the Amami rabbit requires nearly five years to mature.

Such an extreme delay in growth has previously been documented only in extinct island-dwelling mammals from regions such as the Mediterranean and the Ryukyu Islands, making this the first discovery of its kind in any living mammal species (Note 2).

On islands, where predators are scarce and food resources limited, animals are known to follow a “slow-rearing” strategy—producing fewer offspring and investing more in their survival. Previous studies had already suggested that the Amami rabbit exhibits this unusual reproductive mode (Note 3). The present study further demonstrated that the species also takes significantly longer to reach maturity than its relatives. Slow growth may reduce reproductive output and make it more difficult to sustain populations, thereby potentially increasing the extinction risk of the Amami rabbit compared with other rabbit species.

The researchers also found that the bone tissue of the Amami rabbit is exceptionally dense and robust compared to other rabbits. This feature is considered an adaptation of the skeletal structure to efficiently navigate the island’s steep and rugged terrain.

Finally, this study demonstrates that even for rare species that are difficult to monitor through long-term ecological surveys, paleontological bone histology techniques can provide valuable insights into their ecology and life history using skeletal specimens.
 
In the Amami rabbit, a much larger number of growth arrest lines (red lines) can be observed compared to the Japanese hare. The presence of multiple growth arrest lines even in young adults—whose gonads have matured and whose bones have just fused—demonstrates that the Amami rabbit requires several years to reach full maturity.

The Ryukyu Islands have long been isolated from other islands and continental landmasses, resulting in ecosystems with virtually no mammalian predators and unique ecological dynamics. In such environments, unusual life-history and evolutionary patterns—previously known only from fossil species in Europe, North America, and other regions—can still be observed in living species. The findings of this study not only enhance our understanding of these distinctive evolutionary traits found in the animals of the Ryukyu Islands, but also highlight the importance of conserving such rare and ecologically unique species.

Notes

(Note 1) Bone histological analysis (bone histology)
A method in which bones are thinly sliced and observed under a microscope to infer growth patterns and rates. It is widely used in the study of fossil species, where only bones remain, and has been especially important in dinosaur research.

(Note 2) Example: Growth delay in fossil mammals of the Ryukyu Islands
Similar to the Amami rabbit, the extinct Ryukyu deer—believed to have colonized the islands more than a million years ago—also exhibited extremely delayed growth compared with its relatives (Hayashi et al., 2013. Variation and process of life history evolution in insular dwarfism as revealed by a natural experiment. Frontiers in Earth Science, 11:1095903. https://doi.org/10.3389/feart.2023.1095903).

(Note 3) Unique reproductive traits of the Amami rabbit
Unlike other rabbits, the Amami rabbit gives birth to only 1–2 offspring at a time, and breeding seasons are limited. It is thus one of the least fecund species in the order Lagomorpha (Hamada, F. & Mizuta, T., 2020. Unique reproductive traits of the Amami rabbit Pentalagus furnessi: an endangered endemic species from southwestern Japan. Mammal Research, 65: 805–813).