Project Purpose

　Canis lupus familiaris is the oldest domesticated animal, having lived with humans for about 40,000 – 50,000 years. In the course of their cohabitation, humans and dogs have developed a special relationship, with dogs being found widely throughout human society as the animal most familiar to us. To date, there have been innumerable reports of the physical and mental benefits to humans of living with dogs. For example, living with dogs has been reported to be beneficial in cases of depression in war veterans and others, the care of children with autism, pain control, and digestive disorders. Thus, it has been shown epidemiologically that many health issues faced in modern human society can be ameliorated by living with dogs, and there is no cause to doubt its effectiveness—indeed, in Europe and North America, there are currently health insurance premium discounts for those owning a dog.

　Elucidating the biological mechanism underlying these epidemiological effects is not merely useful for understanding the coevolution of humans and dogs. Due to the aging population and decline in the birthrate in Japan, pet ownership has increased such that there are currently more pet cats and dogs than 15-year-olds. In the current situation, clarifying the symbiotic mechanism involved in living with beneficial animals is an urgent issue that will promote human physical and mental health, help reduce soaring medical costs, and ameliorate adverse effects of developmental disabilities and the increase in nuclear families. The neuropeptide oxytocin is considered a strong candidate for the beneficial effects of animal cohabitation on humans. Oxytocin has been found to relieve anxiety, depression, and stress response and to suppress excess tension. It is also known to have analgesic effects, while also functioning to improve symptoms in children with autism, inhibit obesity, improve cardiovascular disorder, and stabilize the digestive system. Using eye gaze between humans and dogs, our university demonstrated a positive feedback loop for oxytocin and affinity behaviors leading to bond formation and showed that this function of oxytocin was acquired during the process of coevolution of humans and dogs (Science, 2015). In light of these findings, it is thought that activation of the human oxytocin system is involved in the benefits to mental and physical health obtained through cohabitation with dogs. One other potential signal is the effect on bacterial flora brought about by dogs. A resident bacterial flora is indispensable for the maintenance of vital functions and immune system acquisition in mammals (hosts), including humans. This mutualistic symbiotic relationship developed through a lengthy evolutionary process. As a result, research is showing that changes in bacterial flora can greatly change the physical functioning of humans and animals. The discovery of microorganisms allowed an unprecedented development of the microbiology of pathogens and the establishment of methods for controlling pathogenic microorganisms. On the other hand, as it has long been a challenge to grasp the entirety of bacterial flora due to its diversity, the symbiotic mechanisms between bacterial flora and their host animals and their correlations with health and disease have been unexplored. In recent years, metagenome analysis and comprehensive bacterial flora analysis have become possible (Ley et al., Science, 2008). With human resident bacterial flora analysis as a forerunner, researchers, including those at our university, have revealed that various bacterial flora are related to certain diseases and resistances (Nature 2011, Nature 2013). One particularly famous effect of various microbial flora on the mind and body, known as the “hygiene hypothesis,” is the alleviation of allergic reactions. Studies of the involvement of microbial flora in the mental and physical health of humans have demonstrated, for example, that intestinal flora are one reason pet ownership actually reduces the incidence of childhood asthma and improves such conditions as obesity and hypertension. In other words, the advantages of living with dogs may include shared microbial flora in addition to social exchange through eye gaze, etc., developed through a history of symbiosis.

　Thus, in this research we will apply molecular biology and behavioral science in an effort to elucidate the mechanisms of the relationship between humans and dogs, representing animals, which has previously been discussed anecdotally, from the perspective of human health in order to develop a united science of human-animal symbiosis and contribute to the realization of a healthy human society.