Bat mothers invest incredible amounts of energy in raising a pup. Typically, bats mate in the autumn and hibernate over winter; however, the females do not become pregnant until the following spring and give birth in the summer. To sustain pregnancy, bats feed on insects, such as midges and flies that can become rarer when poor weather sweeps in. In addition, younger bats need to hone their insect-catching skills which can also impair their ability to raise pups. Bianca Stapelfeldt from the University of Greifswald, and colleagues from the Bat Research Project, Germany and the Nossentiner/Schwinzer Nature Park, Germany, set out to determine how weather affects the reproductive success of adult female Natterer's bats (Myotis nattereri) of different ages.
Beginning in 1990 and continuing every year since, researchers have collected Natterer's bats from bat and bird boxes in the Nossentiner/Schwinzer Nature Park in Mecklenburg-Pomerania, Germany, between July and September. The researchers measured the body mass, forearm length (measurement of body size) and identified the bat's age (whether they were adult or juvenile), before marking the bats with forearm bands to keep track of individuals through the years. In addition, the scientists noted whether the adult females had given birth that year by assessing the fur around the nipple; if there was fur, then the bat likely did not have a pup, while a lack of fur meant that she had given birth to a pup that season. Over the decades, the team also monitored archived climate data from a nearby meteorological station in Goldberg, Germany, and identified the average and lowest temperatures, the daily precipitation levels and the overnight wind speeds and total precipitation levels from 2003 to 2020.
The team found that over the 17-year period, the younger adult female bats had less success in birthing and rearing baby bats, compared with older adult females, likely because the younger mothers may be unable to provide themselves with the immense amount of energy required during pregnancy and when feeding their pups. The researchers speculate that younger adult bats may forgo pregnancy to avoid investing energy into a pup that they may be unable to raise. Interestingly, wetter springs also resulted in lower birth rates in younger adult bats, which was not seen with the older mothers. The researchers suspect that rainy periods make it difficult for the bats to echolocate and hunt for food and to stay warm. Therefore, poor weather may cause the bats’ body weight to reach a critical low, so that younger bats may forgo pregnancy altogether.
In addition, the researchers identified a 2-week window (from the end of April to 11 May) where poor weather had the greatest negative impact on the females’ pregnancies. It is unknown when Natterer's bats become pregnant, but this 2-week critical period is the same time pregnancies begin for other bats across Europe. Since pups have little time to grow and prepare for hibernation, adult females cannot delay their pregnancies any later, and so may abandon pregnancies if they experience adverse weather during this pivotal period.
Stapelfeldt and colleagues have successfully shown the impact of poor weather on the pregnancy and baby rearing by bat mothers using long-term monitoring of a bat population and the weather conditions they encounter. Climate change is an imminent threat worldwide and many European countries are likely to become increasingly wet during this critical period for bat pregnancies, which could dramatically affect the ability of bat populations to reproduce. Increases in rainfall during the spring may be a particular threat for European bat species at risk of extinction.