Vol.:(0123456789) 1 3 Journal of Comparative Physiology B https://doi.org/10.1007/s00360-018-1175-0 ORIGINAL PAPER The energetics of torpor in a temperate passerine endemic to New Zealand, the Rifeman (Acanthisitta chloris) Brian K. McNab 1  · Kerry A. Weston 2 Received: 8 March 2018 / Revised: 6 June 2018 / Accepted: 17 July 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Compared to other birds, passerines, refecting their small mass, have a narrow set of behavioral characteristics. One difer- ence is that few enter torpor, especially in temperate environments. The few that do include swallows, none of which live throughout the year in cold-temperate environments, because their food, fying insects, is not available in winter and no passerine is known to hibernate. They seasonally migrate to warm-temperate and tropical environments. We present data on the energetics of the Rifeman (Acanthisitta chloris), a small, insectivorous member of the Acanthisittidae, a passerine family endemic to temperate New Zealand. This family is considered to be the sister taxon to all living passerines, which raises the question whether its physiological and behavioral characteristics refect its evolutionary status in a manner that distinguishes it from other passerines. Only two of the eight known species in this family survive; four of the extinct species were fightless, a condition that evolved independently three times and is almost absent from other passerines. The Rifeman readily enters torpor, which is facilitated by its small mass. It enters torpor at ambient temperatures that are commonly encountered in its wet, cool-to-cold environment. As a result, its body temperature and rate of metabolism are highly variable. An estimate of the basal rate of metabolism is similar to that expected from body mass. Unlike some torpor-prone birds, the Rifeman is a permanent resident in a temperate environment. This residency is possible, because the Rifeman gleans insect prey from surfaces, which does not require insects to have high body temperatures for activity. Its only living relative, the endangered, insectivorous Rock Wren (Xenicus gilviventris), is a permanent resident at altitudes from ca. 1000 to 2500 m in the mountains of South Island, New Zealand. There it faces severe winter conditions that are not avoided by descent to lower altitudes. Its response to these conditions may be an extended period of torpor. The repeated evolution of a fightless condition possibly refects some distinctive property of the acanthisittids. The evolution of torpor and a fightless condition in acanthisittids may have facilitated their survival on a geographically isolated, temperate landmass, and these character states permitted by the absence of endemic mammalian predators. Keywords Acanthisittidae · Energetics · Flightless · New Zealand wrens · Rifeman · Rock Wren · Temperate climate · Torpor Introduction Passerines constitute about 60% of all bird species, but they have a narrower set of characteristics compared to other birds refecting their small mass and the consequentially restricted diversity in behavior. For example, torpor occurs in many birds, including todies, swifts, doves, mousebirds, caprimulgids, and hummingbirds (McKechnie and Love- grove 2002; Scheucher 2004). However, few passerines enter torpor and most that do have a tropical distribution, e.g., frugivorous manakins (Pipridae) and nectarivorous sun- birds (Nectarinidae). Passerines most prone to enter torpor are characterized by a small mass (McNab 1989) and the Communicated by G. Heldmaier. * Brian K. McNab bkm@uf.edu 1 Department of Biology, University of Florida, Gainesville, Fl 32611, USA 2 Department of Conservation, Government of New Zealand, Christchurch, New Zealand