Female reproductive success naturally declines with increasing age, however, whether the cost of reproduction reduces longevity of an individual is still a matter requiring greater investigation. Reproduction and lactation are highly energetically demanding processes likely sequestering energy from mitochondria. I investigate how mitochondria are affected by reproduction and how that ultimately influences longevity. To do this, I maintain a colony of Mus musculus in semi-natural enclosures and monitor the body temperatures for reproductive and non-reproductive female mice, and I test their mitochondrial respiratory capacity, physical fitness, and look for markers of oxidative damages, senescence markers, and the types of mutations present in the liver, brain, and skeletal muscle tissues. I also collect the same data from animals of different age groups in a more traditional laboratory setting to understand how ageing influences mitochondrial function and performance.