February 15, 2011

Cows, Horses, and Dogs: Domesticated

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Domestication as a form of evolution was a primary theme of panel discussions during the week of events known as Darwin Days 2011. On Wednesday, February 8, attendees pondered the evolutionary fates of the cow, the horse, and the dog during the discussion “Evolution of Barnyard Animals,” which was presented by three specialists from the College of Veterinary Medicine and led by Prof. John Hermanson, biomedical sciences. The dairy cows of modern dairy farms evolved from primordial oxen, like yaks, that gave only paltry amounts of milk. Compared to their ancestors, today’s cows have been bred, selected and managed to be milk-making machines, producing continuously higher volumes of milk per cow since the 1940s. Prof. Daryl Nydam, population medicine and diagnostic sciences, argued that the higher efficiency of dairy cows today –– measured as the energy needed to make a gallon of milk –– leads to a more efficient use of resources. All cows need about the same amount of energy for maintenance, but they need increasing amounts of energy to make milk. When individual cows produce more gallons of milk, the maintenance energy they need is divided over every gallon they make, called “the dilution effect.” Nydam explained, every gallon of milk requires less energy overall. More efficient use of energy means less environmental impact, Nydam said, citing a controversial study published by Cornell Animal Science Professor Dale Bauman’s group in 2009 that looked at the total energy inputs needed to produce milk. “The carbon footprint of the average U.S. dairy cow has indeed doubled,” Nydam said. “The carbon footprint of a gallon of milk has been reduced by two-thirds … Productivity is the reason for that.” The horse, on the other hand, has been domesticated for performance. “Horses are very good at running fast in straight lines,” equine surgeon Jonathan Cheetham, clinical sciences, explained.  He detailed the impressive physiology of the horse during the discussion. Horses outshine human performance athletes like marathon runners in one important aspect: their respiratory changes during peak exercise. The rate of breaths is linked to racehorse’s stride. They take a breath with every step, unlike humans, whose respiratory frequency goes up as they exercise. As a result, horses have to breath in 18 L of air in a quarter of a second when running at full speed, through a long, delicate respiratory system about 1.5 meters long. Cheetham explained the consequence is that “a very small dysfunction in the upper airway of a modern horse leads to a very large change in performance.”Horses’ legs are also adapted to make them super athletes. Their muscles are concentrated on the top of the leg, while the bottom part of the limb is mostly bone and tendons, resulting in a long lever arm that can generate more force at the bottom of the leg. Horse’s front legs function as springs, as the tendons on the bottom and the muscles on the top absorb forces greater than 9 g’s of deceleration as the horse lands, and then push the leg back up. “People have used the analogy that horses are on a pogo stick on their front legs, and that they’re being propelled by their hind legs,” Cheetham said. Another such relationship is that of dogs and humans, which began a very long time ago. Bones of dogs found in archeological sites in Russia indicate dogs, as a new species, were associating with humans up to 17,000 years ago, explained Prof. Gregory Acland, medical genetics, during the final presentation of the discussion. Acland traced the trajectory of dogs’ evolution and argued that they coevolved with humans. Dogs evolved from small Southern wolves from Southeast Asia, India, and China, not from the large gray Northern wolves most people imagine. Wolves coexisted with other species of human ancestors like Homo neanderthalensis and Homo erectus for thousands of years, even a quarter of a billion years, but no evidence suggests these species contributed to the domestication of dogs. Homo sapiens like us, however, caused an almost instantaneous divergence. According to Acland, based on available archeological data, about 42,000 years ago, “modern humans walked out of Africa, met wolves, and the process of divergence took place instantly, in evolutionary terms.”One of the most important reasons that allowed dogs to evolve from wolves once they met H. sapiens, Acland claimed, is the behavioral commonalities between wolves and modern humans. Modern humans had similar running-based hunting strategies like wolves, unlike Neanderthals.  Wolves, too, understand subtle eye gazes humans use to communicate. The white foreground of human eyes allowed wolves to follow them much better than they could follow the eyes of other human ancestors, posited Acland. “The emergence of gracile, hairless, white-eyed long legged wandering humans from Africa provided an ecological niche for which a subset of wolves began to evolve into what we now know as dogs, and that happened extremely rapidly, ” Acland concluded.

Original Author: Daina Ringus