Plant secondary compounds as complementary resources: are they always complementary?

Abstract

Generalist herbivores typically grow better on mixed- than on single-component diets. This response has been attributed to food complementarities that either enhance the utilization of nutrients or dilute the negative impacts of plant secondary compounds (PSC). For instance, when animals choose between foods that contain diverse PSC, they eat more than animals offered a food that contains just one PSC. In addition to their negative impacts on herbivore fitness, recent evidence suggests that at appropriate doses PSC may provide beneficial effects to herbivores (i.e., by reducing parasitic infections). Thus, complementarities among diverse PSC may not only influence an herbivore's ability to consume food but also reduce the incidence of disease. We assessed the complementary effects of two PSC by offering sheep (Ovis aries) a choice of foods containing condensed tannins and saponins while challenged with a parasitic (Haemonchus contortus) infection. Animals offered a choice ate more than animals just offered tannins or saponins in single rations. However, sheep offered choices displayed greater fecal egg counts (an indirect measurement of parasitic burdens) than sheep offered single rations. Thus, saponin- and tannin-containing foods were complementary resources regarding nutrient intake but antagonistic regarding effects on parasitic loads. The nature of the relationship among PSC may depend on the dimension (i.e., nutrient intake, disease) where the interaction occurs. A unifying currency such as growth or reproductive output may help understand the trade-offs between costs (disease) and benefits (nutrient and medicine intake) for herbivores grazing multiple PSC.

Defensive mixology: combining acquired chemicals towards defence

“…In sheep, mixing foods containing tannins and saponins allowed increased food consumption compared to eating diets containing one PSM or the other (Copani et al . ). However, individuals that had consumed the mixed diet had greater faecal parasite loads (Copani et al .…”

“…However, individuals that had consumed the mixed diet had greater faecal parasite loads (Copani et al . ). Single PSMs have been variously shown to enhance (Singer, Mace & Bernays ; de Roode, Lefèvre & Hunter ), or reduce (Haviola et al .…”

Calibration of a hand-held instrument for measuring condensed tannin concentration based on UV- and red-excited fluorescence

“…as protection against herbivores, pathogens or ultraviolet (UV) radiation (Skarpe and Hester 2008, Tharayil et al 2011, Tremblay et al 2012. The roles of condensed tannins in plant-herbivore interactions include reducing plant palatability and intake or digestion by herbivores (Basha et al 2013) and reducing internal parasite loads in herbivores (Copani et al 2013). Furthermore, some condensed tannins reduce methane and nitrous oxide emissions of ruminants (Hassanat and Benchar 2013).…”

Beneficial Effects of Temperate Forage Legumes that Contain Condensed Tannins

“…Condensed tannins and saponins cross-react and bind in the gastrointestinal tract, nullifying the effects of both compounds [86]. For example, goats increase intake when shrubs contain a combination of CT and saponins relative to when animals are offered single shrubs [75], and sheep offered a choice between saponin-and CT-containing rations ate more feed than animals only offered CT or saponins in single rations [87]. However, the in vivo antiparasitic effect of the CT-saponin combination was found to be less than that of the single rations [87].…”

“…For example, goats increase intake when shrubs contain a combination of CT and saponins relative to when animals are offered single shrubs [75], and sheep offered a choice between saponin-and CT-containing rations ate more feed than animals only offered CT or saponins in single rations [87]. However, the in vivo antiparasitic effect of the CT-saponin combination was found to be less than that of the single rations [87]. Thus, a reduction in the negative impacts of CT or saponins on the herbivore (i.e., through inactivation) can be carried over to the endoparasite, resulting in beneficial effects for both the herbivore and the parasite.…”