As if the Easter Island statues weren’t enigmatic enough, a few of them are wearing hats—6.5-foot-wide, 13-ton cylinders of cindery red volcanic rock called scoria. The hats are as much of an enigma as the statues themselves. For starters, archaeologists aren't actually sure they're supposed to be hats at all.

Their shape—ranging from a straight-sided cylinder to a tapering cone, with a smaller cylinder on top—is similar to a style of woven grass hat that some historians say was once popular in New Caledonia. Carvings found on some statues in Hawai'i could represent similar hats, if you look at them from the right angle. But that same general shape could also represent a traditional Polynesian hairstyle for men of high rank: long hair bound up in a topknot, called a pukao, which is what gives the hats their name.

Archaeologists still aren't sure which version, hair or hat, the statues' builders intended, or why fewer than a hundred of the island's several hundred statues, called moai, seem to have been visited by a giant milliner. The hats, or topknots, could be a sign that some statues (or the people, spirits, or gods they represented) were a much bigger deal in Rapanui religious life than others. Maybe the ones with the red hats are just prehistoric Linux fans.

"It is a matter of speculation at this point. Dramatic population declines due to disease and slave raids on Rapa Nui historically led to the loss of likely many oral traditions," Sean Hixon, a graduate student in anthropology at Pennsylvania State University, told Ars.

Giant statues and population collapse

That population decline is another long-standing mystery of Easter Island: what happened to the people who built the statues? There are about 3,000 Rapanui people today, many of whom live on the Chilean mainland. They’re the descendants of the Polynesian seafarers who first settled Easter Island (also called Rapa Nui) in the 1200s, built the moai and their stone hats, and then nearly dwindled into obscurity. They’re now actively working to preserve the statues and other aspects of their cultural heritage and control their own ancestral lands.

Anthropologists have tried for years to link the moai to the collapse of the Rapanui population. Easter Island isn’t much of a place to build a culture: it’s just 63.2 square miles of nutrient-poor soil and very limited surface water—not great conditions for agriculture. That makes the Rapanuis' feats of monumental architecture seem even more impressive, but it also raises questions about the practicality of the whole enterprise.

One hypothesis, popularized by Jared Diamond and others, suggests that the Rapanui poured all their limited resources into building statues, while overpopulating the island until they ran out of food or water. But Hixon and his colleagues, among others, contend that Easter Island’s soil has always been bad for farming and its population has always been small, despite the giant statues. Rats and wildfires, not human clearing, doomed the island’s palm forests, while European diseases and slavery doomed its people.

Regardless of the cause, the lost traditions took with them our only way of knowing why some of the pukao are decorated with straight and curved lines or why some massive cylinders of scoria seem to have been abandoned part way to their intended destinations, left lying on their sides like giant stone hay bales. But Hixon and his colleagues think they've solved one mystery about the Easter Island statues' hats: how the statues' builders put them on.

How do you don a 13-ton stone hat?

When a Spanish expedition visited Easter Island in 1770, the explorers marveled at how precariously balanced the hats seemed to be atop the 33-foot stone statues.

“The diameter of the crown is much greater than that of the head on which it rests, and its lower edge projects greatly beyond the forehead of the figure, a position which excites wonder that it does not fall,” wrote A. de Aguera y Infanzon. And in fact, that has been one of the lingering mysteries of the pukao: without cranes or elevators, how do you get a 13-ton stone cylinder to the top of a 33-foot stone statue and then put it down so that it stays in place?

Archaeologists had long ago worked out how the early Rapanui got the cylinders from the island’s scoria quarry, a volcanic crater at Puna Pau in the southwest corner of the island, over 8 miles to the stone platforms, called ahu, where the statues waited: they rolled them. Several of the pukao have wear marks on their sides that are consistent with being rolled several miles across the island, and the handful of cylinders left lying along the route from the quarry to the statues back up the idea.

The more difficult question has been how the Rapanui lifted the hats into place. Over the years, demonstrations have indicated that there were several ways they could have gotten the job done: ramps made of spruce logs would work, as would lashing the hats to the statue while it lay on the ground, then using a wedge of stone rubble to set the statue upright, hat and all. But Hixon and his colleagues wanted to find an explanation that matched the archaeological evidence.

