Little beasts of Polish meadows
It was as early as 400 years ago that common shrew was mentioned in Edward Topsell's book The History of Four-legged Beasts. Yet, it was as late as this year that the biologist of NCU and AMU confirmed that it is the third species of a venomous mammal living in Poland.
People often mistake common shrews for mice, whereas they are not even rodents – they belong to a separate order of insect eaters. At the first sight, they resemble a mole. They have their snout similarly lengthened, their head small, not very prominent neck, a short tail, and silky fur. Together with Eurasian water-shrew and Mediterranean water-shrew, both of which belong to the family of Soricidae, they are the third venomous species of mammal inhabiting our country.
They produce toxins in their submandibular salivary glands. The venom of water-shrews has first of all crippling properties, whereas those of common shrew venom are hematolytic, which means that the poison dismantles red blood cells, which transport oxygen. Therefore, the most visible result of the venom being introduced into the body of the prey is cardiac arrest.
– The discovery that the common shrew may be venomous was to some extent accidental – says Dr. Krzysztof Kowalski from the Faculty of Biological and Veterinary Sciences of Nicolaus Copernicus University in Toruń. – A few years ago, together with Dr. Paweł Marciniak and Prof. Dr. habil. Leszek Rychlik from Adam Mickiewicz University in Poznań, we were the first to have established the composition of the poison of Eurasian water-shrew. We also managed to examine quite well its crippling and cardiotoxic properties. Knowing that the poison of other animals, i.e. snakes, is also hematolyticly active, we wanted to check if it is similar in the case of shrewsmice' venom. If the common shrew is the most common species of shrewmice in Poland and Europe, we decided to sample its glands, prepare an extract and check the properties of its saliva. It turned out that not only is the venom of water-shrew hematolyticly active, but also the one of a shrewmouse. This has obliged us to classify these animals as venomous. The results of our research were published in "Zoological Letters".
Shrewmice and water-shrews are animals which like wetlands with slowly flowing water streams, situated on delicate slopes, as well as water reservoirs. The shrewmice can be seen in gardens, deciduous and mixed forests, and on wet meadows with thick vegetation.
Both species are not dangerous for the human. Compared to snakes or scorpions, their venom is rather weak. In nature, they are rather timid with a hidden lifestyle. They will prefer to avoid and escape from the man. Definitely, they will not attack people. Water-shrews are more active after dark, whereas shrewmice remain so all day long.
- Working on shrewmice water-shrews and shrewmice, you cannot avoid being bitten – says the biologist from Toruń. A water-shrew in captivity is still trying to bite the human. When I was bitten by a shrewmouse, I did not notice any signs, but after having my thumb bitten by a water-shrew, my finger was swollen and stiff for two days.
This does not mean, however, that all species of shrewmice are dangerous for a man. In January 2022, a team from China confirmed venomousness of a shrewmouse called blarinella quadraticauda. In their work, they contained pictures of a man bitten in his foot. The reaction was similar to being bitten by a snake. The subfamily described by the Chinese is most probably more venomous than the one living in Poland.
A titbit for a common shrew is caterpillars, but the animal will not despise insects, spiders or snails. Shrewmice are also able to hunt bigger victims, including vertebrates such as small frogs and other rodents. Such a prey provides food for much longer, which for such animals as shrewmice is very important. They are little starvelings which in fact must eat all the time. A shrewmouse which is devoid of food for two hours dies of hunger. The venom is not to kill the victim but to paralyze it. An incapacitated victim is still alive and remains fresh. It does not decompose, which guarantees food for much longer. Due to this, water-shrews and shrewmice do not have to leave their hiding place to hunt.
- We do not know how long the effect of the venom lasts, but the research conducted on northern short-tailed shrew which lives in North America has shown that once the paralysis is over, the mammal bites its victims again in order to apply the venom and incapacitate the preys once more – explains Dr. Kowalski. – What else has been observed is that the animals were colleting paralyzed frogs in the coolest part of their hiding places. If the temperate rose, they moved the victims into another place. In this way they were making a sort of a larder.
As long as 400 years ago, it was presumed that common shrew might be venomous. It was the first species of mammals suspected of having toxins in their saliva. The work by Edward Topsell the History of four-legged beast in 1607 was the first written source where it was mentioned. The thesis however was based only on observations as, in those days, there were no tools or techniques allowing analysis of the content or properties of those animals' venom. The first research on this matter was conducted in the 40s, 50s and 60s of the previous century, and the first work on northern short-tailed shrew was made in 2000.
- Further research, in 2017, resulted in our publication, we were the first in the world we defined the content of the venom of the water-shrew – says Dr. Kowalski.
Whereas water-shrews and shrewmice like wetlands, in the vicinity of built-up areas we can spot a white-toothed shrew. At the moment, it is not known if they are venomous. On the Canary Islands there lived endemic species of Canary white-toothed shrew. Scientists noticed that the lizards which were bitten by the animal showed signs of paralysis. This lets us suppose that the white-toothed shrew may be venomous, too. So far, however, no one has analyzed the properties or content of its venom. If it was confirmed that Canary white-toothed, it could turn out that the lesser white-toothed shrew and bicolored shrew, both of which exist in Poland, are also venomous, as well as other shrews: Eurasian pygmy shrew, Laxmann's shrew (called Białowieża) or Alpine shrew.
– This we would like to examine – says the NCU biologist. – In the order of insect eaters, there is also European mole, and yet we still do not know whether it is venomous or not. It hunts caterpillars which it paralyzes. At this stage, it is however difficult to assess if it is the effect of biting, which is a mechanical damage to the neurological system, or of the toxins contained in the saliva.
The biologists are also interested in the seasonal changeability in the venom of water-shrew and shrewmice. Unlike other rodents, these mammals do not hibernate, they remain active also in winter, and their body size decreases at this time. In order to save energy, they shrink, their skull and brain also get smaller.
- We have examined the properties of the venom of the water-shrew and shrewmouse, initially determined its composition, but we still do not know anything about the activity of the its toxins – confesses the NCU researcher. We still have not discovered what happens on a molecular level, and what mechanism is responsible for the paralysis of the muscles. Its discovery is extremely important as it would allow considering if similar toxins could be used in production of medicines in the future. What we know is that one of the components of the venom of northern short-tailed shrew may be effective in treatment of ovarian cancer, prostate cancer or breast cancer.
The biologist is also examining the ecological context: what is the purpose the animals produce venom, how it works with reference to the preys, can preys become resistant to the venom, and what dietary conditions may affect its content and properties.