Abstract
Long-distance migration is a rare phenomenon in European bats. Genetic analyses and banding studies show that females can cover distances of up to 1,600 km, whereas males are sedentary or migrate only short distances. The onset of this sex-biased migration is supposed to occur shortly after rousing from hibernation and when the females are already pregnant. We therefore predicted that the sexes are exposed to different energetic pressures in early spring, and this should be reflected in their behavior and physiology. We investigated this in one of the three Central European long-distance migrants, the common noctule (Nyctalus noctula) in Southern Germany recording the first individual partial migration tracks of this species. In contrast to our predictions, we found no difference between male and female home range size, activity, habitat use or diet. Males and females emerged from hibernation in similar body condition and mass increase rate was the same in males and females. We followed the first migration steps, up to 475 km, of radio-tagged individuals from an airplane. All females, as well as some of the males, migrated away from the wintering area in the same northeasterly direction. Sex differences in long-distance migratory behavior were confirmed through stable isotope analysis of hair, which showed greater variation in females than in males. We hypothesize that both sexes faced similarly good conditions after hibernation and fattened at maximum rates, thus showing no differences in their local behavior. Interesting results that warrant further investigation are the better initial condition of the females and the highly consistent direction of the first migratory step in this population as summering habitats of the common noctule occur at a broad range in Northern Europe. Only research focused on individual strategies will allow us to fully understand the migratory behavior of European bats.
Highlights
Strong seasonal variations in climate and resource availability impose increased energetic pressures on animals
We predicted that the sexes are exposed to different energetic pressures in early spring, and this should be reflected in their behavior and physiology
Females and males of a single species may use varying strategies to cope with seasonal environmental fluctuations, which can be expressed in differences in behavior, physiology, morphology as well as ecology often resulting in sexual segregation [2, 3]
Summary
Strong seasonal variations in climate and resource availability impose increased energetic pressures on animals. Once they rouse from hibernation in the spring, females start a race against time to use the short, but resource rich warm season to gestate offspring that can weigh up to 1/3 of maternal body weight at birth [11], and nurse offspring until they reach independence at nearly 90% of adult size During this time females should reduce the energy saving strategy of daily torpor, which is frequently used by males and non-reproductive females as it interferes with the development of the embryo and milk production [12,13,14,15]. In non-migrating parti-coloured bats (Vespertilio murinus) large differences between the sexes in activity, habitat selection, migration distance and home range size have been reported [6, 16] and are likely a result of this high energetic pressure on females
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