Feeding ecology and diet

Lagomorphs are, with rare exception, strict herbivores. Unlike many rodents, they are unable to hold or process food

Herbivores Colorado
An American pika (Ochotona princeps) sunning among rocks in Colorado, USA. (Photo by John Shaw. Bruce Coleman, Inc. Reproduced by permission.)

with their paws, instead relying on the clipping of vegetation and side-to-side mastication of vegetation using only their mouths. Lagomorphs are rather generalized herbivores, with a diet consisting primarily of grasses and herbs, but also fruits, seeds, roots, leaves, buds, and bark of trees. As no lagomorphs hibernate, their feeding ecology must accommodate the need to harvest vegetation year round, or in the case of pikas to store food when access to vegetation is restricted.

Most pikas live in northern or high altitude environments blanketed by snow during winter. They adapt to these conditions by harvesting vegetation during summer, a time of abundance, and storing it into a cache or hay pile to serve as a source of nutrition during winter. The folk story that pikas lay out their hay on rocks to cure before carrying it to their hay pile is untrue; pikas harvest their hay with great economy, sometimes making hundreds of trips per day to deposit their loads directly on their hay pile.

Although generalized herbivores, pikas can be highly selective of the foods they eat. During summer most rock-dwelling pikas maintain a heavily grazed zone in the meadow adjoining the talus (on which they hold their territories). This zone allows pikas to graze continually on growing grasses, and also to better see approaching predators. At the same time pikas forage for their hay piles at greater distances from the meadow-talus edge, and they primarily select forbs to carry back to their cache. Forbs are larger and more economical to carry than grasses, although grasses can also be gathered up and deposited on haypiles. Plants selected for haying are generally higher in nutrition than non-selected plants, and in most cases they are less likely to be poisonous. The exception is that some rock-dwelling pikas harvest plants that, while poisonous, also inhibit bacterial growth in the hay piles, thus preventing them from rotting. This is an apparent win-win situation for the pikas, as these plants help to preserve the hay piles and later can be eaten after the toxic chemicals degrade.

Pika hay piles can be huge or sometimes inconspicuous when tucked under large boulders. There can be variability in how hay piles are constructed even within a single population. Burrowing pikas that do not have rocks under which to tuck their hay sometimes make giant stacks over a yard high. These are exposed when the snow otherwise covers the landscape, and Mongolian herdsman in the range of the Dau-rian pika (Ochotona dauurica) take advantage of these efforts and graze their livestock preferentially in areas occupied by pikas. Some pikas apparently do not make hay piles, and normally these species live in areas without snow where they can forage year-round. Even pikas that make hay piles often construct snow tunnels to allow foraging on nearby vegetation blanketed by snow.

One of the only known instances in which a lagomorph survives by eating meat is found in an isolated population of the collared pika (O. collaris). These pikas live on nunataks, or small islands of rock in the middle of a sea of ice, protruding from the Seward Glacier in the Yukon, the world's largest non-polar ice field. It is remarkable to find pikas in this environment, and even more so that they can survive on the few plants that also hang on in this bleak place. These pikas eat the brains from birds that die while flying overhead and fortuitously fall on the nunataks giving them their slim margin for survival.

Leporids are generally feeding opportunists, eating a smorgasbord of plants from those that are available in the habitats in which they live. Plant resins have been identified as a deterrent in lagomorph herbivory and may play a role in the 10-year population cycle of snowshoe hares (Lepus americanus). Close inspection of the area surrounding creosote bushes (Larrea tridentata) in the deserts of the southwestern United States yields many clipped but uneaten branches. Apparently each branch contains different levels of alkaloids, and black-tailed jackrabbits (L. californicus) prune the shrub, taste to assess the alkaloid level, eat the palatable branches, and discard the remainder.

