Parental care increases endothermy of striped hamsters (Cricetulus barabensis) rearing different litter size.

Endothermy is one of the most distinctive physiological traits of mammals, especially during lactation. In this phase, heightened metabolic heat production strengthens endothermy and improves parental care effectiveness, thereby increasing reproductive output. However, it also raises the risk of life-threatening hyperthermia. The precise trade-off governing this balance remains poorly understood. We measured parental care behavior, endothermy, and small intestine digestive enzyme activity and cytochrome c oxidase (COX) activity in brown adipose tissue (BAT) and liver, in striped hamsters with litter sizes ranging from 0 to 12. Maternal body temperature increased significantly with larger litter sizes, accompanied by parallel increases in energy intake, metabolic rate, and milk energy output. These changes were associated with significantly heavier small intestines, kidneys, and mammary glands, as well as elevated digestive enzyme and liver COX activities. Larger litters also led to more time spent suckling, resting and feeding. These variables established a relatively linear relationship, reflecting an adaptive mechanism in the digestive tract and related organs that optimizes energy allocation for parental care, enhances endothermy, and increases reproductive success. Notably, all these variables reached a plateau, while BAT thermogenesis declined with larger litter sizes. These findings align with the parental care model for evolution of endothermy, while also matching the key predictions of heat dissipation limit theory. A trade-off model may be employed: parental care necessitates substantial heat generation, yet limited heat dissipation capacity requires downregulating thermogenic activity. This balancing act may be key to endothermy evolution, increasing reproductive success while avoiding maternal hyperthermia.