What has higher energy output than a hummingbird?
December 17, 2012
I happened to be browsing Gerald L. Woods superb Guinness Book of Animal Facts and Feats (3rd edition) this morning, and happened across this fragment on page 76:
Not surprisingly, hummingbirds have the highest energy output per unit of weight of any living warm-blooded animal.
The wording struck me as strange: highest of any living warm-blooded animal? Is Wood just being redundant here, or is he implying that there are cold-blooded animals with a higher mass-specific metabolic rate? The idea seems inherently contradictory, doesn’t it?
I wondered whether he might have insect flight in mind?
Am I making a mistake in conflating “energy output” with “metabolic rate”?
Sauropod Vertebra Picture of the Week 
December 17, 2012 at 1:17 pm
A bumblebee flies across a sunny meadow on a hot July day. The temperature of its body is pretty much the same as the temperature of the body of a nearby hummingbird.
What are the physical reasons that one creature should have a higher metabolic rate or energy output per unit mass than the other? Do birds employ chemical reactions which are more efficient than those used by insects? I’m sure the bird’s respiratory system is much better at moving oxygen to distant tissues than the bee’s — but the bee doesn’t really have any tissues which are truly distant from the atmosphere. Are there studies showing that the oxygen concentration in bird cells is higher than that in insect cells?
December 17, 2012 at 1:41 pm
Energy output is a strange term here, as the animal is *using* energy to output force, the rate of which would be power. If what the author really means is power, then yes, ectotherms do better.
December 18, 2012 at 12:47 am
It would take a long time to measure the mechanical power output of all the known arthropods, and possibly longer to find and measure the unknown ones. If you don’t know, you can’t compare.
I have serious doubts about a hummingbird’s endothermy having any noticeable effect on its power output per unit anything. I would not be surprised to find that muscle efficiency (vs. input, or vs. mass) differs between arthropods and endotherms, or between different arthropod species or different endotherm species, or between one individual of either and its sibling, but I would be very interested to learn the magnitudes and signs of such differences. I might guess that arthropods with a generation cycle much less than a year would do better on any such measure than endotherms and slower-breeding arthropods, just because they have had more opportunities to improve. But maybe a plateau is reached after a million or two generations, and the endotherms’ higher commitment per individual makes efficiency improvements more desirable vs other considerations, such as growth rate or low-temperature performance.
December 18, 2012 at 2:21 am
I think that Woods prob is being redundant but was poss covering himself in case there was a freaky insect of which he was unaware. I assume that by ‘energy output’ Woods means the total of that which is expended in maintaining a hummingbird’s temperature, moving around, feeding, etc. It wouldn’t be quite the same as ‘energy input’ as some of that may be diverted to fat reserves altho’, in the case of hummingbirds, I don’t think that there is much of that going on.
Given that a hummingbird’s daytime internal temperature is around 40-42 °C, it would indeed be quite a hot day in that sunny meadow if it was the same temp inside and outside of that hummingbird (and, obviously, north of the equator).
I do know that a hummingbird’s extravagance is not solely due to its high activity and high surface area-volume ratio. It doesn’t have downy feathers and is therefore not as well insulated as other birds, losing a greater percentage of its energy input as waste heat.
To somewhat offset this, when it is resting at night (and not feeding), it goes into torpor, dropping its temp down to around 21-22 °C and slowing a number of other metabolic processes.
In answer to Mike’s Q, I’ve generally understand metabolic rate to mean the amount of energy an animal expends in maintaining itself at rest. So, the energy used to move about, even tho’ that is essential if the animal is to gather additional energy, is supplementary to, and separate from, metabolic rate.
However, that is more strictly called ‘basal metabolic rate’, so it’s prob fine to conflate the two unless Woods is referring to just the energy expended while flying and hovering, which seems unlikely.
December 19, 2012 at 8:14 pm
Yeah, it’s insects. Here are some numbers. There’s a lot of literature on metabolic rates, so these are just the first sources I found, possibly not the best. Also, metabolic rates are generally given as volumes of oxygen consumed (presumably because that’s how they’re measured) rather than in something convenient like W/kg. 1 cc O2/g/hr = 1 mm3 O2/mg/hr = 5.7 W/kg.
Oliver Pearson’s The metabolism of hummingbirds (1950) reports metabolic rates of 10-16 cc/g/hr resting, 1 cc/g/hr torpid, 68-85 cc/g/hr flying, and “31 cc/g/hr over a period of 157 minutes” average (wow). It also cites “a flight metabolism of about 9.5 cc/g/hr for blowflies, 21 cc/g/hr for fruit flies, and 100 cc/g/hr for butterflies. These were 190, 13, and 170 times, respectively, the resting rate of each species.” (Insects have humungous peak/resting ratios.)
Niven and Scharlemann’s Do insect metabolic rates at rest and during flight scale with body mass? (2005) has (in its supplementary data) more numbers for insects, with metabolic rates of 0.11-5.0 mm3/mg/hr resting and 11-194 mm3/mg/hr flying. (It also finds higher flying metabolic rates for larger insects, perhaps because flying is harder for larger animals.) So some insects have higher peak metabolic rates than hummingbirds, but not necessarily higher average metabolic rates, since insects consume much less power at rest (less than even torpid hummingbirds).
Human peak oxygen consumption is something like 2-5 cc/g/hr. Hummingbirds and insects have crazy metabolisms.
December 19, 2012 at 8:28 pm
Many thanks, Vetebrat, that is really helpful and detailed, not to mention insane. Butterflies can consume twenty times as much oxygen as exercising humans!
December 23, 2012 at 3:45 am
This doesn’t really answer the question, though. Oxygen consumption is a measure of power input, but some fraction of that is wasted, bound into oxides without producing ATP, or producing just heat that must be dissipated, or assisting in dissipating heat.
But those butterflies are pretty astonishing. It must not be very easy to measure the oxygen consumption of a butterfly.
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