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Feeding Horses: Back to Evolution

Rebecca L. Remillard

KEY TERMS

• Extinct refers to a species no longer in existence.
• Extant refers to a species still in existence; surviving today.
• Ungulates are even-toed or odd-toed hoofed mammals.
• The perissodactyls (odd-toed ungulates) extant examples are horses, rhinos, and tapirs.
• The artiodactyls (even-toed ungulates) extant examples are pigs, hippos, camels, deer, giraffes, antelope, cattle, and sheep.
• A “hand” is a non-international standard unit of measurement equal to 4 in. commonly used to measure the height of horses from ground level to the top of withers in many English-speaking countries.

KEY POINTS

• A review of equine evolution is essential to understanding their behavioral, nutritional, and dietary requirements.
• Horses evolved over the past 50 million years in response to environmental changes from dense humid forests to open arid grasslands.
• Extant horses adapted a form of cecal fermentation.
• The genus likely originated in North America and migrated to Old World and South America. The North American ancestors became extinct in the last American ice age but were re‐introduced in the sixteenth century AD by European settlers.
• All breeds of Equus ferus caballus are derived from a 13–14 hand, gregarious, steppe‐dwelling animal that thrived on high‐fiber/low‐protein/low‐starch forages.

1.1 Introduction

Charles R. Darwin produced one of the first illustrations of an evolutionary “tree” in his seminal book The Origin of Species [1]. An evolutionary (phylogeny or phylogenetic) tree is a branching diagram outlining evolutionary relationships. Over 150 years later, evolutionary biologists still use tree diagrams to depict evolution because such figures effectively convey the concept that speciation occurs through adaptation and splitting of lineages. The tree is a visual representation of the relationship between different organisms, showing the path through evolutionary time from a common ancestor to different descendents. Important to note is that many lineages, or branches of the tree, are not successful when the environment changes and therefore become extinct. Ideally, each true species should have a name different from every other species. Specie classification has become more dynamic and may be automatically generated based on completely sequenced genomes [2]. The scientific classification of Equus, the horse, is outlined in Table 1.1 and Figure 1.1.

Within the family Equidae, Equus is the only recognized living genus with seven living species (Figure 1.1). All domestic horses (miniatures to draft size) are the same subspecies, Equus ferus caballus, including the feral horses of Australia, the Western United States, and Canada [4]. A true wild subspecies, Equus ferus przewalskii, is native to the steppes of Central Asia. Other extant members of the genus are wild asses and zebras. The domesticated donkey is Equus africanus, subspecie asinus. The other wild asses, kiang, onager and kulan, and zebras have not been domesticated. Equine species can crossbreed with each other. The most common hybrid is the mule, which is a cross between a male donkey and a female horse. A related hybrid, a hinny, is a cross between a male horse and a female donkey. Most hybrids are sterile and cannot reproduce. Other hybrids include the zorse, a cross between a zebra and a horse, and a zonkey or zedonk, a hybrid of a zebra and a donkey [5].

Table 1.1 Taxonomic classification of equids.

Figure 1.1 Cladogram of genus Equus speciation.

Source: Based on Vilstrup et al. [3]. a Extinct in 1909, b Critically Endangered, c Extinct in 1883.

1.2 The Evolution of Equus

1.2.1 Environmental Changes

It is generally accepted that 66 million years ago (mya), an asteroid hit the earth and changed the atmosphere and vegetation. This ecologically devastating event was responsible for the loss of half of the species of plants and insects on the North American (NA) continent at that time. The subsequent colossal loss of vegetation was responsible for the eventual demise of the dinosaurs. However, small rodent‐like mammals survived in part due to their small body size, living underground, and high reproductive rates. In the absence of dinosaurs, mammals thrived, evolved, and diversified to occupy land, sea, and sky in the millions of years following the asteroid. Around 56 mya, a massive volcanic rift opened in the north Atlantic sea increasing atmospheric greenhouse gases (CO2, CH4) which resulted in global warming. During this warming period, the NA continent became a lush, dense, rainforest reaching as far north as Alaska providing mammals with ample food and shelter for millions of years.

However as the volcanic rift cooled over 20–30 millions years, global temperatures decreased and the NA environment changed again from a tropical to the temperate climate we know today [6]. For the next several million years, still in the absence of human beings, mammals adapted to this temperate climate and again prospered. It was during this time when NA was changing from rainforest to grassland that Equus evolved from a small fox‐size forest‐dwelling creature to the large, odd‐toed hoofed, prairie‐dwelling herbivore we know today. The evolution of the horse during this time has been described primarily based on changes occurring to their musculoskeletal and alimentary systems [7].

1.2.2 Musculoskeletal System Changes in Response to Predation on the Open Plains

The evolution of the horse began some 50 mya. See examples at specific points in time in Table 1.2. The common ancestor of the domesticated horse was Hyracotherium (Eohippus or the “Dawn horse”). Hyracotherium was a small mammal about 1 ft tall weighing about 15 kg, the size of a small dog, with large upper and lower canine teeth, four toes on the front limbs, and three on the hind limbs, appropriate for walking in the swamp‐like rainforests of NA. See Sidebar 1.1. Eocene ancestors, Orohippus and Mesohippus, were larger and adapted to woodlands and meadow‐like environments. Miohippus, Epihippus, and Parahippus (20–35 mya) were adapting to open grasslands as the forest and woodlands died off, and were, therefore, more vulnerable to predators. Anatomical adaptations for running became a matter of survival. The size of Miocene ancestors approached that of current day horses with an enlarged middle toe but two smaller toes on each foot. See Sidebar 1.2.

Table 1.2 A simplified version of Equus evolution in North America.