For mammals, a good start in life predominantly depends on the quality of a mother's milk, which in turn is strongly influenced by maternal nutrition and energy allocation. Consequently, nutritional supplements for nursing mothers may improve the survival of juveniles in the long term. While this effect has already been reported in birds, only few data related to this question have been collected in wild mammals. In addition, it is questionable whether effects of maternal nutrition on offspring traits and fitness act either short term, in the first stage of life, or persistently, with consequences on succeeding stages in life and later survival.
To address these questions in wild, free-living mammals, Tricia Kerr from McGill University in Canada and her colleagues in both Canada and the United States studied North American red squirrels near Kluane National Park, in south-western Yukon, Canada, for nearly 2 years. While all of the squirrels involved both in the `Kluane Red Squirrel Project' and in Kerr's study were marked and monitored for survival and reproduction, some of them were also provided with peanut butter and sunflower seeds over winter and during reproduction, from autumn to the following spring when their babies were born. When the offspring appeared above ground for the first time later in the spring, all offered food items were removed because the aim of the study was to exclusively manipulate maternal nutrition without altering the food supply of the adolescent, weaned offspring. Kerr and her team took advantage of the squirrels' territoriality, which allowed them to target specific adult mothers with food supplements while ensuring that other squirrels didn't get any extra food.
The team found that maternal food provisioning in red squirrels greatly and persistently improved juvenile survival from their birth to the onset of reproduction, which happens a year after they are born. On average, 78% of young from supplemented mothers survived from birth to when they first emerged from the nest, whereas only 54% of juveniles from unprovided squirrels survived the same period of time. After removal of the food delicacies, total survival of young between first coming out of the nest and territory settlement continued to be higher in privileged offspring descended from food-supplemented mothers. As the young grew up, the difference in survival magnified over time: 94% of privileged young survived the first winter to the following spring, whereas only 62% of the offspring from control squirrels survived the first winter.
When further analyzing the differences between supplemented and non-supplemented mothers, the squirrel research group observed that the two groups of females produced litters of a similar size, with offspring of similar weight. However, females provided with goodies gave birth 18 days earlier. The potential benefits to the supplemented mothers and offspring could include improved immunocompetence, metabolic benefits and reduced stress hormone levels, but these have yet to be uncovered. Despite the unknowns, the authors highlight the fact that because prenatally food-privileged squirrels were born 3 weeks earlier in the year they were ahead by a nose in all respects, giving them a long-term advantage over their less well-nourished peers.
