But why do they group together in such a large number? There must be some biological reason?

Hatching from a den's eggs and growing largely in one spot? Doesn't sound logical.

Grouping together for a mass snake orgy? Sounds far more likely.

I really hate snakes. Scare me probably more than anything. Maybe spiders do as well. Especially in my hair or down the back of one's neck.

I would freak if I came across a mass of snakes like that.

The closest I have seen was an old concrete and stone water tank we had underground on a property we used to have in the scrubby land across the ranges in the rain shadow dry lands between the ranges and the Murray River East of here. The water tank was designed to catch run off water and fill and store water from winter or other rains. The inlets were at or below ground level and snakes in hot weather would seek shade and coolness in the large inlets, and slip and fall into the dry tank. Usually all brown snakes. I reckon maybe up to six or so down in there. Us kids used to throw rocks at the horrible creatures.

I think our snakes are much more solitary. I wonder if the purpose of the rattle on the rattle snake is not to warn other creatures but instead to hear, locate and bring together rattle snakes for mating purposes. Or whatever other purpose these groups share/