Piglets are sensitive to cold and shiver to maintain their body heat. Researchers at Uppsala University have uncovered a genetic reason why these newborns are less tolerant of the cold than other newborn mammals. It turns out that the gene that codes for the protein UCP1 was inactivated some 20 million years ago in the evolutionary line to which pigs belong. These findings, available online (http://dx.doi.org/10.1371/journal.pgen.0020129), are presented in the peer-reviewed, open-access journal PLoS Genetics.
Brown fat helps newborn mammals maintain their body temperature by burning fat, which converts into heat. The protein UCP1 (Uncoupling Protein 1) has a key role in this energy conversion, which takes place in the cell mitochondria.
No brown fat or UCP1 protein has been found in domesticated pigs, however. In their study, Berg and colleagues show that the UCP1 gene was shut down about 20 million years ago in an ancestor of the wild boar. They identified four different mutations, each of which would be sufficient to knock out the function of the protein.
This ancestor of pigs thereby lost the ability to use brown fat to maintain body temperature after birth. A reasonable explanation for this is that brown fat was not essential during a period in the evolution of pigs, in which they lived in a warm climate, says Leif Andersson, who directs the research team.
The ancestor of the domesticated pig, the wild boar, is the only pig that lives in cold climates. All other species inhabit tropical or subtropical climates. The wild boar has compensated for the loss of brown fat by a series of adaptations for survival in a cold climate. It is the only hoofed animal that builds a den when it is time to give birth see (http://www.plos.org/press/related-press-image-1), and its young shudder to maintain their body temperature. In modern pig production, heat lamps are used to help the newborn piglets retain their body temperature (see http://www.plos.org/press/related-press-image-2).
The findings show that an important biological function can be lost if it is not vital to life during a period in the evolutionary history of a species; and that if the living conditions once again change, compensatory mechanisms can be developed. The findings we present are fully consistent with the theory of evolution. An important trait can be lost if it is not absolutely necessary to life during the development of a species, says Leif Andersson.