Longest annual round-trip migration by a terrestrial animal

Measuring in a straight line there and back between the end points (aka the Euclidean distance), the longest annual round-trip distance (RTD) for a terrestrial animal migration is that of the caribou (Rangifer tarandus). Two separate Arctic herds - a Canadian one called the "Bathurst Herd", and one from Alaska, USA, called the "Porcupine Herd" - have each logged a maximum RTD migration of 1,350 kilometres (839 miles). This straight-line end point to end point RTD for a given population or specimen of a species should not be confused with that same population's or specimen's total cumulative annual distance (TCAD), which is the sum of all annual movements in any direction and of any distance.

Apart from a grey wolf (Canis lupus) pack in Canada's Northwest Territories that was thought to track caribou and therefore be migratory rather than territorial, and which yielded a Euclidean RTD of 1,016 km (631 mi), the caribou is the only species known for Euclidean RTD migrations exceeding 1,000 km (620 mi). These include certain Old World examples, where this species is known as the reindeer.

Historically, the migration distance of the caribou (particularly the aforementioned herds of North America) has been widely reported as significantly higher but it's now suspected that those measurements were based on TCAD movements rather than the length of the migratory route.

All of these figures were collated and published in the journal Scientific Reports on 25 October 2019 in a paper led by Kyle Joly of the Arctic Inventory and Monitoring Network of the National Park Service.