Conventionally, dynamic energy budget (DEB) models operate with animals that have maintenance rates scaling with their body volume, and assimilation rates scaling with body surface area. However, when applying such criteria for the individual in a population level model, the emergent behaviour of the conventional model apparently only reflects juveniles and not adult animals. This paper discusses the relevance of what level assumptions are made on, and the subsequent impact on interpreting the animal (top-down or bottom-up). The alternative DEB model has maintenance scaling with body area, and assimilation with body volume—the opposite of the conventional energy budget animal. Likewise, scaling of organism function to body mass is emphasized to take into account the different challenges organisms face when growing in size. It is emphasized that homoeostasis and its challenges are continuously changing, and cannot be assumed constant. The perspective is finalized by a discussion on perceiving animals as machines, and how it can maybe serve as a lingua franca for physiologists and modellers alike.