Sustained energy intake and metabolic rate(Sus EI/MR) play an important role in geographical distribution,survival adaptation and reproductive success of animal species,but in some situations it is constrained.Sus EI/MR is not limited extrinsically by food availability or intrinsically by the capacity of animals to intake,process or absorb energy.The limits on Sus EI/MR may be imposed peripherally by the capacity of tissues to utilize energy,the so-called "peripheral limitation" hypothesis,or acted by the capacity to dissipate heat,that is the so-called "heat dissipation limitation" hypothesis.In addition,a trade-off of reproductive investment between seasons may be employed to maximize reproductive output,namely "seasonal investment" hypothesis.Some results have been reported to provide the evidences for these hypotheses.However,the mechanisms underlined the constrained Sus EI/MR are not fully explained by any of the above hypothesis.Finally,the possible research trends in this field are proposed.