Common commensal rats, such as the Norway rat (Rattus norvegicus) and the Roof rat (Rattus rattus), can endure periods without food. Generally, a rat can survive for approximately one to two weeks without eating, provided it has a consistent source of water available. This survival time is not absolute and is influenced by various factors. Understanding this time frame requires separating the roles of food and water, as the lack of each resource affects the animal differently.
The Critical Difference: Survival Limits With and Without Water
Survival without water is significantly shorter for a rat than survival without food. Most rats cannot survive more than three to five days without water, even if they have access to food. Water is an immediate necessity for all basic physiological functions, including digestion, nutrient transport, and temperature regulation.
Rats have a high metabolic rate, requiring a constant supply of water to prevent rapid dehydration. While they can extract some moisture from their food, this is often insufficient to meet their daily hydration needs (about 15 to 30 milliliters). The kidneys of the common brown rat are not as efficient at concentrating urine as those of some desert rodents, making them vulnerable to dehydration. In environments where both resources are scarce, the lack of water will cause death much more quickly than the lack of food.
Key Factors Influencing Starvation Tolerance
The exact duration a specific rat can survive is highly variable and depends on several inherent and environmental factors. A rat’s body size and existing fat reserves are the most significant internal determinants of starvation tolerance. Larger, well-fed adults with greater adipose tissue stores provide a bigger energy buffer, allowing them to draw on these reserves for a longer period.
Species variation also plays a role, as the larger Norway rat generally has a greater capacity for fat storage than the smaller Roof rat. Age is also a strong predictor of survival, with younger rats having lower fat reserves and a diminished ability to survive prolonged fasting. Environmental temperature impacts survival because colder temperatures increase the rat’s metabolic rate to maintain body heat, burning through energy reserves more quickly.
The Physiological Response to Food Deprivation
When a rat is deprived of food, its body initiates metabolic shifts to conserve energy and sustain life. In the initial hours of fasting, the body first utilizes its most readily available energy source: glycogen, stored primarily in the liver and muscles. This glycogen is quickly depleted, prompting a transition to the next phase of energy production.
After glycogen depletion, the metabolism switches to lipolysis, the breakdown of stored fat reserves (lipids) into fatty acids for fuel. The length of time a rat survives is directly related to the volume of this fat reserve, as this process provides a substantial and sustained energy source. Once fat reserves are significantly depleted, the body enters its final phase, involving the catabolism of structural proteins, mainly from muscle tissue, to create glucose. This breakdown leads to rapid muscle wasting, organ failure, and death due to severe energy depletion.