Final answer:
Newborns and young children have considerable amounts of brown fat tissue, which helps in regulating body temperature through heat production. In adults, it is less common and mainly found in those living in cold climates. The significant role of this tissue is most critical in the neonatal period for nonshivering thermogenesis.
Step-by-step explanation:
Females and males have significant amounts of brown fat tissue primarily during the newborn phase and in early childhood. This is because newborns and young children rely heavily on nonshivering thermogenesis for regulating their body temperature due to their relatively thin layer of subcutaneous fat, which provides less insulation. The brown adipose tissue, abundant in neonates, is rich in mitochondria and possesses the protein thermogenin, which uncouples oxidative phosphorylation, resulting in high heat production instead of creating large amounts of ATP.
In adults, the presence of brown fat is much less common, typically found in small amounts in the neck and clavicular regions. However, adults living in consistently cold climates can retain higher levels of this tissue. Brown fat functions by automatically breaking down in response to cold exposure, which is vital for newborns whose bodies lose heat rapidly because of their larger surface area to mass ratio.