The relative energetic investment in reproduction between the sexes forms the basis of sexual selection and life history theories in evolutionary biology. It is often assumed that males invest considerably less in gametes than females, but quantifying the energetic cost of gamete production in both sexes has remained a difficult challenge. For a broad diversity of species (invertebrates, reptiles, amphibians, fishes, birds, and mammals), we compared the cost of gamete production between the sexes in terms of the investment in gonad tissue and the rate of gamete biomass production. Investment in gonad biomass was nearly proportional to body mass in both sexes, but gamete biomass production rate was approximately two to four orders of magnitude higher in females. In both males and females, gamete biomass production rate increased with organism mass as a power law, much like individual metabolic rate. This suggests that whole-organism energetics may act as a primary constraint on gamete production among species. Residual variation in sperm production rate was positively correlated with relative testes size. Together, these results suggest that understanding the heterogeneity in rates of gamete production among species requires joint consideration of the effects of gonad mass and metabolism.

Aim The existence of a body size hierarchy across trophic connections is widely accepted anecdotally and is a basic assumption of many food-web models. Despite a strong theoretical basis, empirical evidence has been equivocal, and in general the relationship between trophic level and body size is often found to be weak or non-existent.Location Global (aquatic). MethodsUsing a global dataset for fishes (http://www.fishbase.org), we explored the relationship between body size and trophic position for 8361 fishes in 57 orders.Results Across all species, trophic position was positively related to maximum length (r 2 = 0.194, b = 0.065, P < 0.0001), meaning that a one-level increase in trophic level was associated with an increase in maximum length by a factor of 183. On average, fishes in orders that showed significantly positive trophic level-body size relations [mean = 51.6 cm Ϯ 11.8 (95% confidence interval, CI)] were 86 cm smaller than fishes in orders that showed no relation [mean = 137.1 cm Ϯ 50.3 (95% CI), P < 0.01]. A separate slopes model ANCOVA revealed that maximum length and trophic level were positively correlated for 47% (27 of 57) of orders, with two more orders showing marginally non-significant positive relations; no significant negative correlations were observed. The full model (order ¥ body size) explained 37% of the variation between body size and trophic position (P < 0.0001). Main conclusionsOur results support recent models which suggest that trophic level and body size should be positively correlated, and indicate that morphological constraints associated with gape limitation may play a stronger role in determining body size in smaller fishes. Differences among orders suggest that the nature of the trophic level-body size relation may be contingent, in part, on evolutionary history.

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Aim We test the 'energetic equivalence rule' (EER) -the idea that the amount of energy used by a population per unit area per unit time is independent of body mass -in meio-invertebrate communities from a series of natural, multitrophic aquatic 'rock pool' microcosms. Our study represents the first rigorous test of the EER at local scales of observation in a community of naturally coexisting species.Location Discovery Bay, Jamaica.Method We estimated population energy use (PEU) for every occurrence of every species of meio-invertebrate fauna found in each of 29 microcosms (233 observations of 31 species) using estimates of population density obtained in January 2005 in combination with published metabolism-mass relations for closely related taxa. ResultsIn the rock pool system as a whole, population density decreased ( ancova : b = -0.38 (-0.55 to -0.19), r 2 = 0.19, P < 0.001) and PEU increased with body mass ( ancova : b = 0.55 (0.36-0.73), r 2 = 0.28, P < 0.001). Main conclusionsThe positive PEU-body mass relation found here suggests that larger organisms are energetically dominant and points to the importance of sizestructured competition in these systems. Our results contrast those obtained in the few other previously published tests of the EER and challenge the idea that all species use similar amounts of energy regardless of their size.