Animal welfare: should we change housing to better accommodate the animal or change the animal to accommodate the housing?

Abstract Animal welfare (well-being) can be generally defined as 'a state of harmony between the animal and its environment, characterized by optimal physical, behavioural and psychological functioning and high quality of the animal's life'. Since its inception in the early nineteenth century, through further development during the twentieth century, and continuing through today, the animal welfare/wellbeing movement has had a significant impact globally on monitoring and regulating the practices of the modern livestock production industries. Before the animal welfare movement brings animal agriculture to the next stage, it is necessary to understand the finely tuned balance between animals, especially domestic farm animals, and their environments. It has been known for centuries that some species, strains and/or individuals of animals adapt to their environments better than others. The outcome of the adaptive process, i.e. increased or decreased in terms of welfare, is dependent on the animals' biological characteristics (genes), environmental factors and genetic- environmental interactions. The animal-or-environment dilemma associated with animal welfare has also raised multiple semantic variants among human beings according to a person's education, ethical viewpoint, socio-economic conditions, culture, religion and political beliefs. This review describes the current status of hypotheses and research in the area of the effects of genes, environments and genetic-environmental interactions on animal welfare in the modern livestock production industry.