Ground squirrels, birds, shrews, and bears all regulate body temperature. Their environments differ — arctic tundra, temperate forest, tropical canopy. Their sizes differ by orders of magnitude. Their metabolic strategies differ — some hibernate, some maintain continuous endothermy, some use torpor. The surface area to volume ratios, insulation mechanisms, and behavioral responses are all different.
FitzGerald, Levesque, Humphries, and Bhatt (arXiv:2504.00359) find that the core control structure is the same. The thermoregulatory dynamics across all these species follow the same mathematical framework. The controller is universal — the same feedback architecture, the same stability properties, the same response characteristics. What differs between species is how the controller couples to the environment — the sensitivity parameters, the gain coefficients, the setpoints.
This is convergent control, not convergent mechanism. The animals don't thermoregulate the same way. They use different effectors (shivering, panting, vasodilation, behavioral modification), different sensors (peripheral and central thermoreceptors with different distributions), different setpoints (34°C to 42°C). But the control architecture — the relationship between error signal, controller output, and environmental coupling — is the same structure.
The implication: thermal control in endotherms was not independently invented multiple times with coincidentally similar solutions. It was constrained — by physics, by the mathematics of feedback control, by the requirement that body temperature be maintained against perturbation — to converge on the same dynamical structure. The control problem has a small number of stable solutions. Evolution found the same one repeatedly because the problem only has one good answer.
The general observation: when a control problem is tightly constrained by physics, the solutions converge across independent evolutionary lineages. The universality is not in the mechanism but in the architecture. Different parts, same blueprint. The constraint is the explanation.