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First published online April 17, 2009
Journal of Experimental Biology 212, 1237-1247 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.023945

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Atmospheric oxygen level affects growth trajectory, cardiopulmonary allometry and metabolic rate in the American alligator (Alligator mississippiensis)

Tomasz Owerkowicz^1,*, Ruth M. Elsey² and James W. Hicks¹

¹ Ecology and Evolutionary Biology, University of California, Irvine, CA 92697, USA
² Rockefeller Wildlife Refuge, Louisiana Department of Wildlife and Fisheries, Grand Chenier, LA 70643, USA

View larger version (24K): [in this window] [in a new window]   Fig. 1. Comparison of alligator hatchlings incubated under three different oxygen levels (12%, 21% and 30%). (A) Mass measurements: total, body and yolk masses. Hypoxic hatchlings are significantly smaller than their normoxic and hyperoxic siblings, but the remaining yolk sac of hypoxic animals is significantly larger. (B) Length measurements: total, snout-to-vent and head lengths. Hypoxic hatchlings are significantly smaller than their normoxic and hyperoxic siblings. (C) A pair of anaesthetised alligator siblings, incubated under hypoxia (above) and normoxia (below). Note the diminutive hatchling size and the protruding yolk sac in the hypoxic animal. The yolk sac is completely incorporated into the abdominal cavity and the umbilical scar closed, but the abdominal skin is stretched thin and a pronounced left umbilical vein is seen. The height of the yolk sac exceeds the length of the limbs, making locomotion cumbersome. Statistical significance between groups was calculated by ANOVA with post hoc Tukey–Kramer (*P<0.05). Bar height and error bars indicate the mean ± s.e.m. for each group.

View larger version (8K): [in this window] [in a new window]   Fig. 2. Growth curves of alligator juveniles under three oxygen levels (hypoxia, normoxia and hyperoxia) over 3 months post-hatching. (A) Body mass and (B) total length growth of alligators. Hypoxic animals grew slowest, and hyperoxic animals grew fastest in terms of body mass and total length. Symbols with error bars indicate the mean ± s.e.m. for each group.

View larger version (16K): [in this window] [in a new window]   Fig. 3. (A–C) Absolute wet masses of major visceral organs at hatching and 3 months later: (A) liver, (B) lungs and (C) heart. All organs are significantly smaller in hypoxic alligators at both ages (ANOVA with post hoc Tukey–Kramer test, *P<0.05). Symbols and error bars indicate the mean ± s.e.m. (D–F) Ontogenetic allometry of major visceral organs in alligators reared under hypoxia, normoxia and hyperoxia. (D) Liver scales to Mb0.80 (Mb, body mass) in all groups, but is significantly smaller in hypoxic animals. (E) Lungs scale similarly (Mb0.73) in normoxia and hyperoxia, but exhibit a significantly steeper slope (Mb1.44) in the hypoxic group. (F) Heart scales with slight positive allometry (Mb1.07) in all groups, but is significantly larger in hypoxic alligators.

View larger version (4K): [in this window] [in a new window]   Fig. 4. RV/LVS ratio of the right ventricle (free wall only) to left ventricle (free wall and interventricular septum) in hatchling and juvenile alligators. In both age groups, the ratio is significantly higher in hypoxic animals than in either their normoxic or hyperoxic siblings, but no significant difference exists between the last two groups. Statistical significance between groups was calculated by Wilcoxon/Kruskal–Wallis rank sums test with post hoc Tukey–Kramer (*P<0.05).

View larger version (3K): [in this window] [in a new window]   Fig. 5. Haematocrit levels in hatchling and juvenile alligators reared under chronic hypoxia, normoxia and hyperoxia. Haematocrit level is significantly higher in hypoxic animals, but not significantly different between normoxic and hyperoxic animals. Statistical significance between groups was calculated by Wilcoxon/Kruskal–Wallis rank sums test with post hoc Tukey–Kramer (*P<0.05).

View larger version (8K): [in this window] [in a new window]   Fig. 6. Differences in metabolic rate of juvenile alligators in different oxygen atmospheres under absorptive and post-absorptive (standard) conditions. Absorptive (AMR) and standard (SMR) metabolic rates are expressed as mass-corrected oxygen consumption rate. Absolute (AMR–SMR) and relative (AMR:SMR) metabolic elevation due to digestion/absorption are also plotted. Metabolic rates and absolute metabolic elevation are highest in hyperoxic alligators. Statistical significance between groups was calculated by ANOVA with post hoc Tukey–Kramer (*P<0.05). Bar height and error bars indicate the mean ± s.e.m.

View larger version (9K): [in this window] [in a new window]   Fig. 7. Breathing rate (left) and mass-corrected oxygen consumption per breath (right) of 3 month old alligators in different oxygen atmospheres under absorptive and post-absorptive (standard) conditions. Hyperoxic alligators show significantly lower breathing rates than their siblings in normoxic and hypoxic groups. Conversely, each breath supports greater oxygen consumption in hyperoxic animals than in other groups. Statistical significance between groups was calculated by ANOVA with post hoc Tukey–Kramer (*P<0.05). Bar height and error bars indicate the mean ± s.e.m.

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