Active season body mass patterns of Little Brown Bats and Northern Myotis: Raw and fitted mass values, environmental conditions and inflection point estimates

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.