To explain the glory that is Paul McCartney singing with John Lennon, John Coltrane playing with Elvin Jones, and a number of other symbiotic duos performing together, Ben Ratliff’s new book Every Song Ever: 20 Ways to Listen in an Age of Musical Plenty wades into the scientific discipline of biomusicology and comes back with the term “entrainment.” It “means two oscillating bodies vibrating synchronously, creating resonance. It is one organism adjusting internally to another. It’s what fireflies do when they flicker in tandem.”

The members of the band Animal Collective might be familiar with the concept. Sometimes when listening to the intertwined voices of singers David Portner (known as Avey Tare) and Noah Lennox (known as Panda Bear), you might think of them as two insects dancing midair. At other times in their songs, you literally hear the sounds of insects. Over 15 years, Animal Collective has cemented its status as one of the most influential and divisive indie-rock bands of the aughts, in large part because they seem to operate by some hidden, inscrutable logic. But at the core of their unruly pop experiments might just be this concept of entrainment. Their music both encapsulates and is about the idea of organisms hitched together through sound, and other forces.

In the heyday of Animal Collective’s acclaim, when they released an uninterrupted string of phenomenal albums from 2004’s Sung Tongs to 2009’s Merriweather Post Pavilion, their songs veered between stomp-along celebrations and eerie, all-enveloping atmospheric passages. Sometimes the instrumentation sounded prehistoric, little more than a drum circle; sometimes it was futuristic and gleaming; often it was both things in the same track. But the vocals have always been key to their appeal. Lennox sings in a high keen that routinely earns the description “angelic,” while Portner delivers harsher, punky croons and yelps. The pairing would seem to lend itself to metaphorical contrasts between sacred and profane, and some of Animal Collective’s songs do suggest that idea. But more often, the joy of the music comes from the two main vocalists working toward the same goal, either when one harmonizes for the other or when the pair trade off on a topline, showing that a strong tune can survive two very different delivery systems (not to mention layers of studio gunk).

Reviews of their new studio album, Painting With, have largely spun a narrative about onetime experimentalists settling into middle age and creating solid but less-challenging music. That’s because the songs are uniformly short and uptempo, adhering to verse/chorus/verse/chorus; the band has said it’s their “Ramones record” and that they purposefully avoided having any ambient passages. But another way of thinking about the album is as extreme rather than safe: a 100-percent concentration of not only the sounds and styles that have defined the band, but also of the mentality that has guided its singers. For anyone interested in the possibilities of human voices working in tandem, Painting With is worth at least one listen, because the vocals are, simply put, insane.

The opener and lead single, “FloriDada,” is a good introduction to the album’s big motif: Avey Tare and Panda Bear trade off every few syllables, completing each others’ sentences while while also independently writing their own. It’s melody rendered as an EKG. This shtick, of course, is not new: You might be reminded of barbershop quartets, church carolers, the Beach Boys, or Caroline Shaw. But Painting With’s collaging of vocals is unusually dense even in the context of history, and rather than render it in clean a capella or with simple pop arrangements, Animal Collective slathers on squelching synthesizers, surreal samples, and unusual instruments (much of it courtesy of the band’s other active member, Brian Weitz a.k.a. Geologist).

The results often feel like optical illusions rendered in sound, which certainly fits the band’s long-running visual aesthetic. On “Hocus Pocus,” the two singers very slightly overlap words in the verses, creating a freaky echoing effect, and then for the chorus get together for a hymnlike and strangely emotional call-and-response. “Vertical” creates a swirling effect with singing in rounds as the lyrics suggest the idea of feeling dwarfed by the universe. On the closer, “Recycling,” Avey Tare and Panda Bear literally switch off every other word, and it’s as if the process of forging two human voices into one entity over the course of the album has been completed.

The effect is, to be honest, a bit intimidating. It tires the ear sometimes, and the multiplicity of vocal approaches can all can blur into one polyphonic sameness if you’re not paying close attention. Some critics have accused Animal Collective of showing off, using their vocal ping-pong matches like a bad metal band uses guitar solos. I can’t argue that this is one of their best albums, though even now it sounds better with each listen as new details reveal themselves. What’s inarguble, though, is that the chaos and density allows for powerful payoffs when the two singers stop bantering and instead converge in harmony. The bridge on “FloriDada,” for example, is as glorious a musical passage as I’ve encountered in 2016.

The lyrics of that bridge are, it turns out, about a bridge, which is an image that helps clarify the possible underlying purpose of Painting With. When you sit with the lyrics you realize they’re all, basically, about various ways that people might be connected—geography, family, love, even gender (as on the Bea Arthur-sampling “Golden Gal”). “I wanna discover the key and open the everywhere place:  A mix of sky from Montana dipped in FloriDada,” goes one refrain, a surreal and surprisingly moving display of desire for unity. Connection and community have always been big parts of Animal Collective’s message, but here the concept of oneness is made incarnate by Portner and Lennox’s performance across the album. The phone app that promoted Painting With is even on theme in that it allows users to collaborate on art with other users.

Before they headed out on tour, the members of the band expressed some trepidation about how they were going to pull off the album’s complicated vocals in concert. I was curious, too, when I went to see them at the Royale in Boston earlier this week. Animal Collective shows are notoriously uncompromising events, with the members playing jammy, soupy sets that omit most of their “hits” and often lean heavily on unreleased material. That was somewhat the case here, though much of Painting With did get played. Whenever Avey Tare and Panda Bear had to trade off singing, I watched closely. To my surprise, it appeared they were mostly doing it live—casually throwing their voices to each other across the stage, with precision that seemed born less of practice than of  biological connection.