Killing behaviors in rodents: from predation to infanticide

Killing is the most extreme behavior that an animal can perform toward another. Rodents are one of the best examples of evolutionary adaptation, being the mammals with the highest number of species and having developed an impressive variety of forms and behaviors, which allow them to live and thrive in the most disparate habitats across all continents except for Antarctica. Moreover, rodents are the most widely employed animal model in biological research. An appropriate knowledge of the ethology of rodents is of particular importance both for basic science and for the employment of these animals as animal models. Since they are common targets of both terrestrial and aerial predators, rodents are frequently viewed merely as prey animals. Nevertheless, it is less known that rodents can also be proficient killers. In fact, most rodents are opportunistic predators. Additionally, rodents perform several non-predatory killing behaviors. However, due to historical reasons (according to the early traditional views, rodents were considered small, exclusively herbivorous animals lacking killing instincts), the killing behaviors of rodents have long been overlooked. In this article, we present an overview of rodent killing behaviors, considering a wide variety of species and genera. Killing behaviors are classified into four major categories: predation (upon invertebrates, fish, amphibians, reptiles, birds and mammals), antipredatory killing, social competition-induced killing (intraspecific and interspecific), and infanticide (non-parental and parental). Notably, killing behaviors are present in the majority of rodent species and can be found in all five rodent suborders (Myomorpha, Sciuromorpha, Hystricomorpha, Castorimorpha, and Anomaluromorpha). The present work is the first in which these diverse types of rodent killing behaviors are described together. This not only allows a synoptical view of killing behaviors, but can also aid in identifying the boundaries between the different categories, making the classification of the specific killing behaviors easier. In the final section, we discuss ethical issues related to research on rodent killing behaviors, and we propose methodological options for addressing such issues in future investigations of these behaviors.