Experiments were conducted to establish the effect of litter size, maternal experience and the presence of the father on laboratory mouse development. Behaviour was recorded from day 3» at the earliest, to day 30» at the latest, with the minimum of disturbance to the subjects. Daily observations were made to establish the proportion of time the pups spent exhibiting certain activities. On day 30» open field tests were conducted and the pups* body weights were measured.
The main experiment involved a 2 x 2 x 2 design so that the main effects of the three factors, and their interactions, could be assessed. Although all three factors influenced pup behaviour, paternal presence was the factor with the greatest number of significant main effects. Paternal presence had five main effects and litter size had four. Maternal experience had only one main effect, but was involved in all eight of the significant interactions. It was concluded that both the presence of the father and the small litter size increased the developmental rate of mouse pups, whereas maternal experience was involved in indirect effects.
Two further experiments were conducted to investigate routes by which the father's effect might be operating. The Split Litter experiment exposed pups to the father, in the absence of the mother. The father displayed a range of care taking activities, very similar to the mother's, and the results and general observations indicated that the father was directly influencing offspring behaviour. The Split Gage experiment showed that the mother's caretaking behaviour was affected by the presence of the father and was associated with changed pup behaviour.
It was concluded that the father affected pups directly, mainly by providing them with thermal insulation and tactile stimulation, and also affected them via a maternal mediation route by eliciting an increased display of maternal behaviour
