Horses, with their acidic stomach, may be prone to gut disturbances if there is too much naturally occurring latex in their diet, a researcher suggests.
Vidya Rajan, with the Department of Medical and Molecular Sciences at the University of Delaware, discussed the consumption of angiosperms by herbivores. Angiosperms are flowering plants that bear their seeds in fruit.
About 10% of angiosperms, an estimated 20,000 species, produce latex from common isoprene precursors.
Latex, an aqueous suspension of rubber particles and other compounds, functions as a deterrent against herbivores eating them. It is soluble in neutral to alkaline pH, but coagulates in acidic environments.
Rajan, writing in the journal Life, proposes that foregut-fermenting herbivores such as ruminants, kangaroos, sloths, insect larvae, and tadpoles have adapted to latex in forage with the evolution of alkaline anterior digestive chambers — the initial chambers in which digestion begins.
However, they consequently become susceptible to the action of Bacillus thuringiensis crystalline delta-endotoxins and related bioinsecticides which are activated in alkaline environments.
By contrast, hindgut-fermenting herbivores, such as horses and rabbits, have acidic initial digestive chambers, in which latex coagulates and may cause a gut blockage, but in which Bacillus thuringiensis is not activated.
“The latex-adapted foregut herbivore versus latex-maladapted hindgut herbivore hypothesis developed in this paper has implications for hindgut-fermenting livestock and zoo animals which may be provided with latex-containing forage that is detrimental to their gut health,” Rajan said.
Rajan noted that plants produce a slew of chemicals as deterrents to herbivores. Although latex functions to plug wounds in the plant body, it is primarily considered to be a deterrent to herbivores.
Latex-containing plants span 43 families and 20,000 species.
“Despite clear evidence of latex’s deterrence of herbivory, and the abundance and diversity of plants that produce latex, the question of how the two major categories of herbivores, foregut fermenters and hindgut fermenters, tackle the small or large amounts of latex that must inevitably enter their diet has not been explored,” he said.
Rajan proposes that the deterrent effect of latex lies in the variable behavior of rubber particles in the latex in the alkaline or acidic conditions encountered in the initial digestive chambers of foregut and hindgut fermenters, respectively.
He notes that areas with abundant latex-containing plants correlate with the origins of foregut-fermenting animals. This correlation indicates that latex may have exerted a selective pressure for gut alkalinity.
Rajan said his primary hypothesis is that the neutral-to-alkaline pH of the initial digestive chambers of foregut-fermenting herbivores has a straightforward evolutionary purpose — to enable tolerance of the widespread plant metabolite, latex.
Foregut-fermenting (often polygastric) animals have higher first-chamber pH than hindgut-fermenting (monogastric) animals. This allows the former to tolerate small amounts of latex in the diet, whereas the latter has to avoid latex-containing forage.
Awareness of this correlation is important anywhere captive animals are provided with forage, such as in farming operations or zoos, he said.
The choice of food for animals who are confined, as in zoos or domesticated or not able to range on their own, is critical for their health. Domesticated herbivores are also at risk if food is offered without foresight, he said.
“Apples and persimmons, among many other fruits, contain latex as a ripeness gatekeeper, and may be commonly planted in pastures. It is known that horses should not eat a lot of unripe apples.”
Since horses are hindgut fermenters with acidic first chambers, latex in feed will coagulate rather than break down, potentially causing obstruction and pain.
Additional work to examine the latex-adapted foregut herbivore versus latex-maladapted hindgut herbivore hypothesis could potentially impact the kinds of feed and forage provided to livestock and captive animals, he said.
Further, providing guidelines for Bacillus thuringiensis use to minimize off-target impact will benefit foregut-fermenting animals already stressed by environmental or captive conditions.
“In summary, the alkaline anterior chambers of foregut-fermenting herbivores are consistent with being an evolutionary adaptation to the widespread presence of latex in forage.
“Since latex is liquid in neutral to alkaline pH, but solidifies to rubber in acidic pH, it is latex itself, not the additional chemicals it contains, that is the primary herbivory deterrent.
“Protection from latex coagulation simultaneously makes foregut-fermenting animals more susceptible to the action of alkaline-pH-activated Bacillus thuringiensis toxin, which is widely used as an insect biocontrol agent.”
Rajan, V. An Alkaline Foregut Protects Herbivores from Latex in Forage, but Increases Their Susceptibility to Bt Endotoxin. Life 2023, 13, 2195. https://doi.org/10.3390/life13112195
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