The ordinary lard is more than familiar to most of us. However, mammals also have a different fatty tissue called brown fat, involved in generating heat. A new study made at New York Medical College and published in the journal BMC Biology has discovered why birds lack this tissue. In the end, birds are actually living dinosaurs, and the lack of heat-generating brown tissue may have been one of the factors that led to the extinction of dinosaurs.

Humans and mammals have these two kinds of fatty tissue, white fat and brown fat. The former type has the role of simply storing energy reserves for the body, while brown fat burns lipids, thus releasing heat.

Bears, for instance, store a lot of brown fat before entering their dens for hibernation. Human infants possess much more brown fat than adults do, only when compared to their body size, as this tissue is meant to defend them against hypothermia.

Many researches are now investigating possible pathways of turning adult white fat into brown fat, which rather burns than stores lipids. Brown fat has been found to originate from the same stem cells that muscles are made of, while ordinary white fat originates from the stem cells generating the skin. Mammals have a gene called UCP1 that is involved in the heat-generation of brown fat, while birds do not possess this gene.

The team made a specific strain of stem cells in chicken embryos that produce differentiated cells mimicking brown fat in structure and activity. When a mouse UCP1 gene was inserted in the genome of these chickens, their cells could even activate it.

It appears that the brown fat tissue emerged in a common basal reptilian ancestor of both birds and mammals. The UCP1 gene seems to have been lost in diapsida reptiles, the group that gave birth to lizards, crocodiles and dinosaurs, the ancestors of birds, after breaking apart from the lineage that led to mammals (lizards too lack the UCP1 gene in their genome).

This explains why lizards do not have brown fat but also points to the fact that dinosaurs, from which birds evolved, were devoid of brown fat, too.