In Northern ecosystems, life history strategies and accompanying animal behaviours occur in the context of extreme annual thermal seasonality. Like many mammals, the behaviour of temperate hibernating bats, such as Myotis lucifugus and Myotis septentrionalis is influenced by seasonality and intraspecific life history differences. For these species, the imperative to achieve sufficient pre-hibernation fat stores must be balanced with the costs of summer reproductive activity. There is evidence that the phenology and rate of mass change differ within and among these species, but such differences have not been adequately quantified. One way to track reproductive and pre-hibernation behaviours may be body mass, which is affected by reproductive and physiological activity. I therefore performed time series clustering on the mass variation patterns of bats captured between 1999 and 2019 in Eastern Canada to illustrate differences among their behaviour. I predicted that pregnancy in females and spermatogenesis in males would yield notable differentiation among groups during the early summer, but that the shared need to gain mass in fall would lead to less distinct differences. When clustered according to dynamic time warping distance, mass variation patterns were most distinct in the early summer, and least so in the late fall. Sex-specific differences were also most pronounced early in the year. These results demonstrate that disparate reproductive investments are detectable in the mass of these species, and that all ages and sexes of bats demonstrate similar mass-sequestration patterns prior to hibernation. The lack of differentiation between juveniles and adults in fall furthermore indicates that some external factor likely limits the rate with which juveniles can gain mass prior to hibernation.