The Forward-Looking Goat

Eyes are one of the most important sensory organs animals use to perceive their surroundings. Across the animal kingdom, eye placement and structure vary depending on ecological needs. Among mammals, herbivorous species typically place their eyes on the sides of the head. This lateral positioning expands the field of view, allowing animals such as cattle, goats, and horses to monitor a wide area for approaching predators.

Przewalski's horse, showing the eyes positioned on the sides of the head（圖片來源：Claudia Feh，採用 CC BY-SA 4.0 授權。）

Carnivores, by contrast, usually have forward-facing eyes. Because the visual fields of the two eyes overlap extensively, each eye receives a slightly different image due to the distance between them. When the brain integrates these two images, it generates depth perception, producing the three-dimensional visual experience known as binocular vision. This visual system enables predators to judge distances accurately when pursuing prey.

Based on this general pattern, hoofed herbivores are often assumed to lack extensive binocular vision. However, the fossil record reveals an intriguing exception. A goat-like animal once existed whose eyes faced forward and possessed binocular vision. This species was the Balearic cave goat, Myotragus balearicus.

Skeleton of Myotragus balearicus（圖片來源：Joan Gené ，採用 CC BY-SA 4.0 授權。）

Myotragus balearicus lived on the Balearic Islands in the western Mediterranean Sea south of Spain. It represents the youngest known species of the genus Myotragus. To date, six species of this genus have been identified. These species form a chronological sequence, indicating that the lineage evolved continuously through time as a single evolutionary branch. Each species replaced its predecessor, so that only one species of Myotragus existed at any given period.

The origin of the genus is thought to date back to more than five million years ago during the Messinian Salinity Crisis. At that time the Strait of Gibraltar closed, cutting off the Mediterranean Sea from the Atlantic Ocean. As the Mediterranean basin partially dried, land connections appeared between regions that had previously been separated by water. The ancestors of Myotragus likely dispersed from mainland Europe to the Balearic Islands during this interval. When the Mediterranean later refilled with seawater, these animals became isolated on the islands and began their own evolutionary trajectory.

Skull of Myotragus balearicus with forward-facing eye sockets（圖片來源：Olaf Tausch，採用 CC BY 3.0 授權。）

Over long periods of evolution, the morphology of Myotragus changed dramatically. Early members of the lineage resembled typical caprine mammals, but later forms gradually evolved a smaller body size, shorter limbs, a broader abdominal cavity, fewer teeth, a shortened snout, and increasingly forward-facing eye sockets.

These unusual changes are widely interpreted as the result of the unique ecological conditions of island environments. On the Balearic Islands there were few other herbivorous competitors, which intensified competition within Myotragus populations. At the same time, the absence of predators allowed the population to grow without strong external regulation. Because island ecosystems have limited resources and relatively low ecological carrying capacity, population expansions could quickly exhaust available food supplies. When this happened, famine would follow, leaving only a small fraction of the population to survive. Such repeated population crashes likely imposed strong selective pressures over evolutionary time.

Body size comparison between Myotragus balearicus and an adult human（圖片來源：Hemiauchenia，採用 CC BY 4.0 授權。）

When the ecological context of the islands is considered, the anatomical changes of Myotragus become easier to understand. The reduction in tooth number and the enlargement of the abdominal cavity likely improved the animal's ability to process the island's vegetation, including toxic plants such as boxwood (Buxus). The shortened limbs and altered skeletal structure reduced its ability to run or leap quickly, but they also lowered energy expenditure. In an environment without predators, high locomotor performance was unnecessary.

The development of forward-facing eyes and binocular vision may also have been advantageous in this environment. Enhanced depth perception would have helped these animals move across the rugged terrain of the islands. Since there were no predators to detect, maintaining a wide panoramic field of view was no longer necessary.

Reconstruction of Myotragus balearicus, showing its forward-facing eyes rather than lateral placement（圖片來源：Xavier Vázquez，圖片為公有領域。 ）

After millions of years of evolutionary change, Myotragus balearicus appeared during the Late Pleistocene as a species highly specialized for life on these Mediterranean islands. Under natural conditions such an organism might have continued to persist there indefinitely.

Yet around five thousand years ago, humans arrived on the islands. Archaeological evidence suggests that Myotragus balearicus disappeared extremely rapidly after human settlement, possibly within only a century. At the same time, the only other native mammals of the islands—the shrew Nesiotites hidalgo and the giant dormouse Hypnomys—also became extinct.

From that moment onward, the world no longer had a forward-looking goat.

Author: Bai Leng

References:

1. Köhler M., Moyà-Solà S. (2004). Reduction of Brain and Sense Organs in the Fossil Insular Bovid Myotragus. Brain, Behavior and Evolution.

2. Mas, G., Maillard, A., Alcover, J. A., Fornós, J. J., Bover, P., Torres-Roig, E. (2018). Terrestrial colonization of the Balearic Islands: New evidence for the Mediterranean sea-level drawdown during the Messinian Salinity Crisis. Geology.

3. Köhler, M., Moyà-Solà, S. (2001). Phalangeal adaptations in the fossil insular goat Myotragus. Journal of Vertebrate Paleontology.

4. Alcover et al. (1999). The diet of Myotragus balearicus Bate 1909 (Artiodactyla: Caprinae), an extinct bovid from the Balearic Islands: evidence from coprolites. Biological Journal of the Linnean Society.

5. Bover, P., Valenzuela, A., Torres, E., Cooper, A., Pons, J., Alcover, J. A. (2016). Closing the gap: New data on the last documented Myotragus and the first human evidence on Mallorca (Balearic Islands, Western Mediterranean Sea). The Holocene.

6. Valenzuela, A., Torres-Roig, E., Zoboli, D., Pillola, G. L., Alcover, J. A. (2021). Asynchronous ecological upheavals on the Western Mediterranean islands: New insights on the extinction of their autochthonous small mammals. The Holocene.