Mice are renowned breeders. Leave two alone in a cage, so the joke goes, and expect to return to a dozen.
But the ability of mice to procreate has been revealed by a novel and shocking experiment.
In January wildlife biologists embarked on the next phase of what has been described as history's largest rat-eradication program
Researchers took two house mice, one male and one female, and placed them on a small island where no mice lived. The idea was to see if the mice could establish a population on the island, and how long it would take.
Amazingly, these two mice didn’t just set up home and start a family. They bred at such a rate that their offspring colonised the whole island. And it took just five months.
The experiment is one of the first to objectively measure in real time how long it can take an invasive species of rodent to dominate an alien land.
Invasive species are a huge problem for conservation. Alien species can predate on local species, threatening their survival, or they can change the local ecology.
These problems are exaggerated on islands. In extreme cases, invasive species, usually transported by human activity, have driven native species to extinction.
The island had previously been colonised by two populations of alien rodents
For example, in the last 500 years, species such as rats, cats and mice have driven over 70 bird species to extinction, according to BirdLife International, the global conservation alliance.
These problems continue today. In January wildlife biologists embarked on the next phase of what has been described as history's largest rat-eradication program, on the remote island of South Georgia in the far south Atlantic. There, introduced rats have wiped out almost 90% of the seabirds, including wandering albatrosses and the diminutive Wilson's storm petrel. The rats raid the birds’ nests for eggs and chicks.
But scientists have only been able to make educated guesses about these alien rodent invasions, reconstructing what they think must have happened from the available evidence.
Now a team of researchers based in New Zealand have taken a different approach. Helen Nathan of the University of Auckland and colleagues intentionally introduced two mice (Mus musculus) onto a small island to see what would happen.
The researchers chose Saddle Island (Te Haupa), which has an area of six hectares.
Located 950 metres offshore from mainland New Zealand, the outer island is covered by sandy beaches, dunes and steep cliffs. The interior is forest and there is no permanent source of fresh water.
In New Zealand, mice become sexually mature at about six to eight weeks old
The island had previously been colonised by two populations of alien rodents.
Invasive rats (Rattus norvegicus) had been present on the island until they were eradicated in a ground-based poisoning operation in 1989, write the scientists in the journal Population Ecology.
Shortly after, mice were also found on the island. After studying this new population for eight months, scientists eradicated this population as well.
The two new mice were therefore introduced into a pristine environment.
Over the following months, the researchers set traps to monitor any growth in the population. Within just five months, the mice had reached what scientists call their seasonal carrying capacity, which describes the maximum number of animals that could survive in the environment, given the resources available.
In New Zealand, mice become sexually mature at about six to eight weeks old. Each litter they produce has an average of six baby mice, and the mice can produce new litters every 20 to 30 days.
Most of the mice were inbred, and many would have been produced by the founding female mating with her sons
The adults live up to 18 months in the wild.
After two months, the two mice on the island had become 14. After five months, 68 mice were living there.
What makes this achievement more remarkable is that the adult male was released at the northern end of the island, while the female was released at the southern end of the island; a distance of 400 metres apart.
So they first had to first find each other before they could even begin breeding.
An analysis of the rodents' genes revealed that the founding female contributed to most of the population explosion, siring 14 of the surviving mice, with her descendants producing the others. So most of the mice were inbred, and many would have been produced by the founding female mating with her sons, or her sons with her daughters.
But the studied also revealed the difficulty in keeping islands free from invading rodents.
Once the experiment was concluded, the mice were again eradicated from the island
The genetics showed that not all of the mice counted were descended from the founding male and female.
A few appeared to have been descended from a third unrelated female that had independently arrived on the island at some point during the study.
Mice are poor swimmers. Saddle Island is popular with recreational boaters as a picnic and fishing site.
“We consider it most likely that the mouse was inadvertently transported to the island on one of these vessels,” the researchers write.
Given how quickly the population of mice exploded, the study also highlights the importance of stopping invasions of alien species early, before they can take hold.
Once the experiment was concluded, the mice were again eradicated from the island, according to agreed ethical guidelines, returning it to its unadulterated state.
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