Horse cognition and behavior from the perspective of primatology
This article aims to show recent, novel approaches to the study of horses. It also describes why readers of the journal Primates would do well to start considering horses in relation to the study of human and nonhuman primates. In our recent research, my colleagues and I have advanced the study of horse cognition using a computer-controlled touch panel (Tomonaga et al. 2015) and also the behavioral study of wild horses, in the Serra D’Arga in Portugal (Ringhofer et al. 2017). A parallel approach using both laboratory work and fieldwork is intended to provide a better understanding of horses from a holistic perspective.
The primary motivation to study horses comes from the idea of understanding humans not just from a primate perspective, but from a broader mammalian perspective. In the classification of organisms, Primates is the order that includes Homo sapiens. Research has often been driven by questions about human evolution, such as where did we come from? How did we come to be as we are today? This is often the grand design or main unifying framework in the study of nonhuman primates. However, less attention has been given to the question of primate evolution in relation to other mammals. How did nonhuman primates come to be as they are? How did nonhuman primates evolve? We have to think more about primate evolution in relation to other mammals, not solely focus on human evolution relative to other primates. This is the key issue. It is also the main reason why I wrote the article “From four hands to two feet” (Matsuzawa 2016), to describe human evolution from a broader, mammalian perspective.
Mammal species number about 4000–6000, depending on the method of classification used. The common mammalian ancestor existed in the Mesozoic, 252–66 million years BC. This ancestor is believed to have been a small, nocturnal, terrestrial creature, similar in form to present-day rats and mice. The important point is that this common ancestor was a four-legged animal that walked on the ground.
Let us examine the history of planet Earth. A huge shift in climate took place on a global level 66 million years ago. A mass extinction occurred, and by the Mesozoic, dinosaurs were extinct. After the mass extinction, the common mammalian ancestor diverged into different environmental niches, adapted to a new environment and, thus, its morphology changed.
Let us consider bats, which fly through the air. Two of their four legs evolved into something similar to birds’ wings. Not many people are aware that almost one quarter of all mammalian species are bats. Bats have been highly successful in terms of speciation, hence the high number of different bat species. This may be partly due to the fact that bats are the only mammal that can move through the air by powered flight. In this sense, bats monopolize a huge niche; they have no competing mammalian species except for other species of bat.
Let us consider whales and dolphins, which swim through water. Here, four legs evolved into something similar to the fins of fishes. Dolphins, whales and other aquatic mammals have been incredibly successful in the water, exploiting an unusual habitat and niche for mammals. Incidentally, not many people recognize that whale and dolphin species are the most successful species on this planet, by one measure, collectively occupying a larger surface area than other species of mammal, including humans. One might counter that there are more than 7 billion people crowded onto the planet, but it is important to remember that humans inhabit land. There are no humans living in the sea. The surface area of planet Earth is about 510,065,600 km2, of which land occupies about 147,244,000 km2 (28.9%) and sea 362,822,000 km2 (71.1%). The ratio of land to sea can fluctuate slightly, but is ca. 29:71. Species of whales and dolphins are found everywhere in the oceans including the Arctic and Antarctic. Therefore, they occupy a larger surface area of the planet than humans.
Let us consider nonhuman primates, which evolved to live in the trees—an arboreal life! That is the crucial point. Not in the air, not in the water, not on the ground, but in the trees. We need to recognize different modes of life of nonhuman primates in different habitats and niches: avian, aquatic, and arboreal, as well as terrestrial. The trees provide an exceptional niche for primates; orangutans, the largest of all the arboreal animals, spend the majority of their lives in trees. Many other mammalian species continue to live on the ground, such as species of horse, wolf, lion, elephant, etc. During the process of speciation from the common mammalian ancestor, many species remained terrestrial, but others flew into the air, dove into the water or climbed into the trees. This is important to keep in mind when seeking to understand the origin of primates that live in trees. We can conclude that the four feet of primates turned into four hands in the course of primate evolution, which enabled efficient grasping of tree trunks and branches.
Primates are the mammals that favored living in the trees. They are characterized by four hands pitted against the four feet of mammals that continued to live on the ground. Having four hands was an adaptation to the new niche in the trees. During the course of human evolution, our ancestors started to spend more time down on the ground. Thus, we evolved two feet from two of our ancestor’s four hands: the evolutionary scenario of humans evolving from four-handed animals. This is in contrast to the popular understanding of “four feet to two hands”, i.e., the idea that four-legged animals instead started to stand up over the course of human evolution during which hands were created that are unique to humans. This is definitely not the case! We humans have hands because we are also primates, previously adapted to an arboreal life. Humans evolved two feet as an adaption to allow them to run and walk, and for stamina over long distances. This is the story depicted by the four hands to two feet hypothesis (Matsuzawa 2016). The key point is the divergence from the common ancestors of all mammals, the original form of which was an adaptation to the terrestrial environment. Then terrestrial life diverged to include mammals adapted to avian, aquatic and arboreal life.
Having studied nonhuman primates for the past four decades, I wanted to find out more about the descendants of the earlier types of four-legged animals that have remained terrestrial. An understanding of four-legged animals may provide us with a better understanding of primate evolution in the context of all other mammals. I selected the horse as the model animal.
I joined the field survey in May 2017. Ringhofer et al. (2017) report that there are currently 208 individuals in 26 groups at the site, and each individual horse has been identified by the research team. The follow-up study this year clearly showed two major factors. First, a high mortality rate for foals aged less than 1 year, some of which are presumably eaten by wolves. Indeed, I observed a foal with characteristic wounds inflicted by attacking wolves. Second, a high frequency of female emigration from one group of garrano to another. The stability of groups of horses may be different to that of nonhuman primates, and continued long-term fieldwork on horses will elucidate the evolution of their social life in comparison with that of nonhuman primates.
The opportunity to examine the life of wild horses parallels that of nonhuman primates in their natural habitats. So far, it has proved very difficult to understand inter-community encounters and relationships in nonhuman primates. However, in the case of the horse, inter–community interactions can be measured and evaluated easily, thanks to the use of drones. Using this novel technique, the spatial distribution of every individual in each group of horses is easy to observe and record. Group decision-making for movement can also be quantified, based on precise data. Thus, the study of horses is likely to complement the study of nonhuman primates in some aspects of behavioral and social studies. The study of horses may open up new horizons in understanding the evolution of the human mind and society, from the perspective of mammalian evolution.
This article is based on my three personal field expeditions to Portugal, from 2015 to 2017. The research described was financially supported by MEXT 16H06283, JSPS Core-to-Core CCSN, and the JSPS Leading Graduate Program U04 (PWS) (to T. M.). For the laboratory work, I would like to express my appreciation to Masaki Tomonaga, Kiyonori Kumazaki, and Carlos Perreira for their collaboration. For the fieldwork, thanks are due to Carlos Pereira, Renata Mendonça, Satoshi Hirata, Shinya Yamamoto, Monamie Ringhofer and Sota Inoue. I am also grateful for the kind assistance provided by the local authority and community of Serra D’Arga in Portugal.