Given a selection of plants, most leporids choose those which are the most succulent, particularly during times of water stress. They also prefer plants in the pre-reproductive or early reproductive stages of development that have the greatest nutritive value. Like pikas, leporids select plants that are comparatively higher in moisture and crude protein. Given that plant quality and availability changes dramatically with the seasons, so does the diet of rabbits and hares. Most jackrabbits, for example, show preferences for shrubs in winter and for grasses and forbs in spring and summer. The tendency of rabbits and hares to gravitate toward the most nutritious plants also is responsible for their love of cultivated areas. Here agricultural crops, bolstered with nutrients from fertilizers, represent a cornucopia to lagomorphs, and often result in dramatic increases in their population density. On the other hand, in natural conditions hares often are close to starvation when available food may be covered by snow and they are unable to harvest twigs sufficiently to meet their needs. In some areas it has been determined that hares will starve in winter when the diameter of twigs used for browse is greater than 1 in (3 mm).

Special dietary adaptations

Because generalized herbivores, such as lagomorphs, have such difficulty in acquiring food of a sufficient quality in terms of necessary vitamins and micro-nutrients they have been forced to develop specialized dietary adaptations. Lago-morphs possess a huge digestive system that appears specially constructed to deal with the large quantities of plant material they eat, much of which is difficult to digest. In particular, they feature a giant caecum up to 10 times larger than the stomach that branches off from the gut between the small and large intestines. This organ, its surface area enlarged by numerous divisions separated by a spiral fold of skin that runs through it, hosts a rich culture of bacteria and other microorganisms. Here, partially digested food (that not absorbed by the animal during passage through the small intestine), is broken down by these microorganisms, and in the process various vitamins (in particular vitamin B complex) and mi-crobial proteins are manufactured. To be able to utilize these products, lagomorphs practice what is called coprophagy which literally means feces-eating. A soft feces is excreted from the caecum (called a caecotroph) and reingested. The reingested soft feces can then be digested in the stomach and small intestine yielding up to five times as many vitamins as in the original food. This process is such an integral part of lagomorph biology that if prevented from reingesting soft feces, a rabbit may develop malnutrition or die.

The caecotrophs, or soft feces, contrast with the normal hard round dry pellets characteristic of lagomorphs. These soft and hard feces are produced by a mechanical separation process, dependent on the size of food particles, in the small intestine. Fine food particles are shunted into the caecum, and large particles, basically those of poor quality, form the hard pellets and are passed quickly. When the separation mechanism ceases to act, fermented caecotrophs are excreted. The difference between the two feces types is not due to whether or not the food has passed once or twice through the gut, as is commonly assumed; some foods may be recycled numerous times. Within the leporids, there are two types of cae-cotroph: capsules (a spherical shape coated with a tough mucous membrane) found generally in rabbits and amorphous (without a surface membrane) most commonly found in hares.

Most of the literature on lagomorphs states that soft feces are excreted at night (sometimes even termed "night feces"), and that hard pellets are never eaten. Recent studies, however, show that lagomorphs regularly eat hard feces and that the timing of coprophagy, with respect to hard and soft feces, can be complex. Most feeding takes place at night in lep-orids, and during that time the separation mechanism is activated so that hard feces are formed and fine food particles are shunted to the caecum. In the morning, when animals cease feeding, hard pellets remain in the large intestine. These are excreted and reingested, after which soft feces are formed and reingested until early afternoon. Then the sepa-

Lepus Nest
A black-tailed jackrabbit (Lepus californicus) in its nest. (Photo by Robert P. Carr. Bruce Coleman, Inc. Reproduced by pernssion.)

ration mechanism kicks in again to form hard feces that reach the anus in a few hours and are reingested in the afternoon. Thus, leporids feed on fresh food and ignore feces during the night, and excrete both hard and soft feces during the day and reingest these. This process is not as well known in pikas. Some caecotrophs are not eaten directly (perhaps when pikas have an abundance of fresh nutritious food and they are not necessary to achieve sufficient nutrition), and are deposited near the hay pile. These may be eaten later, or some grow a fungus after which the entire mass is reingested.

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