‘Cannibal rats’ would actually nosh on mice
News outlets went berserk a fortnight ago (hey, this still counts as ‘topical’, right?) when a story went viral on the possibility of a ghost ship of cannibal rats crashing into the UK’s coast. The sodding prime minister was even asked about it at PMQs.
Whilst containing the premise for the greatest B-movie yet to be pitched, the story was mostly implausible. Yes: there’s an abandoned boat somewhere in the Atlantic, drifting about, but odds are it’s not going to hit us and more importantly from my perspective, rats very rarely become cannibals. They have as much of a cannibalistic tendency as humans, though with a lower capacity for reasoning to override their instinct. The story bought to my mind an interesting paper I read last year though, and seems the perfect prompt to talk about it.
Boats frequently contain not only ships rats but also populations of house mice. So in this potential horror-boat, there are likely protagonists of two different species of small whiskery beasts. It’s been observed before that when efforts are made to reduce numbers of ships rats, the number of house mice increase. The reasonable thing to assume from this is that in some way the rats were keeping levels of house mice low, but how? They were presumed to be exhibiting either what’s known to we ecologists as ‘interference competition’, where a species decreases the level of another through interfering in its natural behaviour and so keeping their numbers low, or otherwise ‘intraguild predation’: when a predator species actually kills and feeds upon another predator species.
A study released last year in the journal Animal Behaviour sought to investigate which of these two scenarios was being carried out on ships worldwide. The researchers deduced that if the rats were suppressing mouse numbers by interference competition it was likely through intentionally aggressive rat-displays scaring away the mice, whereas if it was simple predation behaviour rats would be observed to attempt to kill mice introduced to them.
Wild rats and mice were trapped and introduced to each other in cages in a laboratory, on opposite sides of a mesh screen (putting live animals into a cage together where they are expected to attempt to fight to the death is considered unethical, funnily enough). The rats were observed to show threat-display behaviour to other rats in their side of the cage, but none to the mice on the other side of the mesh, who they instead were observed to show predatory behaviour towards. The mice understandably kept away from the mesh, and when they got close to a rat tended to flee straight away. Almost as if they knew the danger they were in…
To back up these findings it was attempted to view the behaviour of wild rats when directly introduced to seemingly live mice without the barrier. As allowing rats to kill mice in an experiment would be very cruel, a recently euthanized, still-warm mouse was dragged through a cage of rats by a piece of wire. Nine of the twelve rats in the cage then attempted to feed on the mouse carcass, with some of them showing behaviours such as chasing the corpse down, before then holding it in place with their forepaws to kill it, natural behaviour that has been observed when rats capture invertebrates. Rats who were hungry were far quicker in deciding to feed on the fresh mouse carcass. From this it is thought that in the wild and on ships, rats will feed on mice when given the chance, particularly when they are hungry.
Though this seems a slightly bizarre subject to research, it is important in its own way. Rats and mice who escape from ships are some of the most problematic invasive species in the world, causing havoc to isolated island populations of animals such as nesting seabirds, who have no natural defences against these alien predators. As they are often accidentally released at the same time, knowing the dynamics between the two species is important for pest management to protect our vulnerable seabird colonies. And heck, it’s also kind of cool to know and imagine what mini-violence is going on right now in ships out at sea.
And so it appears that if the ‘cannibal rat ghost ship’ is heading towards our shores and still has a colony of rodents scurrying about it, they may well be rats. They might not be eating each other, but they have likely devoured a boatload of their smaller cousins instead. A furry nautical struggle.