Archaeological detective work

So first, they had to look for clues. The statues themselves ruled out one possibility; none of the broken, abandoned statues alongside the island’s ancient roads has hats. According to Binghamton University anthropologist Carl P. Lipo and University of Arizona anthropologist Terry Hunt, the Rapanui once “walked” the statues along these carefully prepared roads from the island's volcanic tuff quarry to their eventual homes, much the way you might move a refrigerator today. If the statues weren’t wearing hats for the trip, then the hats must have been put on once they got to their stone platforms.

The most important clue of all, however, is also the one feature that all the pukao have in common. Their shape varies—some are cylinders, while others are more conical—and those differences in style mostly depend on what part of the island the statue stands in. Archaeologists have spotted similar patterns in other artifacts, like obsidian tools, and they say it probably means the prehistoric Rapanui weren’t a single monolithic kingdom, but several local communities with their own variations on Rapanui culture.

But all of the hats had one thing in common: an indention carved into the bottom, which matches the rounded top of the statues' heads. On each of the 50 hats Hixon and his colleagues studied, a stone rim lined the edge of the indention, and its presence provides the final piece of the puzzle. The Rapanui couldn’t have just slid the hats into place on the statues, because that would have damaged the rim around the indention—none of them is damaged.

That means the hats must have been levered upward and then tilted down onto the head of the statue. But how did they get up there in the first place? Hixon and his colleagues point to a very small number of sites on the island where rounded stones are spread out in front of ahu. These spreads of cobbles, the archaeologists say, may be the remains of stone-and-soil ramps the Rapanui once used to roll giant stone hats to the tops of their iconic statues.

Strength in (small) numbers

Using 3D models of the statues and the hats, along with some modern-day estimations of how much weight a single person can push or pull, Hixon and his colleagues calculated the size and slope of the ramp that would have been required to move the pukao, as well as the number of workers required. With a ramp 75 to 328 feet long, on an incline of 5 to 20 degrees, a crew of just 15 people could have maneuvered one of the pukao into position.

And if that’s the case, then the Rapanui wouldn’t actually have needed a workforce of thousands, under the direction of a powerful central ruling class, to install the hats. A few smaller communities could have done the job, which supports the argument that Easter Island’s population was always small and didn’t drive itself to collapse by building giant statues. Lipo and Hunt had previously come to the same conclusion about moving the actual statues.

That finding goes a long way to exonerate the ancient Rapanui in the case of their own population crash. The statues would have been a big project, but they clearly weren’t ecocidally resource-intensive monuments to irrational cultural hubris, either.

“Instead, we see moai and pukao carving and their transport as vivid cultural expressions of groups in a challenging and competitive environment,” wrote Hixon and his colleagues in their paper.

Crowning achievement

The Rapanui could have rolled the scoria cylinders up the ramp with wooden levers, but Hixon says using ropes to stabilize the pukao and pull it up the ramp, a method called parbuckling, would have been much more practical. That’s not a detail archaeologists have any evidence for yet, though.

And the average volume of material required to build such a ramp is about the same as the amount of material that forms the wings of the stone platforms beneath some of the hat-wearing moai, indicating that once the work was done, the Rapanui put the ramps to another use.

The finished pukao are noticeably smaller than the rough cylinders found abandoned at the Puna Pao quarry or along the way to the ahu, and there are chips of red scoria around the bases of some of the hat-wearing statues. That points to the final carving of the hats taking place on-site, just before the Rapanui put them on top of the statues.

It also seems that the Easter Island statues, like countless monarchs, had to bow their heads to receive their crowns (or hats, or hair). When the Rapanui carved the statues, they left the base slanted, so the statue leaned forward; that made it easier to “walk” the statue to its destination. The statue’s base would have been leveled on-site so it would stand fully upright, and Hixon and his colleagues suggest that it would have made sense to put the statue’s hat on before standing it level.

The indention in the base of the pukao would have kept the hat from falling off when the statue was straightened up, according to Hixon and his colleagues' 3D models. It didn't take an irrationally obsessed, overpopulated island kingdom; it just took some ingenuity.

Journal of Archaeological Science, 2018. DOI: 10.1016/j.jas.2018.04.011 (About DOIs).