Chasing Amy

Dogs are classified under Carnivora, a diverse order that includes more than 280 species of placental mammals, including the suborder Caniforma, which includes canids, and the suborder Feliformia, which includes felines. Unlike cats, however, who are obligate carnivores and primarily (but not exclusively – a large percentage of cats can’t seem to get enough catnip and many of them enjoy “grazing” on grass or sampling the occasional carbohydrate-rich indulgence) eat meat, dogs are omnivores who can thrive on a wide range of diets. Domesticated dogs often eat a combination of meat protein, grains, and vegetables. Having co-evolved with humans for so long, dogs’ gastrointestinal systems differ from their wolf cousins: dogs have gene adaptations for starch digestion, while wolves do not. A study that analyzed the DNA of the remains of 13 ancient dogs tracked a gene involved with starch digestion. The remains, which originated from Asia and Europe and dated as far back as 15,000 years, revealed that between 4,000 and 7,000 years ago, dogs began to have more copies of the Amy2B gene. Wolves have two copies of this gene, while most dogs (excepting Huskies, which evolved among less agrarian societies and exhibit more wolf-like starch digestion capabilities) have between four and 30 copies of it. The Amy2B gene creates amylase, which breaks starch down into sugar. So why are we making our dogs eat grain? Early on during domestication, dogs were initially living with hunters and gatherers. As human civilization gradually moved toward more agricultural, sedentary lifestyles, meat became a smaller percentage of their diet. Farmers were now growing and eating higher percentages of wheat, millet, and corn, and in order to live alongside humans, dogs would need to become accustomed to eating these substances, too. Unlike every other domesticated dog breed, Huskies, like wolves and dingoes, only have two copies of Amy2B. Until very recently, evolutionarily speaking, Huskies have spent most of their time with hunters and fishermen, rather than those eating a more agriculturally-biased diet. Thus, their genetics – and their gastrointestinal fortitude – reflects a different evolutionary trajectory. The authors of the study stated: “This expansion reflects a local adaptation that allowed dogs to thrive on a starch-rich diet, especially within early farming societies, and suggests a bio-cultural co-evolution of dog genes and human culture.”156

Dogs weren’t the only species that experienced changes in their digestive systems with the rise of agriculture; the gut microbes of hunter-gatherers differ significantly from those of agriculturalists, too. A recent study compared the gut microbes of the Hadza of Tanzania, one of the last full-time hunter-gatherer populations, with those of residents from 16 industrialized countries. Microbes are inherited from one’s mother, with no microbes present in utero; the first exposure to microbes is in the birth canal. Microbes co-evolve with us and vary depending upon one’s diet and lifestyle. Thus, the digestive systems of dogs and humans were both changing to reflect their shared divergence from a hunter-gatherer lifestyle to a more sedentary agricultural one.

The rise of agriculture changed the digestive systems of domesticated cats, too. The ancestors of modern domesticated cats – as well as their contemporary wild cousins – eat diets that almost exclusively consist of meat. But during domestication, succeeding generations of cats were born with longer and longer intestines than their wild cousins. These longer intestines aided in the digestion of fibrous material; they were eating more grains, too. While they are obligate carnivores, those cats who chose to share their lives with humans needed to also be able to digest grain materials.

The natural history of a carnivorous or omnivorous species, like Canis familiaris, that has co-evolved with humans for tens of thousands of years, is going to differ from that of an herbivorous one, such as the horse (Equus caballus). Predators and prey experience the world with vastly different umwelts. What is salient to a predator species is not necessarily going to be salient to a prey species. While cats and dogs both share our homes, the dog has done so for three to four times as long – long enough, in evolutionary terms, to exhibit significant behavioral differences from both their feline housemates and from the last common ancestor that they shared with wolves.

And much has happened, evolutionarily speaking, since the ancestors of modern domesticated dogs diverged from the ancestors of modern wolves; so much so that Darwin, and countless other scientists and observers, have marveled at the morphological variation within this species. As man shaped dogs, though, they shaped him, in turn, resulting in increased rates of hunting success and protection from other predators, likely resulting in increased rates of reproductive success. The natural history of the dog, then, is just as much a story about canines as it is about modern humans.

