The common hippopotamus is a large, semi-aquatic mammal native to the rivers and lakes of sub-Saharan Africa. Due to harsh seasonal changes, it is often wondered if the hippo employs a survival strategy like hibernation. Hippos do not hibernate; their survival mechanisms rely on behavioral changes and unique physiological adaptations rather than metabolic dormancy.
The Direct Answer: Why Hippos Don’t Hibernate
Hibernation is a state of prolonged inactivity used by mammals to survive cold periods, involving a dramatic drop in body temperature and suppressed metabolic rate. Hippos, however, inhabit tropical and subtropical regions where cold is not the primary survival threat.
The hippo’s large size and high metabolic rate make hibernation physiologically unfeasible. A large body requires immense energy to cool down and reheat, which would negate any energy savings from a metabolic slowdown. Hippos maintain a high and stable body temperature and lack the biological mechanisms necessary for the sustained hypothermia that defines true hibernation. Their primary survival challenge is not cold but the intense heat and drought of the dry season.
Understanding Torpor and Aestivation
Confusion arises because other forms of dormancy exist for animals in warm environments. Aestivation is a state of dormancy some animals enter in response to extreme heat or drought, essentially acting as “summer hibernation.” This reduces metabolic activity and the need for water during periods when resources are scarce.
Torpor is a less extreme, short-term state of reduced physiological activity, often lasting less than 24 hours, used to conserve energy during food scarcity or cold snaps. While small warm-weather animals like rodents or amphibians may utilize aestivation or torpor, the hippo’s lifestyle prevents it from undergoing prolonged metabolic dormancy. Instead of shutting down their metabolism, hippos rely on behavioral modifications to conserve energy and manage heat.
Hippo Strategies for Surviving Drought
When drought hits their African habitat, water bodies shrink, and the hippo’s survival depends on behavioral adaptations. Hippos, which are semi-aquatic, congregate in the remaining pools of water or mud to keep their skin cool and hydrated, leading to severe overcrowding. This forced proximity can increase aggression and competition for dwindling resources, often leading to higher mortality rates.
The hippo’s skin is extremely sensitive to the sun and drying out, requiring them to spend up to 16 hours a day submerged. When exposed, their subdermal glands secrete a thick, reddish-orange fluid often mistakenly called “blood sweat.” This fluid, containing hipposudoric acid (red) and norhipposudoric acid (orange), acts as a sunscreen and antimicrobial agent, protecting their skin from ultraviolet light and infections. Additionally, hippos reduce their activity and range, traveling short distances at night to graze on sparse grass, minimizing energy expenditure until the rains return.