Some of Singapore's rats may be changing their behaviour to adapt and live more easily.
The Norway or sewer rat, for example, traditionally burrows into the ground to make its home, or lives in sewers.
However, as more buildings have gone up across the island over the past few years, pest busters have noticed increasing numbers of them simply living in gaps in or between building slabs and other structural defects.
To better understand how the rats' behaviours have changed, the National Environment Agency (NEA) intends to begin a research project into the matter, The Straits Times understands.
Details of the project are still being worked out, but it is expected to involve finding out the rat population, how they behave, how that behaviour has changed and how the rodents survive in urban areas.
The study will be timely as companies that provide rodent control services said that the rat situation in Singapore seems to be worsening.
Four of them told The Straits Times that they have received up to 60 per cent more inquiries related to rats in the past year.
Star Pest Control, which dealt with the Bukit Batok infestation that made headlines last month, handles nearly 60 cases a month now, an increase of about 40 per cent from 2013.
Its general manager, Bernard Chan, said: "The scale of the work has also increased. In the past, a mall might want us to deal with just the one food shop that has rats. Now, they will engage us to deal with all of the food shops just in case."
PestBusters now fields about 130 inquiries a month on average about rats, compared with about 80 a month on average in 2013.
Its business development manager, Angela Toh, said that several malls and commercial properties have also requested more frequent inspections and extermination services.
"They used to want them on a monthly basis, but now, when they put out a tender, they want the inspection and eradication to be on a weekly or even twice- weekly basis," said Ms Toh.
"The number of rats has definitely increased. They are usually active at night, so when you see them in the daytime, as in the Bukit Batok case, it's due to overpopulation," she added.
The companies said construction activity may have flushed out the rats from their homes. The growth in Singapore's human population has also led to more food waste that is not disposed of properly, causing the rat population to grow faster, they added.
Increasing connectivity between buildings, such as malls that are linked to one another or to other buildings like hotels, has also allowed the rats to range and spread farther, they said.
Also, the public's tolerance of the rodents may have fallen, said Star Pest Control's Mr Chan.
"Now, with the aid of digital media, people can snap a photo of a rat in a mall and put it online easily. That creates awareness and demand for rat elimination services," he explained.
Tackling the rat problem is not just about hunting them down and killing them.
Infestations could recur if pest busters catch the male rats but not the pregnant female ones, Carl Baptista, technical director of Origins Exterminators, pointed out.
"The pregnant rats don't venture far, so other rats go out and scavenge for them. These may be the ones that get caught," he said.
"All firms should be identifying the sex of the rats they catch. If the pregnant females are not caught, the problem will come back," he added.
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