We describe a new link between physiological development of a highly derived mammalian tissue, and life history constraints. The heads of vertebrates are recognized as potentially being the body part that shows the most diversity. Toothed whales have evolved very complex and unique cranial adipose tissues for acoustic transmission and reception (echolocation). We show that 1) acoustic tissues develop according to complex, pre-determined patterns established in utero; 2) rates of post-parturition development depend on species-specific timing of independence; early weaning species (porpoises,