The parent of economic analysis of law is Jeremy Bentham, and the theoretical foundations of economic analysis can be found in utilitarianism (originally elaborarated by Bentham). Economists like to measure utility in terms of "willingness to pay," which (to be sure) does not perfectly capture utility. But if people are willing to pay a lot for something, we can usually assume that they really want it, and if they are willing to pay little, they probably do not want it much (unless they are poor). In short, economists are best seen as applied utilitarians, trying to operationalize utility, which is ultimately their master concept. The idea of "efficiency" is an administrable approach to utility.
Here is the puzzle: Why have so few economists, and economic analysts of law, shown even a little interest in animal welfare? Bentham, sometimes described as the first economic analyst of law, was a passionate champion of animal welfare, analogizing the current treatment of animals to slavery and arguing that the question is not whether animals can talk, but whether they can suffer. It is true that animals are not willing to pay much to promote their welfare, because they lack money; but children and poor people also lack money, and economic analysts of law have a lot to say about children and poor people.
From private conversations, I know that many economically inclined lawyers (whatever their politics) are quite interested in animal welfare, and (like Bentham) in the reduction of animal suffering. It's clear that for those who care about utility, animal welfare is an important topic. But there's almost no economic literature here, on the positive or normative sides. Why is this?