Insular dwarfism in horses from the Aegean Sea and the Japanese archipelago

Abstract

The horse (Equus caballus) varieties from Skyros and Rhodes islands (Greece) in the Aegean archipelago are extremely small, reaching shoulder heights of only about 1 m. Furthermore, the Japanese archipelago is home to eight small, native horse breeds. We investigated the evolutionary morphology and provided a review of historical documentations of these horses of cultural interest in Greece and Japan, thus providing a comparison of the independent evolution of small size in islands. We integrate cranial data from historical literature with data from newly gathered and curated skulls and analyse a measurement dataset featuring various domestic and mainland horse breeds and varieties. We use non-invasive imaging to study and measure 3D models of the bony labyrinth, housing the inner ear, and the braincase endocast. When considering the effects of allometry by regressing each PC1 scores (for each set of measurements) with the cranial geometric mean from linear measurements as a body size proxy, we show that size explains a large amount of the shape variation in horse crania, the bony labyrinths and brain endocasts. We found high intrabreed variation in all the analysed datasets. Globally, there are at least 30 distinct horse populations on islands, offering the chance to further study processes of convergence in morphological divergence and evaluate the effect of drift and the environment.

Supplementary information: The online version contains supplementary material available at 10.1007/s42991-024-00408-4.

Keywords: Allometry; Bony labyrinth; Domestication; Endocast; Nanism.

Fig. 1

Home ranges of the studied Aegean breeds/varieties (A) and all native Japanese breeds (B). Insular home ranges include the size of the respective islands, excepted for the Japanese mainland. Maps adapted from mapchart.net. Symbol: *, The Miyako breed is the only native Japanese breed not represented by our study

Fig. 2

Examples of living Skyros horses (A), Rhodes horses (B), a Hokkaido horse (C), and a Tokara horse (D). Pictures taken by MRS-V, GLG, and KMM

Fig. 3

Measurements taken on the cranium (A), the bony labyrinth (B), and the braincase endocast (C). Cranial measurements illustrated on Skyros I (NMBE1088728) in ventral and dorsal views and modified from Heck et al. (2018). Measurements of the bony labyrinth of Rhodes A (NMBE1088733) in dorso-anterolateral, anterior, and anterolateral views. The landmarks here serve only to illustrate the measurements taken on the bony labyrinth. Brain endocast of Rhodes A (NMBE1088733) in ventral (A), dorsal (B), and lateral (C, D) views. As the right inner ear was removed for acquisition, missing parts appear on the right side of the braincase 3D mesh because of empty space during segmentation process. Measurements and their corresponding abbreviations are available in Table 2. Further abbreviations: ant. anterior, aquae. aquaeductus, lat. Lateral, post. posterior

Fig. 4

Principal component analyses and loadings of cranial, bony labyrinth and endocranial shape in the investigated horses (PC1-2). Cranial morphospace with raw (= form) (A), log shape ratios (B) and allometry-free shape (C) data. Bony labyrinth morphospace with raw (= form) (D), log shape ratios (E) and allometry-free shape (F) data. Braincase endocast morphospace with raw (= form) (G), log shape ratios (H) and allometry-free shape data (I). Abbreviations for each variable are available in Table 2

Fig. 5

Multivariate regressions of the PC1 scores from the log shape ratios PCA with log(geometric mean) for the cranium (A), the bony labyrinth (B), and the brain endocast (C). Gray spectrum indicates a 95% confidence interval. Illustrated individuals are TU_M33986 and V_000E931_H_piz_C_3_6 for (A) and NMBE_1088739 and OU_1 for (B, C). Color legend refers to Fig. 4