“Sum” is one of the odder collective nouns on this list, and honestly, nobody has fully pinned down where it came from. One theory ties it to the idea of adders being “counted together,” possibly linked to their habit of basking in noticeable numbers on sunny hillsides, easy to tally at a glance. It’s a plausible story, but not a documented one.
What is documented is the adder’s real social behavior. As Europe’s most widespread venomous snake, adders spend the colder months in communal hibernation dens called hibernacula, often shared by dozens of individuals across multiple seasons. Come spring, males compete for mates in a ritualized wrestling display sometimes called the “dance of the adders,” intertwining and pushing each other to the ground.
Adders are also live-bearers. Rather than laying eggs, females carry 5 to 20 young internally and give birth in late summer, another reason a “sum” isn’t a bad way to describe them; a mother adder is, briefly, a countable little population of her own.
Example: “We found a sum of adders basking together on the warm side of the hill.”
Related: Collective Nouns for Reptiles · What Is a Group of Adders Called?