One or two? Common noctule females plan parenthood

Common noctule females "plan" cautiously whether they will give birth to one or two pups. The main criterion for the decision is fat reserves. Research by scientists from the University of Veterinary Sciences in Brno and the Institute of Vertebrate Biology of the Czech Academy of Sciences (CAS) showed that females that had the largest fat reserves after hibernation gave birth to twins, while the lightest ones always had only one pup. Biologists have also found that females of medium weight or size can start pregnancy with two embryos, absorb one of them when stressed, and deliver only one pup.

In mammals in general, large ones live longer and become the parents of several large, slowly developing young, such as elephants. In contrast, small short-lived mammals, typically mice, tend to have large litters of small young that grow and mature quickly. Bats are an exception: although small, they live long lives - even several decades - and reproduce in a similar way to large mammals.

Females of most bat species produce one litter each year, containing one or two pups, which develop slowly over a relatively long gestation period. And it is this variability that has attracted scientists led by Kateřina Zukalová of the University of Veterinary Sciences in Brno.

"A bat pup can weigh up to 35% of its mother's body weight after birth, which is a huge burden for the female. This high investment in reproduction means that the female chooses between immediate reproductive success or the possibility of her own survival and reproduction in the following years. The correct timing and planning of gestation is particularly important in temperate regions where food sources are unavailable during winter and bats go into hibernation. We were interested in what makes a female decide whether to have one or two pups," says Kateřina Zukalová.

Mothers vs. pups by the win-win method

The research was aided by a coincidence when 22 pregnant common noctule females were taken into veterinary care during the renovation of the polyclinic building in Velký Týnec near Olomouc. Scientists seized the opportunity and started an experiment in which they measured the progesterone levels of the females.

"Pregnancy in bats, like pregnancy in humans, is maintained by the hormone progesterone. However, fat reserves, which form an energy reservoir for a given individual and which the female can put into the development of the fetus, are also the building blocks for progesterone production," says Jan Zukal, director of the Institute of Vertebrate Biology of the CAS and co-author of the study.

In some mammals, the level of progesterone corresponds to the number of young. "Based on the pattern of progesterone levels, we found that the amount of this hormone increases in common noctule as the weight of females increases at the end of hibernation," explains Jan Zukal.

Scientists have been able to describe three different reproductive strategies in the common noctule. "Females that had the largest fat reserves after hibernation maximized their immediate reproductive success and gave birth to twins. In contrast, the lightest bats gave birth to only one pup each time. However, according to our results, medium-weight or medium-sized females can start pregnancy with two embryos, and if adverse climatic conditions or other stressful situations occur, they can absorb one embryo and give birth to only one pup," adds Jan Zukal.

The results of the study, published in the BMC Zoology journal, suggest that common noctule females can manipulate the number of pups in a litter so that they do not deplete their energy reserves and die. In this way, they maximise the transfer of their genes to future generations in the long term.

In recent years, research on reproduction in bats has come back to the forefront of scientific interest, particularly in view of the negative effects of white-nose syndrome on North American bat populations.

"This has been aided by the development of non-lethal or non-invasive methods, which our research group is one of the few in the world to use. These include the ability to collect small amounts of blood in small mammals using micropipettes or the use of high-resolution mobile sonography to determine the number of fetuses of pregnant females in the field," concludes Jan Zukal. All of this, he says, opens the door to further research on reproduction even in endangered animals such as bats.

In our latitudes, common noctula (Nyctalus noctula) is most often found in the cavities of old trees, but with the decline of these natural roosts, it has also learned to use crevices in human buildings. Recently, it has been encountered almost more frequently in urban environments than in forest stands, both in summer and winter. Common noctula, like most insectivorous bats in Europe, reproduces in autumn. Viable sperm are retained in the female's reproductive tract throughout hibernation, up to 6 months. In spring, ovulation, fertilisation of the egg and development of the fetus occurs. However, the common noctule female is one of the few bat species that can only produce one or two pups.

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