Lop rabbit
Lop rabbit or lop-eared rabbit refers to any rabbit with ears that droop, as opposed to being carried erect. A number of rabbit breeds (listed below) are characterized by such lop ears. Abnormalities in the skull of a half-lop rabbit were in 1868. Ear carriage. The defining feature of the lop rabbit is its ear carriage. Unlike the erect ear of the majority of domestic rabbit breeds, lop breeds have ears loosely drooping, with the opening of the ear facing the skull. Due to the slightly-raised cartilaginous ear base, the head of many lop rabbits  (with the exception of English Lops) has a small bulge, referred to as the "crown". The head of a typical lop rabbit is said to resemble that of a male sheep in profile, thus the German term for a lop rabbit "(Widder," meaning "Aries [the ram])", the French term "(bélier," meaning "ram") and the Italian term ("ariete", from Latin "aries"). A rabbit's ear, with its blood vessels close to the surface, is an essential thermoregulator, since rabbits cannot sweat. The additional weight of a longer or thicker ear is not always fully supported by the rest of the  ear structure, resulting in ears that droop. The ears of some young lop rabbits may not achieve their full adult droop until the ear growth is finished. The ears of a lop rabbit prevent them hearing predators and maintaining a healthy body temperature. Both of these disadvantages keep them from surviving outdoor conditions. Ear type. Rabbits are known to carry their ears in one of five ways: Ear length. Longer (or thicker) ears are more likely to droop. Some lop-eared rabbits have been deliberately bred for exaggerated ear length. As this can be considered detrimental to the rabbit's  health, requirements in Germany "ban rabbits with ears over , and in Holland, is as long as they can be allowed [in shows] before disqualification." The "Guinness World Records" award for "Longest ears on a rabbit" was given in 2003 to an English Lop in the United States with ears measuring . Breeds of lop-eared rabbit. Though the majority of rabbit breeds have erect ears, lop-eared breeds make up approximately 15% of all the breeds currently recognized by the American Rabbit Breeders Association (ARBA) or the British Rabbit Council (BRC). Such lop-eared breeds include: