Answer: Ants

Ants are, hands down, the most successful animal colonizers on the planet and the only animals, big or small, to rival the sheer scale of human colonization of planet Earth.

The most successful of all ant species is the Argentine ant. Originally native to South America, the ant has slowly spread, hitchhiking with humans, across the entire globe. Argentine ants are well-known for forming vast colonies with hundreds of millions of ants living in an enormous interconnected home.

The largest known Argentine ant colony stretches approximately 3,700 miles along the Mediterranean and Atlantic coasts in Southern Europe. While not all colonies are so large that they cross dozens of international borders, the world is covered by several mega colonies—the state of California, for example, has a colony that stretches approximately 560 miles along the coast.

Even more interesting is that ants from Argentine ant colonies separated by oceans (such as those you would find in France and in California) recognize each other as members of what amounts to a global mega-colony of Argentine ants and will not attack each other. If they could get across the oceans, Argentine ants from any of the continental colonies could travel unharmed through the tens of thousands of miles of underground roads the individual super colonies have built.