Ferns represent one of the most ancient groups of vascular plants, thriving across diverse ecosystems globally. These resilient, non-flowering species have existed for hundreds of millions of years, developing sophisticated defenses against consumption. The question of whether ferns are regularly eaten is directly tied to their safety. While many herbivores avoid them entirely, some animals have evolved specialized methods to tolerate or utilize them as a food source.
Animals That Regularly Consume Ferns
Insects represent the most numerous group of fern-eaters, with specialized species of caterpillars, beetles, and sawfly larvae adapted to feeding on their fronds. These insect herbivores often target specific parts of the plant and have evolved mechanisms to detoxify the compounds present in the foliage.
White-tailed deer and moose occasionally browse on ferns, though they are not a preferred food source. The “mountain beaver,” a burrowing rodent of the Pacific Northwest, is a notable exception, with ferns comprising over 75% of its diet, including bracken and sword ferns. Other small mammals, such as the European wood mouse, feed on fern spores during the winter months, often limiting consumption to less toxic, newly unfurled fiddleheads.
Why Many Mammals Avoid Ferns
Ferns employ a defense strategy that deters most large grazing animals, making them a low-preference food source. Physically, ferns possess tough, highly fibrous tissues that are difficult for many herbivores to digest and provide poor nutritional return. Many species also incorporate significant amounts of silica into their structure, a hard, abrasive compound that causes excessive wear on the teeth of grazing animals.
Ferns utilize chemical defenses to reduce palatability, containing high concentrations of phenolic compounds like tannins. Tannins bind to proteins in the herbivore’s digestive tract, inhibiting the animal’s ability to absorb nutrients. This process reduces the food’s nutritional value and makes the plant taste bitter or astringent, causing animals to seek out more nutritious forage.
The Specific Toxicity Risks of Ferns
The most serious safety concerns stem from specific toxic compounds, particularly in the widely distributed Bracken fern (Pteridium aquilinum). Bracken fern contains two distinct classes of toxins that threaten livestock and domestic animals.
One major compound is thiaminase, an enzyme that causes a severe Vitamin B1 (thiamine) deficiency in non-ruminant animals. Thiaminase cleaves the thiamine molecule, rendering the vitamin biologically inactive within the body. This deficiency primarily affects monogastric animals, such as horses and pigs, leading to neurological issues known as “bracken staggers” in horses. Symptoms include lack of coordination, staggering gait, and eventual convulsions if left untreated.
The second major toxin is ptaquiloside, a norsesquiterpene glycoside and a potent genotoxin. Ptaquiloside is the primary carcinogen in bracken fern and is most toxic to ruminants, especially cattle. When ingested, it hydrolyzes in the digestive tract to form an unstable dienone, a compound that acts as a DNA alkylating agent.
Chronic exposure to ptaquiloside in cattle leads to acute hemorrhagic disease due to bone marrow suppression and chronic bovine enzootic hematuria. The genotoxic effect also causes cancerous tumors, specifically bladder and intestinal carcinomas, which develop after years of consuming small amounts of the fern. Ptaquiloside can also contaminate the milk of affected dairy animals, posing a risk to human health.
Protecting Pets and Livestock from Ferns
Preventing exposure is the most effective way to manage the risks associated with toxic ferns, as treatments for severe poisoning are often limited. In pasture settings, poisoning typically occurs when desirable forage is scarce, forcing animals to consume the less palatable ferns. Removing livestock from fern-heavy areas and improving pasture management to encourage alternative forage growth is necessary.
For pets, many common houseplant ferns like the Boston fern are non-toxic, but others are dangerous. The Asparagus fern (Asparagus densiflorus), though not a true fern, contains a toxic steroid that can cause vomiting and gastrointestinal distress in dogs and cats. The Sago palm, often mistaken for a fern, is extremely dangerous and can cause liver failure and death. If a pet or livestock animal shows symptoms like ataxia, lethargy, or gastrointestinal distress after consuming a fern, immediate veterinary intervention is necessary.