Stopping the rise of rodents? From climate change to refuse collection - more research is urgently needed

If there’s one creature thriving in the modern world, it’s the humble rat. From New York’s subway platforms to Birmingham’s overflowing bins, rodents -rats and mice- are on the rise globally.

And as climate change fuels warmer summers and wetter winters around the world, these urban opportunists are getting more comfortable - and more numerous - than ever before.

But what if we could manage rodent populations without poison, cruelty, or endless arms races between pest control and rodent resilience?

Experts behind the Botstiber Institute for Wildlife Fertility Control (BIWFC) international conference that took place in Barcelona believe that wildlife fertility control could be the ideal scientific solution to humanely and sustainably reduce rat and mice numbers.

Resilient rodents

For centuries, cities have waged war on rats and mice. Yet despite billions spent globally on traps, toxins, and extermination campaigns, the rodents keep coming back, seemingly unbothered by best laid plans delivered by both national and local governments. One reason is the global challenge of refuse collection in fast-growing cities increasingly offering more takeaway food.

“Our overflowing waste bins and broken refuse collection systems are giving rats exactly what they want - food and shelter,” explained Prof Giovanna Massei, Director of BIWFC Europe, based at the University of York in the UK.

In the UK industrial action and striking workers in Birmingham means local communities have to live among huge piles of rubbish on the streets. Similar issues with refuse and rodents are being seen in big cities around the world.

An immediate challenge

Prof Massei said: “This is essentially setting the table for the rodent equivalent of an all-you-can-eat buffet. The maths is simple: if one pair of rats finds a steady food source, they can produce up to six litters per year, with each litter averaging 8 to 10 baby rats.

“Within a few months, a handful of rats becomes hundreds. Multiply that across a city, and the scale of the problem becomes obvious.”

Refuse collection is the unsung hero of public health. Prof Massei added: “When refuse collection works it is invisible. But when it goes wrong - like we are seeing here in the UK - the ripple effect has a huge impact on people. Rubbish just doesn't attract rats, it sustains them.”

Turning up the heat

The planet’s warming climate adds another layer of complexity. Traditionally, cold winters kept rat populations in check. But milder winters mean more adults survive the season, while warmer, wetter conditions create perfect breeding environments.

Prof Massei said: “We’re seeing more reports of rodent activity outside traditional peak seasons. In some urban areas such as New York rat populations are now active year-round.

"Climate change is literally removing the pause button for reproduction.”

An age of extremes

Prof Steven Belmain has worked on rats and other rodents for more than 30 years and attended April's conference. He believes the changing climate is of secondary importance as the key issue is actually people and how our lifestyles are changing.

He explained: “From the research I've seen, and anecdotal evidence from colleagues in the sector, the biggest challenge facing urban areas is that too many people are so careless with their food. We are starting to see evidence of this in major cities such as Paris and Helsinki but to properly tackle the issue more research and hard data are urgently needed.”

The BIWFC conference in Barcelona will bring together new research and data and will help to set the agenda for the future of rodent control.

Learning from experts

We can only plan for a future of human coexistence with urban rats by looking to the past, according to Dr Jens Jacob, from the Julius Kühn-Institute (JKI) Federal Research Centre for Cultivated Plants, Rodent Research, in Germany.

He said: “This has been an issue in our cities for many thousands of years. Poisons work, but they only eradicate rats for a short while. That's one of the reasons both scientists and local authorities are so excited about wildlife fertility control. I hope April's conference will explore successful bait delivery to the animals, because that will be transformational in terms of rodent control.”

Land use is crucial for rodent potential and pathogen transmission, according to Dr Jacob.

He said: “We need to learn more about the effects of climate change on small mammals in agriculture as well as urban settings. My prediction is that citizen science will be so important for this, particularly as our climate changes. Because people really care about this issue.

And of course they really do care about animal welfare too. Rats and mice can be a big problem because they are very clever and adaptable creatures.”

The end of rodenticides?

The traditional way to manage rat populations is using rodenticides - chemical poisons. But this is on the way out because they are indiscriminate, often putting other wildlife, pets, and even predators like owls and foxes at risk. Furthermore, resistance to some rodenticides is increasing. For instance, Barcelona itself, where the BIWFC conference will take place, is struggling with widespread occurrence of rodenticide-resistant mice, which may increase the risk of secondary poisoning of animals preying on this species. That's why the BIWFC conference was so crucial, according to Professor Massei.

She said: “We urgently need new tools to tackle old problems. Wildlife fertility control is where science meets compassion. The principle is simple - instead of killing animals, contraceptives prevent them from reproducing. This is a fast-advancing field, employing novel technologies to develop new effective and humane oral contraceptives for rats and mice. Research groups around the world are engaged in testing new oral contraceptives.

"Others are designing devices to deliver these compounds in baits that target single species. For instance a new bait-delivery system that can be fitted under rubbish bins is being tested in French cities to attract rats to food that will contain contraceptives."

Working with natural turnovers

She added: "Sustainability is key. Fertility control doesn’t create sudden ecological voids that other rodents rush to fill. It works with the population’s natural turnover, creating a steady decline over time. Human-rat coexistence might be difficult to accept but rats are here to stay. We might as well find a way to live together that limits the use of rodenticides as much as possible.

"The irony is that rats tell us so much about our own systems. When waste, housing, and climate resilience fail, rats are the first to thrive. Understanding them better may be the best mirror we have for understanding where we’re going wrong as cities.”