This emphasis on natural selection and individual fitness was accompanied by new research methods. Behavioral ecologists, including Tinbergen later in his career studied large numbers of animals in natural populations. In many cases they marked or tagged individuals. This method allowed the scientists to study variation in behavior among individuals and quantify its consequences for survival and reproductive success. This was quite different from Tinbergen's early study of sticklebacks, which involved observing only a small number of fish living in aquaria.