Animals are expected to adjust their behavioural patterns to improve fitness outcomes, such as fecundity or offspring survival. For long-lived hibernators, decisions made in each annual cycle may reflect considerations not just for concurrent survival and reproduction, but also the pressure to maximize overwinter survival and future reproductive success. We examined how these elements manifest themselves in the body mass variation patterns of North American northern latitude temperate bats, whose size and roosting habits present considerable monitoring challenges. We characterized and compared the summer and fall mass variation patterns of little brown myotis (Myotis lucifugus) and northern myotis (M. septentrionalis) from a historic dataset. In summer, the estimated date of parturition was strongly associated with spring foraging conditions (low wind, low precipitation, warm temperatures), and mass gain associated with female reproduction conferred considerable differentiation between the mass variation patterns of females and males. In fall, differences were most apparent among species, although adults exhibited a greater capacity for rapid mass gain than juveniles. These results demonstrate how reproductive constraints and interannual survival have important influences on the behaviour of temperate bats. Future work should seek to quantify the fitness benefits of patterns identified in this study, such as the rate of prehibernation mass gain.

These data include bat capture records spanning capture surveys between 2000 and 2019 and weather data provided by Environment and Climate Change Canada. The capture data used in this project are the body mass values of bats captured by independent research teams in the Canadian provinces of New Brunswick, Nova Scotia, Prince Edward Island, Ontario, and the island of Newfoundland. In these projects, bats were captured with mist nets (Avinet, Dryden, New York, USA) and harp traps (Austbat Research Equipment, Lower Plenty, Victoria, Australia) by different research groups and assessed for standard morphometric and diagnostic criteria, including sex, age class (adult or juvenile per Kunz and Anthony 1982), and mass (g) to two decimal places. Upon initial screening of the dataset, we chose to proceed with little brown myotis and northern myotis because they were the two species with sufficient data for an informative comparison. The capture dates were recorded as Julian Date (1-365), and individual observations were grouped for analysis according to year, province, age-class (adult/juvenile), sex, and species. We categorized the initial full historical dataset according to capture record timing, location, individual sex, and individual age. We further screened records for suitability in our time series analysis, and the data presented here include those sets that met our critieria for summer and fall analysis: at least nine unique sampling nights and no sampling gaps longer than 20 days.

We have included both the raw mass values and the LOESS nonparametric fitted values (span = 0.75, quadratic fit). We fitted the mass value records to the widest possible range available for the summer and fall groups, which were Julian Dates 158-207 (approximately June 7 - July 26) for summer and 227-262 (approximately August 15 - September 19) for fall. We chose not to fit exclusively between these dates because doing so would lose accuracy provided by captures outside that range for groups with more sampling.

The environmental data here are raw hourly weather conditions at Canadian federal weather stations near bat capture sites. We considered temperature, wind, and precipitation when creating our foraging condition index, and weighed the data from a particular station according to the proportion of bats caught near it.

All data are accessible through spreadsheet software such as Microsoft Excel and are best processed in R.