will generally yield a smaller litter while shorter gestation periods will give birth to a larger litter. The size of a single litter can range from four to 12 kits allowing a female to deliver up to 60 new kits a year. After birth, the female can become pregnant again as early as the next day. The mortality rates of embryos are high in rabbits and can be due to infection, trauma, poor nutrition and environmental stress so a high fertility rate is necessary to counter this. Sleep Rabbits may appear to be crepuscular, but their natural inclination is toward nocturnal activity. In 2011, the average sleep time of a rabbit in captivity was calculated at 8.4 hours per day. As with other prey animals, rabbits often sleep with their eyes open, so that sudden movements will awaken the rabbit to respond to potential danger. Diseases and immunity In addition to being at risk of disease from common pathogens such as Bordetella bronchiseptica and Escherichia coli, rabbits can contract the virulent, species-specific viruses RHD ("rabbit hemorrhagic disease", a form of calicivirus) or myxomatosis. Among the parasites that infect rabbits are tapeworms (such as Taenia serialis), external parasites (including fleas and mites), coccidia species, and Toxoplasma gondii. Domesticated rabbits with a diet lacking in high fiber sources, such as hay and grass, are susceptible to potentially lethal gastrointestinal stasis. Rabbits and hares are almost never found to be infected with rabies and have not been known to transmit rabies to humans. Encephalitozoon cuniculi, an obligate intracellular parasite is also capable of infecting many mammals including rabbits. Rabbit immunity has significantly diverged from other tetrapods in the manner it employs immunoglobulin light chains. In one case McCartney-Francis et al., 1984 discover a unique additional disulfide bond between Cys 80 in Vκ and Cys 171 in Cκ. They suggest that this may serve to stabilise rabbit antibodies. Meanwhile shows high amino acid divergence between domesticated types and ferals derived from them. This can be as high as 40%. Rabbit hemorrhagic disease is caused by strains of rabbit hemorrhagic disease virus (RHDV) including type 2 (RHDV2). RHDV2 was detected for the first time in Washington state, USA in May 2022 and then in August once in Washington and twice in Oregon. Ecology Rabbits are prey animals and are therefore constantly aware of their surroundings. For instance, in Mediterranean Europe, rabbits are the main prey