Basal metabolic rate (BMR) is the amount of energy needed for a specific organism to maintain homeostasis with daily function. While human BMR is mostly relies on lean body mass such that an average man has an average BMR of around 7100kJ per day compared to 5,900KJ per day to an average woman, other species such as mammals, fish, birds, reptile BMR are affected by not just their body mass, but also climate, habitats, diets, gender, environmental temperature, exposure to disease/illness etc… Even some organisms such as bears enters a state of torpor through hibernation in winter to slow down their BMR. Past research shows that total BMR are much higher in larger species such as mammals compared to smaller species such as birds. In one study by Speakman, John “Body size, energy metabolism and lifespan” the Old research such as Rubner (1908) compared BMR and lifespan of guinea pig, cat, dog, cow, and horse show there were not much difference in expenditure per gram per lifespan even though there was a large proportional difference in body mass between cat and a cow. This led to early theory such as rate of living where total energy expenditure per lifespan is a fixed ratio, assuming higher BMR correlates with shorter lifespan. However, in some studies, it shows smaller endotherm (organisms that uses metabolic heat to regulate body temperature) organisms tend to have higher per gram body mass metabolic rate than larger organisms – thus raise the question whether there is a correlation between BMR and lifespan
