Svalbard ptarmigans don't prioritise fighting infection in winter

From economies to elephants, everything runs on a budget – whatever the currency – and at certain times of year the warm-blooded (endotherms) that make their homes in the polar regions have to balance some of the tightest budgets on the planet. When the sun disappears from the sky and temperatures plummet, locating scarce food blanketed in snow in the dark to satisfy their energy demands is particularly challenging. Consequently, overwintering animals prioritise essential bodily functions over other, less critical, processes to conserve energy. However, Andreas Nord from Lund University, Sweden, and Lars Folkow from the University of Tromsø – the Arctic University of Norway, wondered which side of the energy balance equation the immune response might fall. ‘If it costs energy to fend off infection, it seems a bad strategy to mount a stronger immune response when food is the most difficult to find’, says Nord. But then again, being unable to fight an infection would surely prove fatal. Intrigued by the enigma, Nord and Folkow decided to find out how one of the planet's most northerly birds, the Svalbard ptarmigan (Lagopus muta hyperborea), coped when budgeting for an infection.

Keeping 12 fat male ptarmigans in complete darkness at 0°C in the depths of winter – when the birds are usually conserving energy – Nord and Folkow gave the birds a fake infection by injecting them with a fragment from the outer membrane of E. coli bacteria (known as lipopolysaccharide). Then the duo monitored the birds’ metabolic rate, while also keeping track of whether they developed fever by monitoring temperature at different locations on their bodies for almost 10.5 h. After removing the temperature loggers, the pair collected a blood sample from each bird, to find out how well their immune systems were fighting the fake infection. They also monitored how much the birds consumed during their recovery, as most animals lose their appetite when poorly. A month later, Nord and Folkow repeated the process, but this time they plunged the temperature in the birds’ enclosures to a chill −20°C. Once the winter was over, the scientists gradually increased the amount of daylight in the birds’ pens in parallel with the natural day length change in Svalbard until it was light 24/7 in late spring, before retesting the slimmed down birds’ responses to the fake infection at both temperatures.

Not surprisingly, the birds increased their metabolic rate by 9% when there were plentiful supplies in the spring. However, all of the winter birds dropped their metabolic rate by 4% as they fought the counterfeit infection. Instead of prioritising their immune system, the winter birds chose to conserve their energy reserves for true emergencies. However, Nord and Folkow noticed that the appetite of the birds that had developed a bogus infection at 0°C dropped by 50% in contrast to the birds that had been injected at −20°C, whose appetites weren't affected so badly (down by 30%). Meanwhile, Arne Hegemann from Lund University checked how strongly the birds’ blood was fighting the counterfeit infection and found that the coldest winter birds struggled the most.

The birds were not prioritising their energy budget at all costs, although they cut back on their defensive immune system during the winter to eke out their energy reserves, which would be difficult to replenish otherwise. Even though this gamble with their health may seem rash, the team suspects that the risk may be worth taking, as ptarmigans inhabit a region where infections lie dormant much of the year. However, the scientists are concerned that the ptarmigan's unorthodox approach could be threatened if increasing tourism and climate change spread pathogens, allowing them to flourish in the birds’ polar homes.