What is the role of mitochondrial uncoupling proteins in mammals and birds?

The first uncoupling protein (or UCP), identified in brown adipose tissues of Mammals, UCP1, is a transporter from the internal mitochondrial membrane, that dissipates the energy from nutrient oxidations by decreasing the rate of ATP synthesis. Its physiological role in thermogenesis is clearly established, since UCP1 is necessary to maintain body temperature in newborn and hibernating animals. Other uncoupling protein homologues have also been identified in the muscles of Mammals (UCP2 and UCP3), but only one uncoupling protein has been evidenced in birds yet. The interest for these proteins was enhanced recently since they seem to be involved in the proton leak through the internal mitochondrial membrane as UCP1. Thus, they could play a role in the development of several pathologies. However, the mechanisms involved are still debated and the physiological function of these proteins is not completely clarified: a role in thermogenesis, in limiting the generation of reactive oxygen species, against obesity or oxidative damage?