Data summary from individual strain measurements.

Total litter size includes all (live and dead) pups identified. Weight per live pup is the average of all surviving pups for each individual strain. Dead pups were often found desiccated and partially cannibalized and are thus excluded. Survival rate is the percentage of total pups identified that survived until at least postnatal day 3. Maternal weight was measured and recorded following the identification of a copulation plug and at E14.5 to calculate weight gain. At this time point, females were housed in front of the cameras and not disturbed until a birth was recorded. Pregnancy load is the maternal weight gained as a percentage of initial weight following a successful mating.