Table S1: Forward Exclusion Model Selection Procedure. Models Included at Least One Pair

Table S1: Forward exclusion model selection procedure. Models included at least one pair ofecological variable within- and between-species fixed effects and species as a random factor. Covariates were added as fixed effects or as interactions with each of the within-species terms of ecological variables to see if they improved model fit. The effects of ecological variables, body type, ventricle type,and sexwere assessed sequentially. If a more complex model improved model fit (ΔAIC > 2), the model of best fit was carried over to the next step assessing the following effect in the sequence. The best fit model chosen under the above constraints is indicated by two asterisks. Abbreviations: AIC: Akaike information criterion,BC: between-species coupling, BLC: between-species littoral carbon use, BT: body type, BTP: between-species trophic position, rHeart: relative heart size, SPP: species, VT: ventricle type, WC: within-species coupling, WLC: within-species littoral carbon use, WTP: within-species trophic position.

Models / AIC
One, two, and threeecological variable pairs
rHeart = WTP + BTP + (0+WTP|SPP) / -402.03
rHeart = WLC + BLC + (0+WLC|SPP) / -430.75
rHeart = WC + BC + (0+WC|SPP) / -363.97
rHeart = WTP + BTP + WLC + BLC + (0+WTP|SPP) + (0+WLC|SPP) / -457.05
rHeart = WTP + BTP + WC + BC + (0+WTP|SPP) + (0+WC|SPP) / -397.21
rHeart = WC + BC + WLC + BLC + (0+WC|SPP) + (0+WLC|SPP) / -430.45
rHeart = WTP + BTP + WLC + BLC + WC + BC + (0+WTP|SPP) + (0+WLC|SPP) + (0+WC|SPP) / -455.24
Body type
rHeart = WTP + BTP + WLC + BLC + BT + (0+WTP|SPP) + (0+WLC|SPP) / -459.79**
rHeart = WTP*BT + BTP + WLC + BLC + (0+WTP|SPP) + (0+WLC|SPP) / -457.82
rHeart = WTP + BTP + WLC*BT + BLC + (0+WTP|SPP) + (0+WLC|SPP) / -457.92
rHeart = WTP*BT + BTP + WLC*BT + BLC + (0+WTP|SPP) + (0+WLC|SPP) / -455.96
Ventricle type
rHeart = WTP + BTP + WLC + BLC + BT + VT + (0+WTP|SPP) + (0+WLC|SPP) / -458.54
rHeart = WTP*VT + BTP + WLC + BLC + BT + (0+WTP|SPP) + (0+WLC|SPP) / -457.66
rHeart = WTP + BTP + WLC*VT + BLC + BT + (0+WTP|SPP) + (0+WLC|SPP) / -457.98
rHeart = WTP*VT + BTP + WLC*VT + BLC + BT + (0+WTP|SPP) + (0+WLC|SPP) / -457.01
Sex
rHeart = WTP + BTP + WLC + BLC + BT +Sex + (0+WTP|SPP) + (0+WLC|SPP) / -458.74
rHeart = WTP*Sex + BTP + WLC + BLC + BT + (0+WTP|SPP) + (0+WLC|SPP) / -458.82
rHeart = WTP + BTP + WLC*Sex + BLC + BT + (0+WTP|SPP) + (0+WLC|SPP) / -459.54
rHeart = WTP*Sex + BTP + WLC*Sex + BLC + BT + (0+WTP|SPP) + (0+WLC|SPP) / -459.69

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