Torpor in Dark Times: Patterns of Heterothermy Are Associated with the Lunar Cycle in a Nocturnal Bird

Abstract

Many studies have shown that endotherms become more heterothermic when the costs of thermoregulation are high and/or when limited energy availability constrains thermoregulatory capacity. However, the roles of many ecological variables, including constraints on foraging opportunities and/or success, remain largely unknown. To test the prediction that thermoregulatory patterns should be related to foraging opportunities in a heterothermic endotherm we examined the relationship between the lunar cycle and heterothermy in Freckled Nightjars (Caprimulgus tristigma), which are visually orienting, nocturnal insectivores that are dependent on ambient light to forage. This model system provides an opportunity to assess whether variation in foraging

Adaptive thermoregulation during summer in two populations of an arid‐zone passerine

“…These results corroborate other findings that the environment plays an important role in T b expression at an intraspecific level, both within and between populations (McKechnie et al 2007, Glanville and Seebacher 2010, Smit et al 2011, Doucette et al 2012, Glanville et al 2012.…”

Biogeography of time partitioning in mammals

“…The rotation of the Earth partitions time into regular cycles of day and night, and although all points on the Earth's surface receive roughly equal durations of light and darkness over the course of a year, at mid to high latitudes seasonal variation in day length imposes an uneven distribution throughout the annual cycle. During the hours when the sun is below the horizon, there is seasonal and latitudinal variation in the duration of "biologically useful semidarkness" in the form of twilight and moonlight (1), modified by both the lunar cycle and variable cloud cover, providing changing opportunities for animals able to use visual cues for key behaviors including foraging, predator avoidance, and reproduction (2)(3)(4)(5)(6). Activity during both daylight and semidarkness may be further constrained by covariance between the natural cycles of light and temperature; the metabolic costs of thermoregulation place constraints on the time available for activity (7).…”

“…Where energetic and thermal costs are not prohibitive, temporal niche partitioning may occur as species specialize and avoid competition by concentrating their activity within a particular section along the light gradient (9,10). Behavioral traits are associated with a range of specialized adaptations, particularly in visual systems and eye morphology (11) and energetics and resource use (6,12). Thus, some species are apparently obligately diurnal in their peak activity patterns, some obligately nocturnal, obligately crepuscular (active primarily during twilight), or obligately cathemeral (significant activity both during daylight and night), and others make facultative use of both daylight and night (13), or show seasonal and/ or geographical variation in their strategy.…”

Energy savings due to the use of shallow body temperature reduction in overwintering Northern Cardinals

“…However, recent reports on free-living birds support the relationship between torpor and food availability. For example, moon phase, and presumably foraging efficiency, was a stronger predictor of torpor than ambient temperature in Freckled Nightjars [32] and prey availability similarly predicted the use of heterothermy in free-living Owlet-Nightjars [9]. In contrast, others argue that while food availability may contribute, it is not the sole determinant of heterothermy [28].…”