Caring for offspring is energetically expensive, and when the costs of care are sufficiently high, the use of cost-reducing strategies can be favored. Such strategies may include the avoidance of nest construction through nest take-over and the restoration of depleted energy reserves through offspring cannibalism. Despite extensive theoretical and empirical work on parental care, neither the actual energetic costs of care nor the putative benefits of cost-reducing strategies have been systematically measured. Using plainfin midshipman fish, Porichthys notatus, we assessed how energy reserves of caring parents varied with duration of care, offspring cannibalism, and nest take-overs. We show that liver glycogen and lipid contents declined by 58% and 18.7%, respectively, that liver investment (measured via a hepatosomatic index) declined by 32.6%, and that muscle protein content declined by 8.8%. Other measures of body condition and energy reserves, such as hepatic glucose and adenosine triphosphate, remained stable over the extraordinarily long care period (3 months). Experimentally starved fish showed depletions of energy stores similar to caring fish. Fish that took over nests or that cannibalized eggs both had higher glycogen reserves than fish that did not adopt these strategies. These findings show that even when parental care is energetically costly, starvation may not be the dominant driving factor behind parent–offspring cannibalism.