By Jorge S. Gutiérrez & Theunis Piersma:
Almost everyone knows that eating seafood (or a pizza!) makes you thirsty. Seafood is naturally salty and when you have too much sodium the body makes you drink more water to balance the fluids in your cells. Excess sodium and water is filtered out as urine allowing the body to return to equilibrium. Though a sensation of ‘thirst’ is thought to be rather similar among the higher vertebrates, not all of them have powerful kidneys like ours and access to freshwater to carry off the excess salts.
In fact, many species of birds – whose kidneys work much less efficiently than those of mammals – feed in salty environments for most of their lives with limited or no access to freshwater at all. To cope with the salt these birds have a clever trick up their sleeves, or rather on the top of the skull. They have cephalic ‘salt’ glands that extract salt ions from the bloodstream and produce a salt-loaded liquid that is discarded through the nostrils – excreted salt may be visible as ‘water’ drips off the tip of the beak. But this poses another problem: salts are pumped out against the concentration gradient and this takes energy. That’s why birds keep their salt glands as small as they can.
Mollusc-eating birds like red knots eat seawater-packed meat packaged in a shell. Getting rid of excess salt is a challenge as they have to process several times their body mass in seawater each day with no access to freshwater in their main wintering areas in West Africa. Jorge S. Gutiérrez and Theunis Piersma of the NIOZ Royal Netherlands Institute for Sea Research investigated how salt content and freshwater availability influenced food choice in red knots presented with relatively low-salt (Dutch-like) or high-salt (African-like) mud snails.
Impressed by the birds’ ability to choose the low-salt snails during the first trials, the duo went on to scrutinise the birds’ salt glands. By using a non-invasive approach consisting of sliding a finger across a smooth plate prepared with increasing thicknesses that may match those of the saltglands, Gutiérrez blindly scored the saltglands. ‘Initially, I felt their saltglands and notice they were tiny…because they had been eating trout pellets [with low-salt content] with freshwater at will in the lab for weeks’. Gutiérrez and Piersma were also impressed to see that once the freshwater was cut-off, the birds’ saltglands started to swell up and their preference for low-salinity snails eventually disappeared.
So, it seems that birds choose low-salinity prey when they have to. That marine birds could keep track of their salt intake was recently suggested by Jianzhi George Zhang of the University of Michigan in Ann Arbora, who showed that that penguins have evolutionary lost receptors for detecting sweet, umami, and bitter tastes, but still possess those for detecting saltiness (Current Biology 25(4) R142). There is a wide range of situations where a bird can’t afford to lose water. Such behavioural flexibility is rather relevant at a time when fluctuating temperatures and salinity conditions predicted under various climate change scenarios will affect the options open to birds and other aquatic air-breathing vertebrates such as sea turtles, snakes, and mammals.
Journal reference: Ecological context determines the choice between prey of different salinities. Behavioral Ecology: doi:10.1093/beheco/arv185.
Postal address: Department of Coastal Systems (COS), Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, The Netherlands,