Abstract
In a 1998 paper entitled “Guts don’t fly: small digestive organs in obese bar-tailed godwits,” Piersma and Gill (1998) showed that the digestive organs were tiny and the fat loads huge in individuals suspected of embarking on a non-stop flight from Alaska to New Zealand. It was suggested that prior to migratory departure, these godwits would shrink the digestive organs used during fuel deposition and boost the size and capacity of exercise organs to optimize flight performance. Here we document the verity of the proposed physiomorphic changes by comparing organ sizes and body composition of bar-tailed godwits Limosa lapponica baueri collected in modesty midway during their fueling period (mid-September; fueling, n = 7) with the previously published data for godwits that had just departed on their trans-Pacific flight (October 19; flying, n = 9). Mean total body masses for the two groups were nearly identical, but nearly half of the body mass of fueling godwits consisted of water, while fat constituted over half of total body mass of flying godwits. The two groups also differed in their fat-free mass components. The heart and flight muscles were heavier in fueling godwits, but these body components constituted a relatively greater fraction of the fat-free mass in flying godwits. In contrast, organs related to digestion and homeostasis were heavier in fueling godwits, and most of these organ groups were also relatively larger in fueling godwits compared to flying godwits. These results reflect the functional importance of organ and muscle groups related to energy acquisition in fueling godwits and the consequences of flight-related exertion in flying godwits. The extreme physiomorphic changes apparently occurred over a short time window (≤1 month). We conclude that the inferences made on the basis of the 1998 paper were correct. The cues and stimuli which moderate these changes remain to be studied.
Highlights
The bodies of individual animals are in constant material and energy flux, and constantly changing in mass and composition as individuals mature and age and make adjustments to external conditions like weather and food
We present the compositional analysis of a new sample of fueling godwits in comparison with the published in-flight sample, which substantiates the suggestions made by Piersma and Gill (1998) about organ-restructuring for migration
This included the fresh mass of the primary exercise organs, the heart (Figure 1E; F1,13 = 8.4, p = 0.01) and the flight muscles (Figure 1G; F1,13 = 11.4, p = 0.004), which were heavier in bar-tailed godwits sampled during fueling than during flight
Summary
The bodies of individual animals are in constant material and energy flux, and constantly changing in mass and composition as individuals mature and age and make adjustments to external conditions like weather and food. Such changes may reflect seasonally changing “goals and ambitions” – whether these relate to reproduction, molt or migration (Piersma and van Gils, 2011). The greater the variability in external demands in response to the manifold seasonal challenges, the greater the corporeal changes with respect to physiological and morphological phenotypic traits one come to expect. Among the great diversity of avian life forms, the extreme endurance migrants (Piersma, 2011; Conklin et al, 2017) would be expected to embody the most extensive physiomorphic changes
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