Bats are known to be particularly immune to aggressive viruses. But that costs energy. Studies are now showing that migrating bats suppress the energy-intensive cellular immune response during their annual migration. This saves you energy for the exhausting long-term flights. It is still unclear whether the animals have a higher risk of infection during this period.
Superlative hikes: every year in spring and autumn, millions of bats migrate at night between their breeding grounds in northeastern Europe and the wintering areas in southern and western Europe. In terms of number, distance and energy expenditure, these migrations exceed everything that is otherwise usual for mammals – for example, the rough-skin bat (Pipistrellus nathusii), which weighs only seven grams, covers more than 2,000 kilometers every year.
How do bats divide their energy?
But how are these exceptional services possible? “During the migration, bats probably have to weigh the maintenance of costly bodily functions, such as certain types of immune response, against the high energy costs of flying,” explains Christian Voigt from the Leibniz Institute for Zoo and Wildlife Research (IZW). How the animals divide their limited energy into the migration time was not yet known. That’s why Voigt and his team investigated this question more closely, paying special attention to the bat’s immune system
Compared to humans, bats need more energy for their immune defense: To fly, they have a very active metabolism, which leads to increased levels of inflammatory waste. The bats’ highly regulated immune system must counter this and also keep aggressive viruses and pathogens in check. In order to clarify how the immune system is set during the seasonal migration period, the researchers investigated its activity in rough-skin bats before and during the migration period. The team compared the humoral and cellular response of the innate immune system based on the concentration of the protein haptoglobin and the number of certain white blood cells. They compared the initial values of these parameters and examined how they change in the event of a viral attack.
Antibodies are energetically “cheaper”
The investigations showed: “Our results show significant differences between the two periods,” reports Voigt. Migrating bats – like the rough-skin bat – therefore prefer the energetically more favorable humoral immunity – the defense against pathogens by antibodies – over the cellular immunity during migration. Before the move time, the cellular response of innate immunity was significantly higher than during the move time, the scientists found. In the migratory bats, however, this branch of the immune defense was shut down, instead the humoral defense dominated.
If there was a viral attack on the animals’ immune system, the change became even clearer: “The rough-skin bat responds with a strong humoral immune response to a challenge that resembles a bacterial infection,” adds Voigt’s colleague Gábor Czirják. “This reaction actually increases during the train time, while at the same time the cellular response is not activated in such a situation.”
Higher risk of infection during the train time?
The consequences of this are clear to the researchers: “From this we conclude that rough-skin bats take into account the energy requirements of their various immunity branches when their migration period begins,” states Voigt. So when the animals embark on their strenuous journey, they reduce the cellular immune response, which is more energy-consuming than the humoral response – because haptoglobins are faster and cheaper to produce than cellular effectors. This will probably save the bats energy for their long journey.
“The question is whether it also poses a risk for bats to focus on humoral immunity during migration,” explains Voigt. Because the changed ratio of white blood cells and neutrophils indicates a lower level of inflammation in bats during the migration season, according to the researchers – a sign of weaker protection against pathogens. “It is possible that they are more susceptible to certain pathogens during the migration if they cannot trigger an adequate cellular immune response,” continues Voigt. In the future, further experiments should clarify this assumption.
Source: Forschungsverbund Berlin eV, specialist article: Scientific Reports, doi: s41598-020-74473-3