The potential consequences of anthropogenic habitat fragmentation on species diversity and extinction have drawn considerable attention in recent decades. In many cases, traditional island biogeography theory has been applied to explain the observed patterns. Here, we propose that habitat fragmentation as a selective force can be traced in mammalian body length changes. By exploring historical sources, we are able to show that the body length of Danish mammals has altered over a period of 175 years, possibly in response to increasing habitat fragmentation. The rate of body length change was generally lowest in medium-sized mammals, and increased with both smaller and larger body mass. Small mammals have generally increased, whereas large mammals have decreased in length. In addition to habitat fragmentation, some species may experience other selective forces, such as traffic, and may be trapped in an evolutionary tug-of-war, where the selective forces pull in opposite directions.