Friday, April 12, 2024

Are Grapes Toxic to Island Foxes and Gray Foxes?

Recently, this question was posed to Friends of the Island Fox by a wildlife sanctuary that cares for gray foxes. Because gray foxes are wide-spread and considered common, little scientific research has evaluated their daily behavior, including diet. Island foxes, however, have been through periods of captive breeding when they faced near-extinction and there is a growing library of research on their diet and health.

Since island foxes are recently descended from gray foxes, it stands to reason that the two species would have similar responses to toxins. We reached out to the veterinarian members of the Island Fox Working Group and discovered there is no authoritative answer.

Tartaric acid in grapes can be toxic to domestic dogs, damaging kidney function. Since island foxes and gray foxes are part of the canine family it's reasonable to extrapolate that grapes could be toxic to foxes as well. Inquiries to toxicologists and a range of references uncovered no reputable sources that confirmed or denied that grapes are toxic to foxes (or any wild canid). "In the absence of evidence," says wildlife veterinarian and FIF Board member Jessica Sanchez, "it seems reasonable to err on the side of assuming things toxic to dogs will be toxic to foxes." Therefore, grapes, cocoa, and coffee are all substances that should be kept away from gray foxes and island foxes.

island fox in a fig tree

Do wild gray foxes sometimes eat grapes? Yes, one scientific paper, from the last century, reported wild grapes were found in 9.5% of gray fox stomachs. What is unknown is the comparative level of tartaric acid between wild and domestic grapes and whether or not eating grapes impaired kidney function in the wild foxes.


Ironically, in classical Greek and Roman literature, foxes were depicted as raiding vineyards to eat grapes. In Aesop's fables, a fox's craving for sweet grapes turns to disdain when his goal becomes unreachable. Aesop's fox, however, is a red fox and not a close relative of the gray fox and the island fox.

gray fox in a Camarillo backyard

Still, there are anecdotes in Lyndal Laughrin's 1980 paper "Populations and Status of the Island Fox" that recount how the number of island foxes on Santa Cruz Island in 1918 was so great, they "were destroying the grapes in the vineyards." Whether or not the foxes fared well after consuming the grapes, is not reported.

toyon berries are eaten by island foxes

Gray foxes and island foxes are omnivorous. Native fruit can make up more than 50% of an island fox's diet. How do island foxes process toxins found in some of the fruit they eat? Answering these questions for island foxes would also help us understand gray foxes better, and maybe other canines, too.

Friends of the Island Fox supports island fox research

Applications for the FIF 2024 Research Grant 

will be accepted through June 30, 2024  

(Thank you to J. Sanchez DVM and J. Barnes DVM for their investigation of this question.)