Active season body mass patterns of little brown and northern myotis bats

Abstract

Animals are expected to adjust their behavioral 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, and 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 behavior 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.

Keywords: bats; foraging; hibernation; little brown myotis; mass; northern myotis.

FIGURE 1

Fitted summer mass variation patterns of adult little brown myotis (Myotis lucifugus; solid lines) and northern myotis (M. septentrionalis; dashed lines) captured between Julian days 158 and 207. Female groups are denoted in orange, and male groups are denoted in aqua.

FIGURE 2

Hierarchical dynamic time warp distance clustering dendrogram of time series constructed from summer mass variation patterns of adult little brown myotis (Myotis lucifugus) and northern myotis (M. septentrionalis) captured in New Brunswick (NB), Nova Scotia (NS), Prince Edward Island (PEI), Newfoundland (NF), and Ontario (ON) between 2000 and 2019. The distance among clusters is cophenetic, which indicates the point at which a pair of adjoined clusters may be combined.

FIGURE 3

Distribution of the summer daily body mass change in collections of adult male and female little brown myotis (Myotis lucifugus) and northern myotis (M. septentrionalis) captured in Eastern Canada between 2000 and 2019. Each boxplot is comprised of 49 points, each representing the estimated daily change in body mass for the group between Julian dates 158 and 207 (approximately June 7–July 26). Error bars represent the standard error of the estimated mean daily change in body mass.

FIGURE 4

Fitted fall mass variation patterns of little brown myotis (Myotis lucifugus; blue) and northern myotis (M. septentrionalis; purple) captured in Nova Scotia between Julian dates 227 and 262. Adult groups are denoted with solid lines and juvenile groups are denoted with dashed lines.

FIGURE 5

Hierarchical dynamic time warp distance clustering dendrogram of time series constructed from fall mass variation patterns of little brown myotis (Myotis lucifugus) and northern myotis (M. septentrionalis) captured in Nova Scotia between 2009 and 2011. The distance among clusters is cophenetic, which indicates the point at which a pair of adjoined clusters may be combined.

FIGURE 6

Distribution of the fall daily body mass change in collections of little brown myotis (Myotis lucifugus) and northern myotis (M. septentrionalis) captured in Nova Scotia between 2000 and 2019. Each boxplot is comprised of 35 points, each representing the estimated daily change in body mass for the group between Julian dates 227 and 262 (approximately August 15–September 19). Error bars represent the standard error of the estimated mean daily change in body mass.

FIGURE 7

Linear relationship between spring and summer foraging conditions and the estimated date of parturition in little brown myotis (Myotis lucifugus) and northern myotis (M. septentrionalis) captured in New Brunswick (NB), Nova Scotia (NS), Prince Edward Island (PEI), Newfoundland (NF), and Ontario (ON) between 2005 and 2019. Each point represents the estimated date of parturition for one group, whose year of capture is indicated with its province abbreviation.