Rats are often viewed simply as vermin that feed on garbage and waste, but their dietary habits are far more complex. The brown rat (Rattus norvegicus) is a successful global colonizer precisely because of its remarkable flexibility in finding and utilizing food sources. This adaptability raises a fundamental question about its true ecological role: does it merely survive on discards, or does it actively hunt? Understanding the rat’s classification requires clarifying the distinction between two major feeding strategies and examining the behaviors the animal demonstrates in the wild.
Defining Predation and Scavenging
Predation is a biological interaction where one organism, the predator, actively hunts, kills, and consumes another live organism, the prey. This behavior involves a sequence of actions, including the search, pursuit, capture, and handling of a live animal. Predation is a direct means of energy transfer within an ecosystem, where the act of killing is necessary for feeding.
Conversely, scavenging is a feeding behavior where an organism consumes the remains of animals that are already dead. This includes feeding on carrion, which are carcasses left by other predators or animals that have died naturally. While many predators opportunistically scavenge, the primary difference lies in the scavenger’s reliance on passive food acquisition rather than the active killing of live prey.
Evidence of Active Predatory Behavior
Rats possess the capacity for active predation that goes beyond consuming dead matter. They target small, vulnerable vertebrates and invertebrates, demonstrating sophisticated hunting strategies. This behavior is most evident in environments where human waste is less abundant, forcing rats to rely on natural food webs.
One frequently documented predatory act is the raiding of bird nests, where rats consume eggs and nestlings. Studies on island ecosystems have shown rats climbing into nests and killing the young of ground-nesting birds. They also actively hunt and kill invertebrates, such as spiders, beetles, and mollusks, often consuming these in significant volumes.
Rats also prey on small mammals and reptiles, particularly those that are young, sick, or injured. The Norway rat has been observed attacking and killing young rabbits and small lizards. In one study, rats were found to be major predators on hatchling and juvenile diamondback terrapins, killing a high percentage of young by evisceration. This requires an active, nocturnal hunting effort, distinguishing it from simple scavenging.
The Scavenger Lifestyle and Extreme Omnivory
Despite their predatory capabilities, the brown rat’s global success is largely built upon its scavenging tendencies and omnivory. This species is a true omnivore, meaning it can survive on an immense variety of plant and animal matter. Stomach content analyses of wild rats have revealed thousands of different food items, underscoring their dietary flexibility.
In urban environments, rats thrive mainly on discarded human food, which represents a vast, easily accessible energy subsidy. This includes grains, stored crops, raw meat, and cooked food scraps found in sewers and garbage dumps. This reliance on human waste is the primary reason for the popular perception of rats as mere scavengers.
The ability to utilize carrion is another aspect of their scavenging lifestyle, helping them minimize the risk and energy expenditure of hunting. This opportunistic feeding strategy allows them to maintain high population numbers in densely populated areas. Their omnivory, coupled with the ability to consume nearly any organic material, makes the rat an adaptable and widespread commensal species.
The Ecological Generalist
Rats are best described as opportunistic omnivores and ecological generalists. Their feeding strategy is determined by the immediate availability of food in their specific habitat. They are capable predators when the opportunity arises or when live prey is the most abundant food source.
They are also efficient scavengers, relying heavily on human-generated waste in urban settings. This dual capacity allows them to exploit a wider range of resources than most specialized species. The rat’s success lies in its flexibility to switch between active predation and passive scavenging to maximize its energy intake.