Why are domesticated animals small in size

New thesis on domestication

Humans and pets have undergone similar physical changes in their development. In science, however, these are attributed to different processes. An anthropologist from New Zealand is now looking for explanations that apply equally to all affected species.

Much has been written about the development of man on the one hand and the domestication of animals on the other. The assumption that both processes might have something to do with each other was also the subject of research at the beginning of the 20th century. After the racial ideological derailments under National Socialism, apart from the dubious considerations of Konrad Lorenz on the “disgrace” of man, the subject of “man and domestication” has become silent. The anthropologist Helen Leach from Otago University in Dunedin, New Zealand, has now addressed the subject in an essay in the renowned magazine "Current Anthropology" and offers new approaches.

General criteria

Basically, domestication is the process that turns wild plants and animals into useful crops and domestic animals. Domesticated animals still look similar to their wild counterparts, but have changed their appearance and behavior in the course of their breeding. All domestic animals show a similar physical transformation, which can be traced back to an interplay of genetic factors and influences from the environment - to which the animal reacts within a framework defined by its genetic makeup. The respective contribution of these two factors to the individual domestication symptoms, which can be found in different combinations in all domestic animals, is still unclear, according to Leach.

These domestication characteristics include, for example, the bone features, on the basis of which archaeozoologists determine whether finds come from early domestic animals or from wild animals - including a smaller body size, a more delicate bone structure, the shortening of the facial skull and tooth anomalies such as smaller or fewer teeth. Such morphological modifications to the skeleton could, however, also be found in humans from the late Pleistocene thousands of years ago, writes Leach. However, research would not interpret them as the consequences of a domestication process in the current sense of the word.

For Leach, the research here is satisfied with an unsatisfactory solution to the embarrassment, which has resulted mainly from the fact that two different subjects, archeozoology and paleoanthropology, are involved. She is now trying to bring the two strands together and offers new interpretative approaches. In a first step, she describes and compares the most important skeletal modifications of humans and pets in detail. In the following, however, only the reduction in body size will be discussed as an example. Archaeozoologists attribute it to a wide variety of factors. For example, it is said to have come about through the conscious selection of smaller, more manageable animals with lower feed requirements for breeding, or it is explained by the genetic isolation of a small group of animals. Indirect consequences of captivity are also cited, such as a changed or poor diet of pregnant or growing animals as well as a lack of exercise. Turning off natural selection could also have played a role. Natural enemies are lacking in the household. The position of a male animal in the hierarchy - higher-ranking animals are often particularly large and powerful - was possibly less important for reproductive success in captivity.

For humans, according to Leach, almost all of these factors can also be taken into account. According to the scientist, paleoanthropologists associate the downsizing of humans with the development of agriculture, which began at the transition from the Mesolithic to the Neolithic. Arable farming in its beginnings, according to the thesis, only allowed very irregular harvests, humans were fundamentally malnourished, susceptible to disease and reacted to this stressful situation with a reduction in body size. Other researchers also bring climate changes and too small reproductive communities into play as stress factors.

The same applies to the sparrow and mouse

So while some of the driving forces for humans and animals are cited equally, castration or deliberate breeding selection are only possible for animals. Conversely, people like to argue with increasing cultural-technical abilities, especially when it comes to changes in the skull, writes Leach. The scientist criticizes that to this day there has hardly been any attempt to interpret the parallels in the development of pets and humans in a satisfactory way. In the search for generally applicable criteria, Leach therefore includes the house sparrow (Passer domesticus) and the house mouse (Mus musculus), which live in close association with humans. The spread of the house mouse, for example, follows the spread of the sedentary lifestyle as early as the end of the Paleolithic and not just the later arable cultures. However, the mouse and sparrow are not domesticated; they are neither useful to humans, nor has they ever tried to promote or control their development.

Nevertheless, according to Leach, the two species have undergone morphological changes in their development that are considered typical domestication criteria, such as a reduction in body size. The researcher therefore asks which factors that are common to all three groups - humans, domestic animals and commensals - could have caused these modifications. In their opinion, the artificial housing structures and the hoarded food of humans were the decisive factors in the mice. According to the scientist, the same parameters could have been decisive for the development of the human species and later for that of domestic animals.

Leach therefore defines “domestication” in a new, very broad sense as an unconscious adaptation to life in a built environment. She does not only mean buildings, but also gardens and fields, i.e. the entire artificial habitat that has been changed by humans and that offers humans and animals protection from the environment. She positions the beginning of this remodeling of the space earlier than the oldest traces of agriculture in the fertile crescent of the Near East, which are set around 10,000 BC.

Leach's hypotheses do not meet with approval everywhere, as the statements published in the same issue of the specialist journal make clear. The archaeobiologist Melinda Zeder of the Smithsonian Institute in Washington, for example, accuses Leach of serious deficiencies in the evidence and rules out any connection between sedentariness and the domestication of animals. With regard to the majority of domestic animals, for example, there is simply no evidence that they have been switched from their original diet to a changed, “softer” diet in the household - one reason to which the weaker teeth of modern humans are attributed . This explanation is therefore out of the question for the tooth and skull changes in most domestic animals. In their view, domestication and its consequences came about not only through unconscious selection pressure, but at least in part also through conscious intention of humans - namely the will to create a secure and more predictable livelihood for oneself.

Geneviève Lüscher

Source: Current Anthropology 44, 349-368 (2003).