The term “jack” in the animal kingdom does not refer to a single species but acts as a general descriptor applied across multiple distinct classes of animals. This common name is used for various mammals, birds, and fish, often based on a shared physical characteristic or a historical naming convention. There is no biological relationship connecting a creature named “jack” to any other, meaning the word is an example of convergent naming rather than a taxonomic link.
Jackrabbits: Hares, Not Rabbits
The most familiar association with the name “jack” is the jackrabbit, which is biologically a hare belonging to the genus Lepus, not a true rabbit. Jackrabbits are distinguished by their long, powerful hind legs and notably large ears, which can reach lengths of up to seven inches. These outsized ears act as thermoregulators, helping the animal dissipate heat across its extensive network of blood vessels in arid environments.
This species is adapted for speed, capable of reaching sprints of up to 40 miles per hour in a zigzag pattern to evade predators. Unlike true rabbits, which are born altricial, young jackrabbits, called leverets, are precocial, meaning they are born fully furred, with their eyes open, and are able to move independently almost immediately. The common name is believed to be a shortened version of “jackass rabbit,” a nickname given by early settlers who noticed the resemblance between the hare’s large ears and those of a donkey.
Jackals: The Canine Scavengers
Jackals represent a completely different branch of the animal kingdom, belonging to the family Canidae, which includes dogs and wolves. These Old World canines are medium-sized mammals found across Africa, the Middle East, and parts of Eurasia. There are three widely recognized species: the Golden Jackal, the Black-backed Jackal, and the Side-striped Jackal.
Jackals are highly opportunistic omnivores, with their diet consisting of small mammals, insects, birds, and a significant amount of plant matter such as fruit. They are well-known as scavengers, often following larger predators to feed on their leftovers, but they also hunt efficiently in monogamous pairs.
The Simple Definition: Male Donkey
A jack is the common name for an adult male donkey, or ass, scientifically known as Equus africanus asinus. The female counterpart to the jack is called a jenny or jennet.
The breeding of a male jack with a female horse, or mare, produces a mule, a widely used hybrid animal known for its strength and endurance. Conversely, a hinny is the result of breeding a male horse with a jenny.
Other Animals Known as “Jacks”
The Jackdaw is a small, intelligent bird found across Europe, North Africa, and Asia, belonging to the crow family, Corvidae. This bird is identifiable by its dark plumage contrasted with a distinct silvery-grey nape and notably pale eyes, and its name is an onomatopoeia for its harsh, metallic call.
In the marine world, “jackfish” is a common name used for a large group of predatory fish within the family Carangidae, which also includes the trevallies and pompanos. A prime example is the Jack Crevalle (Caranx hippos), a strong, fast-swimming fish common in the Atlantic Ocean. These fish are typically silvery or blue-green and are highly valued by anglers for their fighting ability.