In the second in a series of articles looking at the effects of the changes in Human behaviour caused by the Covid-19 restrictions on pest specie populations and behaviour, we look at the house mouse. At the tail end of March 2020 the UK was plunged into conditions none of us have seen in our lifetime, it has also allowed those of us who study pest activity to see some spectacular behaviours and the true resilience of these amazing animals.
With the aims of reducing transmission of the Sars-Cov2 virus, governments around the globe restricted the movement of people and closed non-essential businesses to the public.
Companies in every sector were affected with most commercial property either left empty or with greatly reduced occupancy and usage. It has been widely reported that these buildings have become a perfect habitat for many types of pest and that pests are running rampant in our cities. However, the impact of removing most human activity from large areas of our environment on pest activity has been more profound and is closer to a process of re-naturalisation.
Mouse activity through the Covid-19 crisis has been a very interesting population to watch. We have looked at several populations in both rural and urban settings. No correlation can be found between behaviours in the 2 groups.
Mice
Mice are by far the most common mammal and man’s closest commensal pest. Commensalism is a behaviour where one specie is reliant on another specie, and the donor specie is neither helped nor harmed by the recipient. The House mouse although classed as a commensal rodent is not exclusively commensal of man so will search for natural forage if necessary. This nonexclusive relationship has afforded the urban populations a degree of flexibility when man effectively withdrew from our cities.
The rural house mouse found on farms and domestic settings has shown virtually no change in behaviour or population dynamics. This is unsurprising in view of the impact the covid-19 lockdown has had on human behaviour. Agricultural and food production was not affected by Covid-19 as the countries need to be fed is constant. If anything, the period we were effectively confined to our homes increased the feeding opportunities for mice in domestic settings. Call out data for the domestic sector has been widely reported in the press and shows a marked increase. However, the population dynamics data does not support in large growth in mouse populations. It is more likely that the increase in reported callouts is due to increased visibility and time spent at home.
The urban populations we have seen particularly in London have shown marked changes. In similarity with the plight of the urban rats. Extensive questing behaviour was noted in the weeks following the Covid-19 lockdown. Mice were exploiting any minor hygiene shortcoming or food storage issue in closed commercial buildings to survive.
Through the lockdown feeding options for mice became very limited, particularly in the second and third months. During this time, some unique behaviours were observed. Mice in London are well known to demonstrate several forms of behavioural resistance. Behavioural resistance is a genetically encoded behavioural trait intended to increase the survival of their species in hostile environments. In London, the most apparent form is presented in the Mouses’ feeding preference. Humphries et al, publish a paper confirming a strong aversion to cereal-based food in 2000. Behaviourally resistant mice populations were posed with the ultimate survival choice. Starve or eat a food source they are genetically programmed not to eat it is unclear what proportion starved but some mice were seen to be unwell and emaciated during this period and analysis of faecal pellets showed an increase in maternal cannibalism. This then further raises questions surrounding the likely outcomes and future population dynamic as we come through the corvid-19 restrictions. Darwinian theorem dictates that this change in environmental conditions will have selected in favour of mice that have adapted to feed on varied forage so we have to ask, have the genes for some forms of behavioural been lost? Selective pressure was responsible for their origins so ultimately could be responsible for their demise. In evolutionary terms the loss of a previously beneficial mutation in favour of a less evolved genotype is a backwards evolutionary step. However, the ability to feed on a diverse range of sources could never be seen as a disadvantage. Only time will tell if these traits are lost but the pest control industry will not mourn their departure as our industry was ultimately responsible for their origins.
The breeding potential of mice will ensure that the surviving strains of mice will rapidly bounce back to pre COVID-19 levels as soon as conditions improve. Mice can have litters of pups very close together and enter a state of Post-Partum Oestrous, amazingly within 28 hours of giving birth. A nursing mother will normally go into a postpartum oestrous allowing her to become pregnant again immediately. There is a tendency in mice to ovulate 12-18 hours after the time of birth, but this can be countered by the tendency to ovulate nocturnally. This trait is often touted as having 1 litter every 3 weeks but in reality, this is not the case. The level of postpartum oestrus fertility is reduced somewhat relative to that achieved during a normal oestrus cycle. So, if a female successfully mates in postpartum cycle litters will be smaller and with a lower survival rate due to the mother having to split her resources between the pups. None the less mice have one of the highest breeding potentials of any mammal. This trait allows them to maximise on opportunities as they arise and is a throwback to their origins.
We can only speculate but it appears that in the first 3 month of lockdown and the first 3 months of “the new normal” the mouse population reverted to dynamics last seen in a pre-industrialised world but historical data for mouse activity during this period is entirely anecdotal. This rapid population dynamic changes seen are a true testament to the resilience and resourcefulness of these incredible animals. To put this into a human context. We “forage” in supermarkets and shops. How would our population have reacted if the shop shelves emptied overnight without warning and stayed empty for months? How quickly could we adapt?
