Why is the ability to drink salt water so rare in land-animals?

The problem is a water budget, not a taste for salt

Salt water is a trap because the body has to spend water to remove salt. Mammals clear excess salt mainly through the kidneys, which concentrate it into urine. If the urine can only be made so concentrated, then a very salty drink forces the animal to lose more water in urine than it gained by drinking. The result is a net water loss, which is why a thirsty land animal that drinks seawater can end up worse off than before [4].

This is why the ability is rare: it depends on kidney performance, not on willingness. A review of saline water in rabbits describes the animal's response as an attempt to consume enough water to meet its needs, with drinking water intake changing relative to the saline content [4]. In other words, the animal drinks more when the water is saltier, but that extra intake only helps if the kidneys can handle the salt load.

Some land animals can use salty water, but within limits

The ability is not absent everywhere. In spiny mice, one species was reported to drink seawater or sodium chloride solution as concentrated as 0.9 N, and the two species differed in their ability to use salt water [1]. That comparison is useful because it shows the trait varies between closely related animals rather than being all-or-nothing across land mammals.

In domestic animals, the limit is a matter of degree. Dairy goats can ingest saline water with levels up to 9500 mg per liter with no effect on feed intake, but higher levels decrease feed intake [2]. That number matters because it marks a practical threshold: below it, the animal keeps eating and drinking normally; above it, the salt starts to suppress appetite, which is an early sign that the water budget is under strain.

Even tolerant animals pay a price in health and production

For farm animals, the cost of salty water shows up in milk and growth rather than in immediate death. A study of dairy goats examined how increased salt water intake affects the production and composition of milk [2]. A review of rabbits looked at productive, reproductive, and physiological traits under saline drinking water [4]. And a systematic review of ruminants in arid and semi-arid regions examined how saline water affects physiological, metabolic, and productive performance, along with the factors that shape whether an animal adapts [5].

These findings fit the same explanation: an animal may survive on saline water, but the salt load draws on the same water and energy budget used for growth, reproduction, and milk. That is why the ability stays rare even where salty water is available. The evidence does not establish an evolutionary reason for the rarity; it documents the physiological and productive costs that make salt water a poor everyday drink for most land animals [2][4][5].

Coastal water is a special case, not a general solution

One study examined Baltic Sea water as a drinking source for cattle, looking at effects on health and production and evaluating the water's quality [3]. This is a narrow, coastal situation rather than evidence that land animals generally can drink salt water. It fits the broader picture: where salt water is used, it is a managed or marginal source whose safety depends on how salty the water actually is.

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