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The animal on the cover of Learning React is a wild boar and its babies (Sus scrofa). The wild boar, also known as wild swine or Eurasian wild pig, is native to Eurasia, North Africa, and the Greater Sunda Islands. Because of human intervention, they are one of the widest-ranging mammals in the world.

Wild boars have short thin legs and bulky bodies with short, massive trunks. Their necks are short and thick, leading to a large head that accounts for up to a third of the body’s length. Adult sizes and weights vary depending on environmental factors such as access to food and water. Despite their size, they can run up to 25 mph and jump to a height of 55–59 inches. In the winter, their coat consists of coarse bristles that overlay short brown downy fur. These bristles are longer along the boar’s back and shortest around the face and limbs.

Wild boars have a highly developed sense of smell; they have been used for drug detection in Germany. It also has an acute sense of hearing, which contrasts with its weak eyesight and lack of color vision. It is unable to recognize a human standing 30 feet away.

Boars are social animals that live in female-dominated groups. Breeding lasts from around November to January. Males go through several bodily changes in preparation of mating, including the development of a subcutaneous armor that helps confront rivals; they travel long distances, eating very little on the way, to locate a sow. Average litters contain 4–6 piglets.

Many of the animals on O’Reilly covers are endangered; all of them are important to the world. To learn more about how you can help, go to animals.oreilly.com.

The cover image is from Meyers Kleines Lexicon. The cover fonts are URW Typewriter and Guardian Sans. The text font is Adobe Minion Pro; the heading font is Adobe Myriad Condensed; and the code font is Dalton Maag’s Ubuntu Mono.