Anecdotal evidence suggests that birds have smaller intestines than mammals. In the present analysis, we show that small birds and bats have significantly shorter small intestines and less small intestine nominal (smooth bore tube) surface area than similarly sized nonflying mammals. The corresponding >50% reduction in intestinal volume and hence mass of digesta carried is advantageous because the energetic costs of flight increase with load carried. But, a central dilemma is how birds and bats satisfy relatively high energy needs with less absorptive surface area. Here, we further show that an enhanced paracellular pathway for intestinal absorption of water-soluble nutrients such as glucose and amino acids may compensate for reduced small intestines in volant vertebrates. The evidence is that L-rhamnose and other similarly sized, metabolically inert, nonactively transported monosaccharides are absorbed significantly more in small birds and bats than in nonflying mammals. To broaden our comparison and test the veracity of our finding we surveyed the literature for other similar studies of paracellular absorption. The patterns found in our focal species held up when we included other species surveyed in our analysis. Significantly greater amplification of digestive surface area by villi in small birds, also uncovered by our analysis, may provide one mechanistic explanation for the observation of higher paracellular absorption relative to nonflying mammals. It appears that reduced intestinal size and relatively enhanced intestinal paracellular absorption can be added to the suite of adaptations that have evolved in actively flying vertebrates.digestion ͉ gut morphometrics ͉ nutrient absorption ͉ paracellular uptake B irds have structural, physiological, and biochemical refinements that adapt them for flight (1), but basic differences in digestive processing between flying and nonflying vertebrates have never been described to our knowledge. The phrase ''eating like a bird'' wrongly suggests that birds have relatively small appetites, whereas in fact the typical wild bird eats about one-third more dry matter each day than does the typical nonflying mammal (2). Flight, a very energetically demanding activity, contributes to high daily energy demands, but its structural prescription for low weight also may shape an aspect of fliers' digestive apparatus in a way that runs counter to that system's role in providing fuel to meet high energy demands.There is anecdotal evidence that birds have relatively shorter intestines than mammals (3), and shorter intestines are associated with less surface area and volume, parameters directly correlated with digestive capacity. Indeed, in both birds and mammals, digestive adjustments to higher feeding rates almost always include an increase in gut size and thus an increase in digestive enzymes and nutrient transporters (4). For birds that fly, however, the size of the digestive tract and consequently the mass of digesta it carries may need to be minimized because the cost of flight ...
Birds show phylogenetic variation in the relative importance of respiratory versus cutaneous evaporation, but the consequences for heat tolerance and evaporative cooling capacity remain unclear. We measured evaporative water loss (EWL), resting metabolic rate (RMR) and body temperature (T b ) in four arid-zone columbids from southern Africa [Namaqua dove (Oena capensis, ∼37 g), laughing dove (Spilopelia senegalensis, ∼89 g) and Cape turtle dove (Streptopelia capicola, ∼148 g)] and Australia [crested pigeon (Ocyphaps lophotes), ∼186 g] at air temperatures (T a ) of up to 62°C. There was no clear relationship between body mass and maximum T a tolerated during acute heat exposure. Maximum T b at very high T a was 43.1±1.0, 43.7±0.8, 44.7±0.3 and 44.3±0.8°C in Namaqua doves, laughing doves, Cape turtle doves and crested pigeons, respectively. In all four species, RMR increased significantly at T a above thermoneutrality, but the increases were relatively modest with RMR at T a =56°C being 32, 60, 99 and 11% higher, respectively, than at T a =35°C. At the highest T a values reached, evaporative heat loss was equivalent to 466, 227, 230 and 275% of metabolic heat production. The maximum ratio of evaporative heat loss to metabolic production observed in Namaqua doves, 4.66, exceeds by a substantial margin previous values reported for birds. Our results support the notion that cutaneous evaporation provides a highly efficient mechanism of heat dissipation and an enhanced ability to tolerate extremely high T a .
Evaporative heat loss pathways vary among avian orders, but the extent to which evaporative cooling capacity and heat tolerance vary within orders remains unclear. We quantified the upper limits to thermoregulation under extremely hot conditions in five Australian passerines: yellow-plumed honeyeater (; ∼17 g), spiny-cheeked honeyeater (; ∼42 g), chestnut-crowned babbler (; ∼52 g), grey butcherbird (; ∼86 g) and apostlebird (; ∼118 g). At air temperatures () exceeding body temperature (), all five species showed increases in to maximum values around 44-45°C, accompanied by rapid increases in resting metabolic rate above clearly defined upper critical limits of thermoneutrality and increases in evaporative water loss (EWL) to levels equivalent to 670-860% of baseline rates at thermoneutral Maximum cooling capacity, quantified as the fraction of metabolic heat production dissipated evaporatively, ranged from 1.20 to 2.17, consistent with the known range for passerines, and well below the corresponding ranges for columbids and caprimulgids. Heat tolerance limit (HTL, the maximum tolerated) scaled positively with body mass, varying from 46°C in yellow-plumed honeyeaters to 52°C in a single apostlebird, but was lower than that of three southern African ploceid passerines investigated previously. We argue this difference is functionally linked to a smaller scope for increases in EWL above baseline levels. Our data reiterate the reliance of passerines in general on respiratory evaporative heat loss via panting, but also reveal substantial within-order variation in heat tolerance and evaporative cooling capacity.
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