Most folks that visit the desert often see the popularized black-tailed jackrabbit. Many are unaware that the desert cottontail also lives in the desert, and that it is the only true rabbit here.
The jackrabbit (Lepus californicus) is actually a hare. Although the two are of the same family, there are several differences between the hare and the rabbit. The hare is typically larger overall than most rabbits, especially their hind legs and ears. Nearly all rabbits live in underground burrows, whereas hares use vegetation for cover. Unlike rabbits, hares are not born in underground burrows; they are born with fur and their eyes open, and able to fend for themselves soon after birth. Desert cottontail rabbits (Sylvilagus audubonii) are born in shallow burrows or sometimes above ground in well-concealed nests, but they are blind, hairless and helpless until about three weeks old when they begin wandering from the nest. Jackrabbits are typically solitary, whereas the desert cottontails are social and often appear in groups, called ‘warrens’. All hares have 48 chromosomes, whereas rabbits have only 44.
How does one tell the difference between the black-tailed jackrabbit and the desert cottontail rabbit? The former averages about 2 feet long and weighs from 4 to 7 pounds. Its ears are 5-7” long and, like the tail, are topped with black fur. Like all cottontail rabbits, the desert cottontail has a shorter rounded tail with white fur on the underside which ‘flashes’ as it runs away. It is also lighter colored than the jackrabbit; light grayish-brown with almost white fur on the belly. Cottontail adults are 13-17 inches long and weigh up to 3.5 lb. The ears are 3-4 inches long and pink on the inside. Their hind feet are about 3 inches in length, whereas the powerful hind feet of the jackrabbit are 5 inches.
Both animals are favored prey of medium to large mammals (coyote, cougar, wolf, bobcat, and sometimes even large squirrels will eat very young rabbits), and large birds of prey, such as owls and eagles. They rely on their speed and jumping ability to escape all predators by leaping (jackrabbits can leap up to 20 feet) and running as fast as 40 mph. Both use a zig-zag pattern to confuse predators and flash the underside of their tails to warn other nearby hares or rabbits. They might occasionally leap exceptionally high to see their surroundings and confuse their pursuers.
Both animals are more abundant in black gamma grassland than in creosote bush shrub lands because they share the same dietary habits. They eat grasses, forbs, shrubs, and even cacti. They rarely need to drink, getting their water mostly from the plants they eat or from dew. Like most lagomorphs, they are herbivores and coprophagic. They re-ingest and chew their own feces. Like other herbivores, lagomorphs have to deal with a bulky diet containing cellulose, a substance which mammalian digestive enzymes are unable to break down. However, the lagomorphs have developed a process of extracting maximum nourishment from their diet. First they bite off and shred plant tissues with their long and sharp incisors, and then they grind the material with their molars. Digestion continues in the stomach and small intestine where nutrients are absorbed.
After that, some food remains are diverted into the cecum, an internal pouch. Here they are mixed with bacteria, yeasts and other microorganisms that are able to digest cellulose and turn it into more simple carbohydrates. Other fecal matter passes along the colon and is excreted in the normally as small dry pellets. About 4-8 hours after the meal, the contents of the cecum pass into the colon and are eliminated as soft, moist pellets. These are immediately eaten by the lagomorph which can then extract all the remaining nutrients in the food. Think of this as efficient recycling.
Rabbit populations tend to follow short and long-term cycles. Factors that influence those cycles are specific to the region, especially the long-term cycle. Observational reports of rabbit population densities in the semi-arid Great Basin suggest that they follow a long-term, aka 6-10 year, cycle as well as a short-cycle, where populations are the highest in the spring and then decrease through the rest of the year. The same was thought of rabbit population changes in the northern Chihuahuan desert, and populations were hypothesized to parallel those with rodents, whose population rates are highly associated with rainfall and plant production.
However, a published paper on a 10-year study (1) in New Mexico suggests that population density of both the black-tailed jackrabbit and the cottontail rabbits in the northern Chihuahuan desert do not follow that pattern. Instead, their data suggest that rabbit population densities here may be strongly regulated by other factors such as disease or predation. One study revealed a 90 percent mortality of populations from a disease called tularemia (2), which may or may not be related to the population changes. These are often referred to as ‘boom and bust’ cycles. Interestingly, these extreme cycles do not seem to be as severe in the northern Chihuahuan desert as they are elsewhere in the western states.
(1) D. C. Lightfoot, A. D. Davidson, C. M. McGlone, and D. G. Parker. Rabbit Abundance Relative to Rainfall and Plant Production in Northern Chihuahuan Desert Grassland and Shrubland Habitats. Western North American Naturalist, 70(4), 2010, pp. 490–499.
(2) Tularemia is a zoonotic disease (aka cross-infects humans and non-human hosts). The pathogenic agent is a bacterium; the vectors are ticks and deer flies.