Birds, unlike mammals, lack a tissue that is specialized to generate heat. A
team of researchers at New York Medical College writes that the same lack of
heat-generating tissue may have contributed to the extinction of ...
Humans, like all mammals, have two kinds of adipose tissue, white fat and
brown fat. White fat is used for storing energy-rich fuels, while brown fat
generates heat. Hibernating bears have a lot of brown fat, as do human
infants, who have much more than adults, relative to their body size.
Infants' brown fat protects them from hypothermia. Clinicians would like to
find ways of making adult white fat behave more like brown fat so that we
could burn, rather than store, energy.
While most mammals have a key gene called UCP1, which is responsible for the
heat-generation function of brown fat, birds do not. The researchers found
they could induce a specific type of stem cell in chicken embryos to produce
differentiated cells that are structured and behave like brown fat. These
chicken cells can even activate a UCP1 gene if presented with one from a
The ability to produce brown fat evolved in a common ancestor of birds and
mammals, but the ability to generate heat was lost in the group that gave
rise to birds and lizards after it separated from the mammalian lineage (the
researchers found the lizard genome similarly lacks a UCP1 gene). This
strongly implies that dinosaurs, which diverged from birds even later than
lizards, also lacked brown fat.
Article: "The brown adipocyte differentiation pathway in birds: an
evolutionary road not taken," by Stuart A. Newman, Ph.D., professor of cell
biology and anatomy, Nadejda Mezentseva, a Ph.D. candidate at New York
Medical College, and Jaliya Kumaratilake, Ph.D., University of Adelaide,