Diet breadth of mammalian herbivores: nutrient versus detoxification constraints

Abstract

Two hypotheses, nutrient constraints and detoxification limitation, have been proposed to explain the lack of specialists among mammalian herbivores. The nutrient constraint hypothesis proposes that dietary specialization in mammalian herbivores is rare because no one plant can provide all requisite nutrients. The detoxification limitation hypothesis suggests that the mammalian detoxification system is incapable of detoxifying high doses of similar secondary compounds present in a diet of a single plant species. We experimentally tested these hypotheses by comparing the performance of specialist and generalist woodrats (Neotoma) on a variety of dietary challenges. Neotoma stephensi is a narrow dietary specialist with a single species, one-seeded juniper, Juniperus monosperma, comprising 85-95% of its diet. Compared with other plants available in the habitat, juniper is low in nitrogen and high in fiber, phenolics, and monoterpenes. The generalist woodrat, N. albigula, also consumes one-seeded juniper, but to a lesser degree. The nutrient constraint hypothesis was examined by feeding both species of woodrats a low-nitrogen, high-fiber diet similar to that found in juniper. We found no differences in body mass change, or apparent digestibility of dry matter or nitrogen between the two species of woodrats after 35 days on this diet. Moreover, both species were in positive nitrogen balance. We tested the detoxification limitation hypothesis by comparing the performance of the generalist and specialist on diets with and without juniper leaves, the preferred foliage of the specialist, as well as on diets with and without α-pinene, the predominant monoterpene in juniper. We found that on the juniper diet, compared with the specialist, the generalist consumed less juniper and lost more mass. Urine pH, a general indicator of overall detoxification processes, declined in both groups on the juniper diet. The generalist consumed half the toxin load of the specialist yet its urine pH was slightly lower. Moreover, the generalist consumed significantly less of the treatment with high concentrations of α-pinene compared to the control treatment, while the specialist consumed the same amount of food regardless of α-pinene concentration. For both groups, urine pH declined as levels of α-pinene in the diet increased. The generalist produced a significantly more acidic urine than the specialist on the treatment with the highest α-pinene concentration. Our results suggest that in this system, specialists detoxify plant secondary compounds differently than generalists and plant secondary compounds may be more important than low nutrient levels in maintaining dietary diversity in generalist herbivores.

Habitat selection and use in the Critically Endangered Sahamalaza sportive lemur Lepilemur sahamalazensis in altered habitat

“…Following definitions by Dearing et al (2000) and Freeland (1991), who stated that herbivores can be classified as generalists when they feed on a wide array of tree species or specialists when they only consume one or a few related trees, we classified the Sahamalaza sportive lemur as a generalist herbivore. In comparison to the white-footed sportive lemur, which was described as highly specialised and mainly feeding on only 3 tree species (Nash 1998), the Sahamalaza sportive lemur with at least 42 different feeding tree species can be classified as a generalist.…”

The Gastrointestinal Tract of the White-Throated Woodrat (Neotoma albigula) Harbors Distinct Consortia of Oxalate-Degrading Bacteria

“…Plants typically produce toxic plant secondary compounds (PSCs) as a means to combat herbivory (1,5,6). One mechanism by which mammalian herbivores may overcome PSCs is to host microbes capable of degrading such toxins (7).…”

Survival and causes of mortality for pygmy rabbits (Brachylagus idahoensis) in Oregon and Nevada

“…When acquiring food, herbivores must consider many factors, such as the nutritional quality of forage, potentially toxic plant secondary metabolite (PSM), and availability of cover for protection from predators and thermal stress (Werner and Hall 1988, Dearing et al 2000, Bakker et al 2005, Hebblewhite and Merrill 2009. However, foraging patches might not always have a sufficient mixture of these factors.…”

“…The classes of PSMs in sagebrush (e.g. monoterpenes, phenolics and sesquiterpene lactones) can disrupt cellular function (Wink 2008, Forbey et al 2011, inhibit digestive enzymes (Kohl et al 2015), irritate mucous membranes (Hedenstierna et al 1983), cause diuresis (Dearing et al 2000), and compromise energy budgets (Sorensen et al 2005). In addition, pygmy rabbits experience high rates of predation from a diverse suite of terrestrial and avian predators (Estes-Zumpf andRachlow 2009, Crawford et al 2010), and they use burrows and sagebrush as concealment cover to reduce predation risk (Green andFlinders 1980, Camp et al 2012).…”