Maintaining adequate immune function can be quite a challenge for birds during winter. The cold season demands extra energy for thermoregulation while food supply is often reduced. Such times of environmental stress may suppress energy-demanding immune responses, increasing the risk of disease. But which parts of the immune system are energetically expensive? Is there a trade-off between different arms of the immune system? And do growth, thermoregulation and immune function all draw from the same resources? These questions are interesting not only for physiological ecologists, but also for chicken farmers as they shift towards farming systems that resemble more natural conditions. In contrast, physiological ecologists can also learn how birds deal with several simultaneous environmental stressors from research into the ways chickens cope. Basav Hangalapura and his colleagues at Wageningen University, the Netherlands, investigated how resources are allocated to thermoregulation, growth and various components of the immune system in chickens. They kept cold-stressed chicks on a restricted diet, and simulated a disease by inoculating the birds with a harmless substance: keyhole limpet hemocyanin.
The team used three genetically selected lines of chickens: two with either high or low specific antibody responses and the third, a control line that resembled the original stock. In two experiments, 26-day old chicks from each line were cold-stressed for 2 or 7 days before immunization with hemocyanin at various time points. During the experiments, the chicks were fed a reduced diet, receiving only 80% of the food needed for normal growth. The researchers took blood samples for a variety of immunological tests at 0, 1, 7, 10 and 28 days after immunization. These tests quantified the responses of different parts of the immune system: the specific antibody response to hemocyanin and cell-mediated immunity as measures of adaptive immunity, and phagocyte activity as an indicator of innate immunity. In addition, the team monitored the growth of the young chickens.
The results showed different effects of cold-stress on various parts of the immune system. Cold-stress did not affect the specific antibody response, but it enhanced cell-mediated immunity, especially when the cold-exposure lasted for 7 days. Testing each group of bird's innate immune response to the cold-stress and time of immunization, the team found that the response is influenced by the time between immunization and application of the cold stress. This led the authors to conclude that innate immunity can be sensitive to environmental stress, depending on its timing, and is less affected by genotype.
Overall, the findings indicate that thermoregulation, growth and cellular immunity draw on the same energy resource: cold-stressed chicks grew slower than control birds, and fast growth came at the cost of a reduced cellular immune response. But in contrast to expectations, no clear trade-off existed between cellular and humoral immune components.
With hundreds of chickens waiting at their beck and call, Hangalapura and his colleagues now plan to subject their birds to a simulated long winter,instead of a short cold spell. They hope that such a prolonged stress will reveal more clearly the chicken's evolved preference for being cold, hungry or sick. Physiological ecologists can't wait to see the results.
