Long-term telemetry of heart rates and energy metabolic rate during the diurnal cycle in normothermic and torpid African blue-naped mousebirds (Urocolius macrourus)

Colies are one of the phylogenetically oldest groups among the modern birds ; the earliest finds are from about 35 million years ago. In states of ener gy deficiency they can undergo torpor during the night when metabolic rate and body temperature are decreased drastically to save energy (up to 90%). Here, we report the first measurements of heart rate (HR) by long-term tele metry, in seven individuals of blue-naped mousebirds (Urocolius macrourus); simultaneously and continuously metabolic rate (MR) was determined. HR at night was about 20% below the range of expected values (246/310 bpm). Mean oxygen pulse (O-2 output/stroke) in normothermic birds was in a range of 0. 019-0.020 ml O-2/stroke; during torpor nights this value decreased signific antly to 0.0086. Mean cardiac output ranged from 724 to 1214 mi blood/kg pe r min; in torpid birds this value fell to 400 mi blood/kg per min. Cardiac regulation of metabolic demand within an activity phase (day or night) is m ainly achieved by chronotropy. Inotropy contributes at most 25% to the diff erences in MR between day and night (ca. 40%). Entry into torpor is brought about mainly by changes in HR (decrease from 240 to 90 bpm); after torpor levels have been reached, there is an increase in HR (to 200 bpm) and a sha rp decrease (-53%) in stroke volume. This regulation by inotropy is also ch aracteristic of arousal from torpor. (C) 1999 Elsevier Science Inc. All rig hts reserved.