When compared to other animal newborns, human infants are born underdeveloped. Human babies cannot walk, feed themselves, or support their own heads. Infants also lack another survival skill: the ability to shiver as a means of thermoregulation.
Thermoregulation allows humans to maintain a stable internal body temperature. Healthy human adults have internal body temperatures between 97°F (36.1°C) and 99°F (37.2°C). If a human’s body temperature is outside of this range, it will enact one or more thermoregulatory mechanisms to restore a normal body temperature. Sweating and shivering are types of thermoregulatory mechanisms.
Humans shiver in response to feeling cold. Shivering causes muscles to rapidly expand and contract. The rapid muscle movements consume energy, which generates warmth. While human children and adults shiver, human infants do not have the shivering reflex. It seems as if babies would need this ability the most, as their larger heads and larger surface areas make them particularly vulnerable to becoming cold. Infants also have an underdeveloped nervous system that responds slowly to external stimuli, such as colder conditions. So why hasn’t evolution given babies the ability to shiver? It is because infants have a better means of thermogenesis, called brown adipose tissue (BAT), also known as “brown fat.”
BAT (see figure below, left panel) is structurally different than white adipose tissue (WAT; “white fat”), the more familiar type of human fat. WAT (right panel) cells contain a single, large fat (lipid) droplet, which stores excess energy gained from consuming too many calories. The body will utilize these energy reserves to maintain proper function during periods of severe hunger or starvation. WAT was particularly useful to our ancient ancestors, who could not rely on a steady food supply. BAT has many small lipid droplets, and more capillaries for greater oxygen consumption. BAT also contains mitochondria, which convert energy in eukaryotic cells. The mitochondria contain iron, giving brown fat its brownish color. Unlike white fat, BAT is an energy-expending tissue, generating heat by prompting mitochondria to consume lipids as energy. When infants are too cold, they activate their brown fat to stave off cold. While brown fat may keep babies warm, it is not intended as a replacement for blankets and warmer clothes. Research suggests that depleting BAT reserves may take away from the calories that babies would otherwise use to grow.
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