Pups call, mothers rush: does maternal responsiveness affect the amount of ultrasonic vocalizations in mouse pups?

In rats and mice, the ultrasonic vocalizations emitted by pups have been suggested to modulate maternal behavior. In the present study we show that the number of calls emitted by mouse pups can reflect maternal responsiveness. Maternal responsiveness towards pups was evaluated on postnatal day 8 using a three-compartment cage test where the mother, to reach the pups, had to cross the central part of the cage containing cues from a potentially infanticidal male. Maternal responsiveness was lower when alien rather than own pups were used as stimulus. Moreover, the administration of morphine (2.5 mg/kg, i.p.) a drug known to disrupt maternal behavior, resulted in an increase of the latency to reach the pups, as well. This behavioral and pharmacological validation supports the hypothesis that this measure can represent an index of maternal motivation. Therefore, we evaluated maternal responsiveness on day 8 postpartum and pups' ultrasound emission during isolation on day 4 and 8 of life, under conditions strongly affecting the amount of maternal behavior received by pups. C57BL/6 mothers scored higher in maternal responsiveness than BALB/c females, and their pups emitted fewer calls than BALB/c pups both on days 4 and 8. Mothers of handled pups scored higher than controls in maternal responsiveness. Handled pups showed a lower rate of calls on day 8, although they did not differ from controls on day 4. These results support our hypothesis that maternal responsiveness, that is mother promptness to respond to pups' needs, is one of the factors tuning the rate of ultrasonic emission of the offspring.