DEB theory’s tetrahedron of life links feeding and reproduction to trophic position of species

Dynamic Energy Budget (DEB) theory’s tetrahedron of life links the inner to the outer world of individuals, beyond energy and mass conservation. It follows mathematically from the some assumptions behind DEB theory, without using new assumptions and links the supply stress, the fraction of assimilation to reproduction and the minimum scaled functional response to reach puberty all to the fraction of mobilized reserve to soma: the cornerstone of life. The supply stress quantifies the supply–demand spectrum: supply-species eat what is available, while demand-species eat what they need. The minimum scaled functional response to reach puberty increases with the supply stress, which implies that the half-saturation coefficient needs to decrease with the supply stress to ensure that the feeding rate is close to its maximum for species near the demand end of the supply–demand spectrum. This coefficient is the ratio of the specific ingestion and food searching rates, so the food searching rate needs to increase with the supply stress. This is in harmony with the recent finding that the factorial aerobic scope increases from 3.2 for zero supply stress to 32 for maximum supply stress; a high food searching rate requires a high peak metabolism. It turns out that the supply stress needs to be low for the allocation to reproduction to be large. The supply stress has direct links with the trophic position and the precociality index, i.e. the ratio of the maturity levels at birth and puberty. The lower the fraction of assimilation to reproduction, the lower the life-time cumulated neonate mass production as well as the precociality index within a variety of taxa within the Add_my_Pet (AmP) collection of energetic data and DEB parameters. The intra-class variation of the precociality index with the fraction of assimilation to reproduction turns out to differ from the inter-class variation. Invertebrates were found to have a low supply stress, have a low trophic position, allocate a large fraction of their assimilation to reproduction and make relatively small eggs. Tetrapods, especially birds and mammals, have a high supply stress, a high trophic position, invest little in reproduction and their neonates are relatively large. This coupling of traits follows from the structure of DEB theory and is empirically confirmed by the 7334 animal species in the AmP collection for a wide range of taxa. Species with a high trophic position must have a food intake close to their maximum and cannot invest much in reproduction. The combined implications for feeding and reproduction avoids or reduces over-exploitation of resources in the food web and might contribute to the dynamic stability of food webs and to the maintenance of biodiversity in ecosystem models, challenging fitness maximization as central theme in evolution biology. The tetrahedron provides strong support for the central position of the supply–demand spectrum in animal energetics.