Pup retrieval is a highly specific and instinctive maternal action. This behavior is a fundamental part of parental care in mice and other altricial species, whose young are born underdeveloped and completely helpless. The primary purpose of pup retrieval is to ensure the survival of vulnerable offspring by returning them to the safety and warmth of the nest.
The Mechanism of Pup Retrieval
The physical act of carrying is a precise, instinctual sequence initiated by the mother mouse. She approaches the scattered or misplaced pup and grasps the loose skin, or scruff, at the back of the baby’s neck using her mouth. This grasping motion requires a high degree of motor coordination and is executed with gentle yet firm pressure.
The mother’s action triggers a specific reflex in the pup, which is scientifically termed Pinch-induced Behavioral Inhibition (PIBI), or transport immobility. The pup immediately goes passive and limp, suspending all movement. This reflex is crucial because any struggle from the pup would make the retrieval attempt more difficult and potentially cause injury during transport back to the nest. The mother then carries the non-responsive pup until she deposits it safely back into the main nesting area.
Triggers for Moving the Litter
A mother mouse initiates pup retrieval for several protective reasons, all rooted in optimizing her litter’s chances of survival. The most common trigger is the perception of danger or disturbance, which includes the presence of a predator or human interference. This protective instinct drives the mother to move her offspring to a more secure location.
Environmental instability also prompts a relocation, such as damage to the nest structure, a sudden change in temperature, or flooding within the nesting cavity. Additionally, the mother may choose to move the litter to a new site closer to a reliable food or water source. In some cases, a mother will strategically create multiple nest sites to separate parts of a large litter, which can help mitigate competition among the pups for resources.
When the Carrying Behavior Stops
Pup retrieval is a temporary behavior that is strictly linked to the developmental stage of the young mice. Newborn mice are altricial, meaning they are born blind, deaf, and immobile, making them entirely dependent on the mother’s retrieval behavior for survival. This dependency begins to fade as the pups develop their own sensory and motor capabilities.
The pups’ eyes begin to open around Postnatal Day 11 and are usually fully open by Day 12. Following this, they start to become more active and begin moving around the nest independently. Once the pups gain sufficient mobility and coordination, typically around two to three weeks of age, they can follow their mother or quickly flee from danger on their own. By the time they are weaned, around Postnatal Day 21, the maternal retrieval behavior is no longer necessary and naturally ceases.