Global variation in avian metabolic rates and the slow pace of life of tropical birds

Efforts to identify mechanistic links between the rates at which birds use energy and their ecology and evolution have often focused on metabolic rates measured under standardized conditions, most notably basal metabolic rate (BMR). It has long been recognized that birds inhabiting tropical regions generally have lower BMR than their temperate-zone counterparts. Two alternate hypotheses have been advanced to explain this difference. On the one hand, the low BMR of tropical species may arise directly from living in warm environments, with modest demands for metabolic thermogenesis and activity reflected in low rates of baseline energy expenditure. This notion is supported by two recent analyses that found negative correlations between BMR and air temperature (White et al. 2007; Jetz, Freckleton & McKechnie 2008). The second possibility is that the low BMR of tropical species is instead mechanistically linked to their generally slow pace of life and lower investment in reproduction (Ricklefs & Wikelski 2002). According to this view, the more modest investments in reproduction by tropical birds compared to fastbreeding, highly fecund temperate-zone species result in lower overall energy demands, which translate into lower BMR via mechanisms such as lower masses and/or massspecific metabolic rates of organs (Wiersma et al. 2007; Williams et al. 2010). Measurements of avian BMR date back to early in the 20th century (Benedict & Fox 1927; Brody & Proctor 1932; Riddle, Smith & Benedict 1932), and the species for which BMR measures exist now number well over 500 (McNab 2009). Although several researchers have gone to considerable effort to measure BMR in tropical species (Hails 1983; McNab 2001, 2005; Wiersma et al. 2007), data for low-latitude species remain disproportionately sparse, particularly when viewed in the light of the enormous diversity of tropical avifauna. This paucity of tropical data has severely limited global analyses of avian BMR, particularly those seeking to investigate the roles of life history versus climate in driving latitudinal metabolic variation. In this issue of Functional Ecology, Londo~ no et al. (2015) present a global analysis of avian BMR based, in part, on new measurements for over 250 tropical species. This study not only greatly increases the number of species for which BMR estimates are available, but also goes a long way towards remedying the latitudinal imbalance in our knowledge of this important metabolic trait. Most of Londo~ no et al.’s data were collected at three field stations at 13°S in Peru, spanning a 2600 m altitudinal gradient from lowland rain forest to montane cloud forest. The study took place over a 3-year period and covered phylogenetically diverse taxa, including tinamous, doves, nightjars, hummingbirds, toucans, woodcreepers, elaenias, manikins, thrushes, euphonias and flowerpiercers. The variation in altitude and hence temperature among the study sites provided the basis for testing the influence of climate on avian metabolic rate. Despite the highest of the three sites being on average 12°C colder than the lowest, BMR did not vary significantly with altitude. This finding suggests that the low BMR of tropical species is not directly linked to temperature but instead represents a characteristic of tropical birds in general, supporting the hypothesis that low BMR is mechanistically linked to a slow pace of life. However, the nature of these links between BMR and life history remains unclear. One obvious potential link concerns the relationship between BMR and field metabolic rate (FMR), the total amount of energy a bird uses on a daily basis. Relatively, few FMR estimates exist for tropical birds, and those that are available do not provide a clear picture. For instance, an analysis of latitudinal variation in avian FMR found that the handful of low-latitude species in which it has been measured often have FMRs comparable to those of species from temperate regions (Anderson & Jetz 2005). In contrast, the FMR of spotted antbirds (Hylophylax naevioides) in Panama measured via heart rate telemetry was much lower than allometrically predicted values (Steiger et al. 2009). Many more measurements of FMR in tropical species are needed to examine the potential link to low BMR. In addition to providing important new insights into global patterns in avian energetics, Londo~ no et al. (2015) also reignite a long-standing debate concerning phylogenetic variation in avian BMR. Several early workers noted that BMR in the order Passeriformes, which includes more *Correspondence author. E-mail: aemckechnie@zoology.up. ac.za