Can Key deer survive drinking brackish water? Interesting deer fact from Save Our Key Deer: There is a widely circulated claim Key deer’s unique adaptations to their island environment includes their ability to survive drinking water with higher salinities than other animals. This statement appears in many places, including recent scientific articles, and USFWS web pages. The claim was taken for granted by the Key deer Refuge management after Irma, when they stopped providing supplemental water (and discouraged residents to continue) after some natural water holes’ salinities dropped to 15 parts per thousand (ppt).  Last March SOKD initiated a project to monitor natural water hole salinities on multiple islands within the Key deer’s range – one of its objectives being to verify or dispute this claim.

So what constitutes “drinkable water”? Body fluids of humans and other mammals contain about 9 ppt salt ions (i.e. close to 1% of you is salt). So drinking water with a higher salt content is counterproductive since the extra salt has to be excreted out either through urine or sweat, and both require more water. Birds such as seagulls have specialized glands to excrete salt (that’s why they always seem to have a runny nose), and manatees and dolphins have longer “Henle” loops in their kidneys, allowing them to produce more concentrated urine. 

The notion that Key deer have adapted to tolerate relatively high salt amounts started in the 1970s when Dr. Klimstra (a prominent Key deer researcher at the time) wrote in one report that he  saw deer drinking out of a water hole with 12-13 ppt. Subsequently, one of his students reported seeing deer tracks at holes with even higher salinity. For comparison, sea water is in the 35-37 ppt range, so the long-repeated belief is that Key deer do just fine drinking half-seawater. This has then been passed on in more recent publications, and is now widely stated as fact without any additional proof. The truth is, however, that seeing a deer drink saline water once provides no information on whether it continued to do so sustainably, and seeing tracks on a mud bank are no proof other than a deer walked there to perhaps just test the water. SOKD’s trail cameras targeting water holes with @ 12-15 ppt salinity have not, in almost a year, documented any repeated drinking by Key deer, even with no other lower salinity water sources in the vicinity. Since there is no published evidence of any physiological differences in Key deer kidneys vs. regular whitetails, there is no logical explanation how Key deer would be able to tolerate an increased salt load.  We have thus become very skeptical of the “salt tolerance” claim. We thus consider the likely long-term drinkable limit at less than 10 ppt. Much more data exist for farm animals’ tolerance of water salinity and they parallel our observations: at 5-7 ppt animals don’t do too well; at 7-10 ppt pigs are the first to get really sick, and at more than 10 ppt everybody (pigs, goats, sheep, cows) croaks after a week or so. For more info and recent results of our salinity monitoring project, go to our web site and click on the “Water Monitoring” tab.

(Photo © 2020 ValPreziosiPhotos.com) Key deer licking condensation from over-turned  boat: