Coping with lactation by an ephemeral forager as pups get bigger: energy expenditure, water flux, diet and foraging activity of lactating fish-eating myotis ( Myotis vivesi )

Bats are characterized by long lactation, and their physiology and behavior change during this period. Notwithstanding, very little is known of how lactating bats adjust as their offspring develop. We examined changes with pup body mass in diet, foraging behavior, daily energy expenditure (DEE) and daily water flux (DWF) of free-ranging lactating fish-eating myotis (Myotis vivesi) in 2016 and 2017 using GPS-tracking, the doubly labelled water, and DNA dietary reconstruction. DEE and DWF were correlated in both years. DEE (and thus DWF) varied widely across individuals and was positively correlated with mother flight time length in both years. There were no significant correlations between pup body mass and their mother DEE and DWF in 2016, but DEE and DWF were positively related to pup body mass in 2017. Mother flight time length (including commuting and foraging flight time length in 2016) was not related to pup body mass or forearm length. Mother diet was composed of California anchovy (Engraulis mordax) and krill (Nyctiphanes simplex) regardless of pup body mass. We found partial support for the prediction that mother´s DEE increases as pup get bigger, but the extent of their nightly foraging movements might be also affected by prey´s unpredictable distribution.