Induction of Metamorphosis Causes Differences in Sex-Specific Allocation Patterns in Axolotls (Ambystoma mexicanum) that Have Different Growth Histories
We tested the hypothesis that salamanders growing at different rates would have allocation patterns that differ among male and female metamorphic and larval salamanders.We raised individual axolotls, Ambystoma mexicanum, on four food regimes: constant high growth (throughout the experiment), constant low growth (restricted throughout the experiment), high growth switched to low growth (ad libitum switched after 140 d to restricted), and low growth switched to high growth (restricted switched after 140 d to ad libitum). Because axolotls are obligate paedomorphs, we exposed half of the salamanders to thyroid hormone to induce metamorphosis. We assayed growth and dissected and weighed gonads and fat bodies. Salamanders that were switched from restricted to ad libitum food regime delayed metamorphosis. In all treatment groups, females had larger gonads than males and males had larger fat bodies than females. The association between storage and reproduction differed between larvae and metamorphs and depended on sex.Abstract
Growth profiles for axolotls grown in four food regimes. Diamonds are LL growth treatments, squares are LH growth treatments, triangles are HL growth treatments, and circles are HH growth treatments. Dashed line indicates the initiation of treatment with thyroxine to induce metamorphosis in half of animals. Open symbols after induction of metamorphosis are growth profiles for larvae (i.e., untreated salamanders). Growth profiles for metamorphs terminate at mean larval period. Error bars are ±1 SE.
Treatment-group means of four allocation variables for larval (i.e., untreated salamanders) and metamorphic (i.e., salamanders treated with TH) axolotls. Because we used six preplanned pairwise comparisons, we calculated a Bonferroni correction to set α = 0.0083 as significance criterion in these paired LSD comparisons. Letters next to means indicate means significantly different. Open squares = LL food treatment; open circles = LH food treatment; filled squares = HH food treatment; filled circles = HL food treatment. Error bars are ±1 SE.
Allocation to reproduction (gonad mass) and storage (fat body mass) in male and female axolotls. Closed symbols are larval axolotls, open symbols are metamorphic axolotls (i.e., those treated with TH). Arrows are drawn from larvae to the metamorphs for sex and food regimes. Arrows are included to emphasize the effect of TH treatment on allocation to reproduction and storage. Diamonds are LL growth treatments, squares are LH growth treatments, triangles are HL growth treatments, and circles are HH growth treatments.
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