Cross-fostering technique in the rat

Edel Holene and Kristin Wear Prestrud
Laboratory Animal Unit, Norwegian School of Veterinary Science


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A cross-fostering procedure was developed in order to create different treatment groups for a toxicity study. Rat dams were orally exposed to a lipophilic chemical during gestation days 10 to 20. Lipophilic chemicals are stored in the animal's body fat and excreted in the milk during lactation. In order to create groups of offspring that were exposed either in utero or during lactation, the cross-fostering procedure had to be performed shortly after delivery and before the offspring started to suckle. The rat is assumed to be an excellent foster-mother. However, disturbances during delivery may elicit cannibalism.

These webpages describe a successful cross-fostering procedure performed shortly after delivery:


Time mated DA/OlaHsd rats from a commercial breeder (Harlan, UK) were used in the study. The day the vaginal plug was observed was assigned gestation day 0.


The rats started to build nests 1-2 days prior to delivery.


Delivery started on gestation day 21. The observer entered the animal room regularly and looked for traces of blood on the nesting paper and/or pups, noting the time of discovery and identity of the dam. The dams were observed only through the transparent walls of the cages.   They were never disturbed or handled until about 3 hours after delivery. The observer also performed the cross-fostering and was familiar to the animals prior to delivery.


The rats generally delivered pups every 10 to 15 minutes and ate the placentas immediately afterwards. 


Various stages of uncleaned and cleaned pups were scattered around in the cage during delivery.
The delivery was generally over within three hours. Cross-fostering was performed when the dam started to gather the pups beneath her for suckling.  Similar size litters in different treatment groups were to be crossed.


The dams were carefully removed from their home-cage and temporarily placed in empty cages equipped with nesting paper and some food pellets.   Each litter was held in the observer's hand and the pups were cleaned with a wet cloth if necessary.


One litter was placed in an artificial nest while the other was handled.  The artificial nest was made of soft paper placed on top of two rubber gloves filled with warm water.


The clean, warm litters were placed in the cage of their foster-mothers and the dams were returned to their home cages. After some reorganisation of the pups and the nest, the dams calmed down and began to lactate.

Twenty-nine litters were successfully cross-fostered in this study.


This presentation is one of the records the NORINA database (record number 5238).

© Copyright The Norwegian School of Veterinary Science 2000.

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