The universe is a place of vastness. You think a star is inconceivably immense, and well, there are billions of it. You wonder about the magic of consciousness – how matter can turn into feelings, thoughts and self-awareness – and here we go again: it is everywhere around us in its most diverse manifestations. We struggle with such unconcealable realities: we know that a bird has all the same sensorial organs that we do, we see them escaping in panic from a predator or approaching friendly acquaintances in excitement. We see manifestations of joy when a family reunites. Many sing at dawn, and some learn words and dance to the rhythm of music. And yet, because consciousness is something so precious to us, many have a hard time granting it to unknown widls beyond the city-walls of our species. 

Yet despite the reluctance to attribute consciousness to other species, most people find it unacceptable to hurt and mistreat an animal (1-3). And here lies one of the most formidable moral paradoxes of our contemporary societies: we coexist with conditions of animal mistreatment that would not be tolerated if imposed overtly on a single animal, let alone to billions on a daily basis. 

The science of animal welfare is one struggle against this dissonance. Although it is notoriously difficult to access the subjective internal states of non-verbal creatures, such as the pain or discomfort felt by an animal, it dares to explore and expose this reality with the tools of science, and develop solutions to a task that feels insurmountable, but also unavoidable if our goal is to reduce as much suffering as possible in the world.

Here we join efforts in this direction. We hope to contribute (humbled by the efforts of so many past and present giants (4,5)) with a framework whereby the loss of welfare experienced by an individual, or population, can be described quantitatively, in a way that enables its comparison across different living conditions, along with the comparative assessment of those courses of action most likely to improve their well-being. The framework focuses on an area that so far seems to have been neglected: the description of negative affective experiences (which we operationally refer to as pain), as perceived by the individuals affected, based on their temporal evolution, around which intensity and other attributes of the experience and its manifestations can be explored. 

This book is the first “road-test” of this method. The first chapter delineates the methodology and the second chapter defines the scope of the analysis, as applied to the understanding of the impact of reforms for improving the welfare of laying hens – a group of farmed animals as abundant as the global human population, and still predominantly raised in one of the most intensive forms of confinement in the animal farming industry. The following chapters then apply it to a range of conditions that negatively impact the welfare of these animals. Each chapter is a case study of one type of challenge to their welfare. The book closes with a chapter where all evidence is integrated to identify the cumulative load of aversive experiences endured by laying hens in different types of housing conditions, providing a quantitative basis to evaluate the welfare impact of the transition from caged to cage-free housing systems. It is our hope that this effort also serves as a blueprint from which other welfare challenges, species and animal production systems can be investigated. 

This book is also a call for the much needed data to explore the welfare consequences of diseases, injuries, and deprivations at the individual level – where suffering takes place. Very few are the welfare challenges for which information about the duration of symptoms or clinical evolution of the injury or disease is available, with even fewer studies exploring the possible intensity of the experience in each case. For example, despite the extensive literature on injurious pecking and cannibalism in laying hens, there seems to be little data on the time course of a typical cannibalistic attack, or how long victims take to die. Similarly, as far as we are aware, no description is available on the clinical evolution of septicemia in chickens and the sensorial and affective consequences for affected animals, even though it is one of the most frequent causes of carcass condemnation in the poultry industry. By highlighting these research gaps, it is our hope to foster the development of the studies needed to help inform the impact of interventions most likely to reduce animal suffering. 

We hope you find the content of this book stimulating enough to get you engaged in providing feedback, corrections and further information, so we can keep refining and expanding this systematic effort together. 

1. 1. Rollin BE. The Ethical Imperative to Control Pain and Suffering in Farm Animals. In: Benson GJ, Rollin BE, editors. The Well-Being of Farm Animals. Oxford, UK: Blackwell Publishing; 2004. pp. 1–19.
   2. Duncan IJH. The changing concept of animal sentience. Appl Anim Behav Sci. 2006;100: 11–19.
   3. Broom DM. A history of animal welfare science. Acta Biotheor. 2011;59: 121–137.
   4. Duncan IJH. The Philosophical and Biological Evolution of Feelings in Well-being. In: McMillan FD, editor. CAB International; 2020. p. 21.
   5. Fraser D, Duncan IJH, Edwards SA, Grandin T, Gregory NG, Guyonnet V, et al. General Principles for the welfare of animals in production systems: the underlying science and its application. Vet J. 2013;198: 19–27.