"Who are you going to see?" my buddy asks.

"Hot Day at the Zoo. Bluegrass. Local, from Lowell. Fun as all hell," is my reply, heading out to face the first blindingly cold night of the year.

"Hot Day at the Zoo. Hmm."

A beat.

"Great name for a bluegrass band."

Indeed, it is, especially one that plays bluegrass like rockers. The music in its purest, string-band forms needs no added emotional heft or dynamics, but just as, say, the Bad Plus approach piano-trio jazz with a rocker's subversion and aplomb, so do these fleet-fingered pickers infuse the music of Monroe, Flatt, Scruggs, and all the rest.

What does a hot day at a zoo sound like? A little like it feels: good, loud, sweaty times, heavy on carnality and outsized personality, a little ripe and stinging in the nostrils. David Amram, an occasional HDATZ sit-in guest and an old-school cohort of Jack Kerouac, is among many who'd agree.

It's also "tasty," as founder and guitarist/harmonica player/vocalist Michael Dion is fond of saying. He likes his bluegrass with forkfuls of Nirvana, Bob Dylan, and urban blues, and he figures you might, too. He also figures you'd dig a group of bluegrass musicians with respective backgrounds in blues, classical, and all kinds of other worldly sounds.

HDATZ sprawls and folds in plenty of flavors without making "polyglot" a necessary adjective. And the three-set show is their specialty: build momentum in the first, slay in the second, exhaust in the third. Everyone goes home tired and satisfied, and each song makes the beer taste a little bit better.

This evening's occasion was the band's fourth anniversary, at a local bar—a cozy, unpretentious drinker's joint. It was was sparsely populated around 9 p.m. but by the time the music kicked off around 10:15 was packed and ready to whoop it up. What I thought of most was that Mountain Tracks/Yonder Mountain String Band recording in which the band pauses to check the "yee haw" factor of the room, and this room was "yee hawing" plenty.

This year has all the makings of a breakout for HDATZ: the band's expanding its national focus to include Colorado (this month), and was recently added to snoe.down, in some primo festival slots to boot. Regular local gigs couple with a gradual, national progression and the booking of key residencies to zero in on the Zoograss-loving heads who'd dig it the most.

The group is heavily Dead-influenced—as a bluegrass unit, more so Garcia and Co. than by the bluegrass forebears who influenced and inspired Garcia. If there's but one downside to HDATZ at this juncture, it's reasonable to think that because of their songwriting skills and extraordinary verve, the members' reliance on the Dead catalog may become a hindrance as they expand their fanbase.

For now, the key Dead and Dead-associated breakouts were useful as customer service tools, and along with the predictable ("Big River," "I Know You Rider," "Cumberland Blues"), they tried on a "Dupree's Diamond Blues" and a lip-smacking "Me and My Uncle."

In the latter tune, especially, so many rock bands go a strict roots and country route, doing little to expand or detour from the Dead's version. HDATZ's lead-up to the first verse was similar to the Dead's, but more of a a tension-and-release build up, as if they'd reached up and pulled the song out of the sky rather than rode up alongside it when its rhythm was already galloping along.

In fact, it's those little twists—and more exciting one, like a wily, cry-in-your-beer version of Nirvana's "Come as You Are"—that make more straight-covered versions of "Rider" and, say, Dylan's "It Takes a Lot to Laugh, It Takes a Train to Cry" pale by comparison. For some groups, "I Know You Rider" is the unquestioned, fail-safe showstopper. For this group, it's actually gilding the lilly.

So, a nudge for HDATZ, as if one were needed? Keep the Dead, but go for some less-traveled choices—it'd be great to see what this crew could do with "Mexicali Blues"—and don't be afraid to make such crackling originals as "Pablo," "Morning Light" and "Wackin' Off" (the night's blustery encore) set pieces.

In the meantime, keep us warm on cold nights (or after a trip down the mountain at snoe.down), and going back for another round. We could use more bands like you.