From the onset of their partnership, the ancestors of modern man and modern dogs both benefited. Dogs had more reliable access to resources, including shelter and safety and food; thus, they were expending far fewer calories than their free-living counterparts. Those early dogs which exhibited the right combination of personality traits also had more opportunities to breed. When they hunted together, humans benefited from dogs’ superior sense of smell, while dogs benefited from humans’ sense of sight, including color vision and a higher vantage point. This was the start of a storied mutually beneficial relationship, with both species benefiting from the unique perspective of the others’ umwelt.

All animals, including humans and dogs, view the world from their own unique umwelts. As primates, humans rely heavily on their sense of sight. But to cohabit successfully with humans, dogs learned, more than any other canine species, to maintain eye contact with people, as that was humans’ preferred modality. Our canine companions, however, rely more upon their superior senses of smell and hearing. A dog’s primary senses include olfaction, hearing, vision, taste, touch, and sensitivity to earth’s magnetic field. A dog’s world is chiefly navigated by olfaction, and approximately one-third of their brain is devoted to it. Recall the canine homunculus, comprised of a very large nose and oversized ears. In comparison to wild dogs, the domesticated dog has a high proportion of gene differentiation in the hypothalamus, which links emotional, endocrinological, and autonomic responses to exploratory behavior.

Dogs are not precocious (developing early and being independent at an early age, like many hooved prey animals are) when they are born; they’re altricial, meaning that they are born in an underdeveloped state in comparison to most prey species. Unlike a prey species, like a horse, which can see, hear, and smell at birth and stands within an hour after being born, dogs come into the world deaf and blind and wholly dependent upon their mothers for many weeks. A dog’s brain isn’t fully developed until the age of two; prior to this time, they lack inhibitory control, much like a teenager. Dogs, like humans, have critical and sensitive periods of development. If they have missed out on socialization opportunities during these times of their development, they will struggle to understand their human family members.

Dogs are neurologically wired to detect the emotions of their human family members. During convergent evolution, dogs’ brains have developed many analogous regions to human brains. To a certain extent, dogs experience many of the same emotional states that humans do, enabling them to relate to humans more. Dogs are able to detect changes in tone and emotional valence and they anticipate our reactions over perceived transgressions and achievements.

There are both costs and benefits to the human-canine relationship. Domesticated dogs benefit from the increased reliability of resources, including mate availability, shelter, and food; increased care of dependent offspring and decreased predation pressure. They incur costs from having to share their food with their human family members, as Ziva had to share with Alec, and from having less choice; they have to abide by the “rules” of another species.

Game theory, as it relates to biology and animal behavior, is crucial to understanding the development of the human-canine partnership. It has three main tenets. The first is that costs and benefits are conditional: the costs and benefits of an individual’s social behavior are relative to the costs and benefits of another individual’s social behavior. The second is that animals don’t necessarily process numbers, at least not the way that humans do, so rules of thumb will arise from this. While the specific circumstances for an organism may be far different from the way they were when these behaviors arose, these rules of thumb will be deeply ingrained. The third is that there are iterative and non-iterative games. For example, an iterative game would be a dog visiting the same dog park every day at the same time and playing with familiar dogs each time. The dog would trust that other dogs would play fairly and return their toys as these would be dogs that they would be likely to interact with again. A non-iterative game would occur at a large city park where the dog had never visited before. There would not be a high likelihood of seeing these dogs again; thus, their interactions would vary widely between the iterative and the non-iterative scenarios.

The social structure of a species is based upon the distribution of resources within their environment. The social structure of the domesticated dog is unique from its closest relative, the wolf, as dogs have benefited from the reciprocal relationship with humans for millennia. For early dogs, cohabitating with humans meant easy access to food, shelter, mates, and potential care of dependent offspring. This impacted how they viewed the members of their household; while wolves have a hierarchy with the father wolf at the top, domesticated dogs view their families as a more amorphous grouping.