In Brief

When researchers switch the litters of rat dams, the behavior of the pups and their foster dams varies based on how old the rat pups are at adoption, finds a new Behavioral Neuroscience (Vol. 118, No. 3) study.

The findings have implications for research studies that cross-foster litters to control for genetic effects, say the study authors, led by psychologist Muriel Darnaudéry of the University of Lille's Laboratory of Perinatal Stress in France. Contrary to the assumption that adoption procedure is a simple control mechanism, they explain, the fostering procedure by itself changes dams' and pups' behavior--introducing a new variance researchers may need to account for.

For the study, the researchers switched the litters of several dams at varying times: within the first six hours of birth, on the fifth day after birth and on the 12th day. The day after the adoption and at least one more time, the pups were separated from their new foster mothers for about eight minutes so that researchers could measure the pups' and dams' reactions to the stressful separation and to being reunited. For each separation of an adopted litter, the researchers also examined a control group of pups being raised by their biological mother.

Previous research has found that early adoption--on day one--increases maternal behavior, such as licking, and shortens pups' release of the stress hormone corticosterone, while later adoptions have little effect on maternal actions but prolong pups' cortico sterone secretion. But no one had examined pup behavior in reaction to the adoption procedures.

The current study found that, during separation:

  • Pups adopted on day one emitted fewer ultrasounds--the equivalent of crying--than the control-group pups after each separation. Moreover, once reunited, the pups' adoptive dams exhibited more maternal behaviors, such as licking pups and spending time in the nest, than the biological mothers in the control group.

  • Pups adopted on day five were more vocal than controls for the separation on day six, but about the same on a subsequent separation on day 13. Their dams spent more time in the nest after the first separation, but not after the second subsequent separation.

  • Pups adopted on day 12 were about as vocal as the control pups when separated the next day. Their adoptive mothers spent no more time licking them than the biological mothers in the control group, but did spend longer in the nest.

"The present experiment provides for the first time clear evidence that postnatal adoptions at different postnatal days affect both dams' and pups' behavior," write the researchers, explaining that the same environmental change--adoption--had opposite effects depending on the development period--day one, five or 12--in which it occurred.

Moreover, the researchers say, the study is the first evidence that the vocalizations of the pups during adoption are associated with an increase in mothers' licking once reunited. It's unclear, they note, whether the mother's behavior affected the pups' vocalizations or vice versa.