Allocation of acquired resources to phenotypic traits is affected by resource availability and current selective context. While differential investment in traits is well documented, the mechanisms driving investment at lower levels of biological organization, which are not directly related to fitness, remain poorly understood. We supplemented adult male and female Anolis carolinensis lizards with an isotopically labelled essential amino acid (13C-leucine) to track routing in four tissues (muscle, liver, gonads and spleen) under different combinations of resource availability (high- and low-calorie diets) and exercise training (sprint training and endurance capacity). We predicted sprint training should drive routing to muscle, and endurance training to liver and spleen, and that investment in gonads should be of lower priority in each of the cases of energetic stress. We found complex interactions between training regime, diet and tissue type in females, and between tissue type and training, and tissue type and diet in males, suggesting that males and females adjust their 13C-leucine routing strategies differently in response to similar environmental challenges. Importantly, our data show evidence of increased 13C-leucine routing in training contexts not to muscle as we expected, but to the spleen, which turns over blood cells, and to the liver, which supports metabolism under differing energetic scenarios. Our results reveal the context-specific nature of long-term trade-offs associated with increased chronic activity. They also illustrate the importance of considering the costs of locomotion in studies of life-history strategies.

Author contributions

Conceptualization: J.F.H., S.P.L.; Methodology: J.F.H., S.P.L.; Validation: J.F.H., S.P.L.; Formal analysis: J.F.H., S.P.L.; Investigation: J.F.H., S.P.L.; Resources: J.F.H.; Data curation: S.P.L.; Writing - original draft: J.F.H., S.P.L.; Writing - review & editing: J.F.H., S.P.L.; Visualization: S.P.L.; Supervision: J.F.H.; Project administration: J.F.H.

Funding

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

Data availability

The data that support the findings of this study are openly available in the figshare repository: https://figshare.com/s/9a0476b96948c29adc5d. The repository has four files: the main dataset of all data used in a CVS file, isotope_ms_R; a text document with details, README; the R code used to conduct the analysis, Ac isotope ms; and an HTML file with a walkthrough of the analyses, Ac-isotope-ms. For additional information, please contact the authors.

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