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

Animals with a slow pace of life and high mass-specific metabolic rates are expected to invest less in innate immune responses. We measured skin inflammation and the resting metabolic rate (RMR) of ruby-throated hummingbirds ( Archilochus colubris) after their immune system was challenged with phytohemagglutinin (PHA) and compared with the response of birds injected a saline solution. The PHA test measures the inflammatory process, a component of the innate response. Ruby-throated hummingbirds belong to a group that is under-represented in avian immunological studies characterized by a slow pace of life and fast metabolic rate. Hummingbirds developed an inflammatory response that lasted <28 h. PHA injection produced a significant increment of RMR (up to ~13%) with respect to RMR values after the injection of the saline solution indicating that immune response involved a metabolic cost for hummingbirds. This increment lies within the range of values previously reported for birds injected PHA (5%–29%).

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The high-energy aerial insectivore lifestyle of swallows does not produce clear thermogenic side effects

The Auk ◽

10.1093/ornithology/ukab022 ◽

2021 ◽

Author(s):

Yufeng Zhang ◽

Kang Nian Yap ◽

Kyle T David ◽

David L Swanson

Keyword(s):

Side Effects ◽

High Energy ◽

Foraging Strategies ◽

Activity Levels ◽

Metabolic Rates ◽

Aerobic Activity ◽

Pace Of Life ◽

Aerial Insectivore ◽

Aerial Insectivores ◽

Temperate Birds

AbstractEcological traits related to pace of life, such as foraging strategies and activity levels, influence daily energy expenditure (DEE) and can affect fitness. A fast pace of life tends to be supported by high-energy aerobic activity and is positively correlated with high DEE and basal and maximal metabolic rates in some endotherms. Given that maximal capacities for exercise and thermogenesis are both functions of aerobic muscle output and are often positively correlated with each other, high-energy aerobic lifestyles might be associated with high aerobic capacities, which would be expected to produce high thermogenic capacities as a side effect. We tested whether the high-energy aerial insectivore lifestyle in swallows is correlated with elevated basal and maximal thermogenic metabolic rates. We measured basal (BMR) and summit (Msum = maximum cold-induced metabolic rate) metabolic rates in 6 species of swallows (Hirundinidae) and combined these data with literature data for additional swallows (n = 10 for BMR; n = 8 for Msum) and non-aerial insectivore birds (n = 215 for BMR; n = 64 for Msum) to address the hypothesis that swallows have higher BMR and Msum than non-aerial insectivores. BMR in swallows was significantly higher than for non-aerial insectivore birds for phylogenetically adjusted analyses after correcting for body mass and region of origin (tropical vs. temperate). In contrast, Msum did not differ significantly between swallows and non-aerial insectivores. Thermogenic scope (Msum – BMR), however, was lower in tropical non-aerial insectivore birds compared with tropical swallows and temperate birds. This suggests that the aerial insectivore lifestyle elevates maintenance costs, but maximum thermogenic capacities are not clearly upregulated, despite tropical swallows having higher thermogenic scope than tropical non-aerial insectivores. These data suggest that the high-energy aerial insectivore lifestyle does not produce strong thermogenic side effects in swallows.

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