First, let us examine what makes rodents so successful.
Rats are amazingly versatile creatures, able to adapt to almost any environment they enter; this ability is derived from their sophisticated array of highly-attuned senses of touch and smell as well as their no-fuss approach to food (consuming practically anything from grains and fruit to meat and rotting scraps - the definition of easy-to-please). In addition to these characteristics, rats have an intelligence that rivals most animals of their size, giving them superior advantage when it comes to avoiding danger (or in the booze-rats case, taking it on) and successfully colonising a new area.
Even with such impressive skills, how have rats managed to appear practically everywhere on earth so quickly? The answer: Us.
Humanities excessive desire to expand our reign has seen the rapid evolution of transport; trains, planes, automobiles, rocket-ships, great oceanic freighters - they all have given man the ability to travel with a speed and effectiveness beyond belief; but this achievement would never just be for us, for many other creatures (cough -rats) too dreamt of seeing the world too, and we gave them the ultimate means to do so.
So how has the booze-rat evolved to it's present majesty?
Our story begins thousands of years ago with the first explorers. At this time in history, most towns and cities were squat and overcrowded, the perfect breeding grounds for vermin of all sorts - namely the black and brown rats. These pesky critters happily co-existed with humans, sharing their germs and fleas, but they soon began to feel unwanted, so as the sailors boarded their shiny new ships and set off in search of new lands, they did too.
The vessels dark bowels with their vast stores of grain and rum were ideal sanctuaries, allowing for the rodents to simply sit back and relax and breed and do whatever else happy rodents do; at each new port, the more curious creatures would disembark whilst the others hung back to sample whatever strange delicacies the sailors brought back onboard, like spices and bananas and copious amounts of tobacco and wine. These are our primary evolvutionaries.
Whilst the fruits were easily ingested by the rats, who were growing larger and more comfortable in their surrounds, the latter had a profound effect on the few that dare drink it. Becoming groggy and disorientated, many died in the hours following; but a few hearty critters lived to drink another day, breeding to create more alcohol tolerant generations.
Ship Captains soon began to notice the un-welcomed stowaways, deciding to recruit dockyard cats as a means of pest control. This worked out surprisingly well for those felines who could withstand the oceans unpredictable swell, getting a regular and hearty feed off the vermin.
One day though, one of these cats came across a group of new generation rats who had bathed in the contents of a ruptured wine cask. Having no chance at escape in their intoxicated state, they was quickly devoured, the cat falling into a drunken stupor from their booze-soaked fur. Now incapacitated and reeking of death, the other rats moved in (not the sort of creatures to shy away from an easy meal…) In the hours that followed, as much as the cat tried to fend away the creatures who nipped and swarmed, it was overwhelmed and eaten with prejudice. It became the first time prey had killed predator on the high seas, all because of the sacrifice of a few drunk rats...
From here, we fast-track our story 500 years.
Sea trade was booming; many thousands of the evolutionary rat generations have been and gone, the vermin having evolved to better withstand the alcoholic wares of the rum/wine barrels beneath deck, becoming confident enough to stand up to any small predator that came their way, having realised that by soaking themselves in booze, anything that ate them would become easy pickings for the remaining rats.
With each ship landing, more and more of these evolved rodents made their way off-deck and into new worlds, most lush with vegetation, sporting impressive vineyards and distilleries... The bigger more powerful rodent quickly overpowered any rival inhabitants, coming to dominate the environment in the space of weeks. They took residency in the vineyard and jungles, feasting on all they had to offer, regularly indulging in grapes and pawpaw and mango, fresh or fermenting, growing ever more tolerant to the effects of the alcohol produced by the various microbes on each. They evolve a partitioned stomach to separate ethanol from the nutritious fruit pulp; this waste was excreted quickly to avoid serious organ damage.
They were still subject to predatory activity and, without the safety of the ships strong alcohol, were unable to defend their nests to the extent they originally could, so rats began to "spray" one another with the excreted ethanol in a bid to replicate the effects of the wine-bathing and protect themselves.
Forward another 500 years...
The evolutionary rats have become fully adapted to their new world environments, the vineyard/jungle dwellers thriving greatly.
The rats no longer need to spray one another; their partitioned stomach has since separated into a separate organ capable of storing larger amounts of ethanol safely (the organ walls covered by a thick mucus) that can be excreted in much the same way if it becomes too full. It is called a booze-bladder.
Predators still threaten the rats, who now possess a more specialised means of protection that better resembles the original wine-bathing. Once a predator is spotted, the older rats leave their nest to confront it, mixing the contents of their stomach and booze-bladder once attacked, causing it to regurgitate the strong ethanol over itself ("bathing") in preparation of being eaten. It's sacrifice allows for the rest of the rat colony to attack the intoxicated predator in the knowledge that due to the brave rodents actions, they will eat as kings.