The evolution of milk composition and lactation strategy of the Columbian ground squirrel, Urocitellus columbianus
Type of Degreedissertation
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The evolution of parental care improved the efficiency of the reproductive process, allowing for the production of fewer offspring with a higher probability of survival. This adaptation is exemplified in the taxon named for their unique and efficient mechanism of nourishing young following parturition. Lactation and milk production is a key feature of the mammalian reproductive strategy contributing to maternal and offspring fitness. Because the quantity and quality of care provided is a balance between maternal cost and benefit, identifying factors contributing to inter and intra-specific variation in lactation effort as well as the costs and benefits involved can provide keen insight into the evolution of mammalian life history strategies. Herein, I determined factors influential in the evolution of milk composition and examine the causes, benefits and costs associated with lactation effort in the Columbian ground squirrel, Urocitellus columbianus. First, I examined factors contributing to the evolution of the diverse nutritional composition of milks observed across mammals using a phylogenetic comparative approach. The biome inhabited, length of the lactation period, and maternal diet were correlated with milk composition. Second, I described the composition of milk produced by Columbian ground squirrels. The concentration of most milk constituents changed over the course of lactation with most exhibiting a distinct peak around 19 days postpartum. Columbian ground squirrel milk was relatively low in lipid concentration but high in protein and calcium concentration and the proportion of energy from protein. Third, I assessed relationships between maternal characteristics and milk composition and impacts on offspring. Variation among females in milk fat, sugar, and protein concentration and energy density was not associated with differences among females in size or condition. Females giving birth around the median parturition date produced milk that was higher in fat concentration and energy than females giving birth at other times. Milk fat, energy, and sugar concentration had a positive effect on offspring survival over-winter. Finally, I examined the proximate and ultimate costs of lactation. Females raising augmented litters had greater rates of energy expenditure, indicating an energetic cost to lactation. Energy expenditure was not related to female survival over-winter or her fecundity the next breeding season, although pups of augmented litters were lighter at weaning, had slower growth rates, and a lower probability of survival over-winter. These results do not support the predictions associated with the cost of reproduction tradeoff.