Selective retrieval of jimpy mutant pups over normal male littermates by lactating female B6CBACa-Aw-J/A-Ta jp mice.

The jimpy sex-linked mutation results in dysmyelination throughout the CNS although myelination in the peripheral nervous system is unaffected. Jimpy pups (jp/Y) weigh less, produce fewer ultrasonic vocalizations, and spend less time engaged in coordinated activity than their normal littermates and rarely live past 30 days of age. Both jp/Y and normal male pups occur within the same litter, making it possible to examine maternal responsiveness to the two types of pups. Using maternal retrieval as a measure of maternal responsiveness, we predicted, in agreement with parental investment theory, that mothers would retrieve normal littermates in preference to mutant pups. When tested for pup retrieval preferences in a T-maze at 2-day intervals from day 3 to day 15 postpartum, mothers retrieved the mutant mouse first on 78 to 100% of the trials and carried him back to the stem section of the T-maze before returning for the normal littermate. We are thus left with the paradox that the mother appears to "prefer" the mutant pup to the normal littermate, despite its smaller size, reduced ultrasonic vocalization rate, and compromised fitness.