When pups are removed for surgery, similarly aged pups can be taken from an outbred mouse (such as CD-1) and transferred to the mother whose pups were taken for surgery. Postoperatively, the surgically altered pups are then placed with the outbred mother for temporary fostering during the recovery period and left with her for a couple of hours or overnight. The litters are then switched so that each mother has her own pups back. Care must be taken to treat both experimental and control pups in the same way to avoid introducing experimental variability.

In any event, the mother’s behavior toward the pup should be observed closely for the first 10–15 minutes after the pup is returned to her and then every 10–15 minutes for the next couple of hours. At the first sign of aggression by the mother toward the pup, the pup should be removed. If other means of caring for the pup (such as fostering or hand rearing) are not available, the pup should be euthanized, as should pups that are not being cared for by their mother.

In higher mammals, neglect and cannibalism are uncommon postsurgical problems. However, the behavior of the dam should be closely monitored after return of the neonate to her.


Two of the most common methods of identifying adult rodents, ear notching and ear tagging, are not useful for neonatal rodents, because they have small ears tightly placed against their heads. Temporary identification of hairless neonates can be achieved with nontoxic indelible markers (for example, Sharpie®). However, this marking rarely lasts for more than a day, because the mothers will lick the color off. More permanent identification can be achieved by marking the tail with a tattoo machine designed for this purpose; with practice, pups can be marked quickly and effectively. According to the Guide, “toe clipping [removal of the first bone of certain toes, corresponding to a predetermined numbering code], as a method of identification of small rodents, should be used only when no other individual identification method is feasible and should be performed only on altricial neonates” (p 46). Under some circumstances, that method of identification may be necessary, but it should be used only with IACUC approval based on appropriate justification in the animal-use protocol.


Many experimental fetal surgical procedures in higher mammals require special procedures or conditions, such as a second surgery for the injection of tracers or producing a lesion, or specialized equipment and facilities. Exposure of a fetus in utero constitutes a major operative procedure as defined by the AWRs and the Guide. In accordance with regulatory requirements for surgery, multiple survival surgical procedures must be justified scientifically by the neuroscientist

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