Hippos are large, mostly aquatic mammals found throughout sub-Saharan Africa. They are known for their barrel-shaped bodies, wide mouths, and sweat glands that produce a unique oily red substance. This has led to the myth that hippos sweat green. In this article, we’ll explore the biology behind hippo sweat and find out if there is any truth to the claim that they sweat green.
Hippo Sweat Glands
Hippos have two types of sweat glands – apocrine and eccrine. Apocrine sweat glands produce an oily secretion thought to be a natural sunscreen, antiseptic, and skin moisturizer. This red-orange substance was once incorrectly thought to be blood, leading to the misconception that hippos sweat blood. However, while apocrine secretions appear red-orange, they do not actually contain hemoglobin like blood.
Eccrine glands, like in humans, produce a watery sweat to help regulate body temperature. Hippo sweat contains trace amounts of acid that turn red-orange when oxidized upon contact with air. So while the sweat itself is not colored, it appears red-orange when it mixes with air.
Composition of Hippo Sweat
In 2012, researchers analyzed the chemical composition of the secretions from hippo apocrine sweat glands. They found that hippo sweat contains:
- Water (59-63%)
- Hipposudoric acid (24–30%) – red pigment that turns red-orange upon oxidation
- Norhipposudoric acid (4-9%)
- Butyric acid (1-3%)
- Hexadecanoic acid (1.5%)
Notable compounds like indoles, quinines, and cyanogens found in sweat secretions of other animals were not present in hippo sweat. The primary pigment hipposudoric acid gives hippo sweat its red-orange color when exposed to air, not any green pigments.
Color Change of Hippo Sweat
When first secreted onto the skin, hippo apocrine sweat is colorless. Upon contact with oxygen in the air, hipposudoric acid oxidizes and turns red-orange within minutes. Exposure to highly alkaline environments such as water can turn the sweat a dark purple. But there are no pigments present in hippo sweat that can turn it green.
|On the skin
As this table shows, hippo sweat turns red-orange upon contact with air due to oxidation of pigments. An alkaline environment like water can turn the sweat dark purple. But it does not turn green under any normal environmental conditions.
Function of Hippo Sweat
Researchers theorize that hippo sweat serves several purposes:
- Sunscreen – Hipposudoric acid absorbs solar radiation, protecting the skin from UV damage.
- Antiseptic – Possibly inhibits bacterial growth on the skin.
- Skin moisturizer – Creates an oily coating that keeps the skin hydrated.
- Pheromone signaling – Specific compounds may transmit chemical signals to other hippos.
Interestingly, hippos have less apocrine sweat glands compared to other mammals. But they have larger and more productive apocrine glands clustered around the eyes, ears, nostrils, and genitalia. This likely helps protect sensitive areas against the sun and infections.
Do Hippos Change Color?
While hippo sweat itself does not turn green, hippos can sometimes appear greenish due to algae growing on their skin. Hippos secrete a viscous, alkaline fluid from their skin which creates a suitable environment for certain algae to grow.
These algal growths are temporary and wash off once the hippo is back in the water. They do not represent a color change in hippo sweat or skin. Some species of algae commonly seen on hippos include:
- Hydrodictyon reticulatum
- Scytonema sp.
- Phormidium sp.
So while hippos may occasionally appear greenish, this coloration is caused by algal growths on their skin rather than any pigments in their sweat.
To summarize the key points:
- Hippo sweat is initially colorless, turning red-orange upon contact with air.
- The primary pigment hipposudoric acid oxidizes and turns red-orange, not green.
- Hippo sweat contains no compounds that can naturally turn it green.
- Temporary greenish coloration is sometimes caused by algal growth on hippo skin.
- Hippo sweat functions as a sunscreen, skin moisturizer, and possibly antiseptic.
So while hippos do have uniquely colorful sweat secretions, they do not actually sweat green. The myth that hippos sweat green is false and not supported by the chemical makeup of their sweat. The distinctive red-orange hue comes from pigments that oxidize upon exposure to air, not any green pigments.
Hippopotamus sweat is a fascinating secretion produced by large hippo-specific apocrine glands. When first secreted, it is colorless, but quickly turns red-orange upon contact with air without any green pigments present. Hippos clearly do not sweat green, despite the myth stating otherwise. Their sweat serves as an all-purpose skin protectant with sunscreen, antiseptic, and skin moisturizing properties. While hippos may occasionally appear greenish due to algal growth, this temporary coloration is not reflective of their actual sweat secretions. The long-standing myth that hippos sweat green is quite simply false and not supported by scientific evidence on the unique chemistry of hippo sweat.