In 218 BCE, Hannibal Barca led an estimated 46,000 soldiers, 37 war elephants, and thousands of horses across the Alps in 15 days. By the time they reached the Italian plain, roughly half the men were dead. What remained of the army went on to defeat Rome at the Battle of the Trebia, but the crossing itself was the campaign’s greatest challenge.
A new study published in PNAS (DOI: 10.1073/pnas.2612764123) by ecologists Emilio Berti and Fritz Vollrath quantifies exactly how punishing that crossing was. Using energy landscape modeling, they calculated the direct energy costs of climbing and descending four proposed Alpine routes, modeling the metabolic demands of men, horses, and elephants based on body mass, terrain gradient, and metabolic rates.
Their conclusion: the Col de la Traversette, the route favored by recent scholarship, was the most energy-efficient option, but it still exacted a severe physiological toll.
The Least Worst Option
The team compared four candidate routes: the Col de la Traversette, Col de Montgenevre, Col du Clapier, and Col du Mont Cenis. The Traversette route required a total of 5.42 terajoules for the entire army, approximately 86 megajoules per man, 176 megajoules per horse, and 1.53 gigajoules per elephant. The other routes required 11 to 19 percent more energy.
Translated into body fat, this means soldiers would have lost approximately 19 percent of their body weight in fat stores during the 15-day crossing, nearly one-fifth of their energy reserves. Horses, with higher metabolic demands relative to their body size, would have lost about 11 percent. The elephants, remarkably, lost only about 4 percent.
The elephants’ efficiency surprised even the researchers. “An elephant moves akin to a four-wheel-drive vehicle,” the authors note. Their thick limbs and plantigrade foot posture make them surprisingly capable climbers, and their large body size means their mass-specific energy costs are lower than those of smaller animals.
What It Means Logistically
The 5.42 terajoules for the Traversette route translates to 232.72 tons of carbohydrate supplies needed for the men alone. The alternative routes would have required 26 to 44 additional tons, supplies the army simply did not have.
The elephants would have needed 5 to 6 hours of extra feeding per day just to maintain body weight, a logistical impossibility during an Alpine crossing. They must have relied on stored body fat. Historical accounts note that most of the elephants died during the following winter; roughly 30 reached Italy, and only one survived the ice storm after the Battle of the Trebia.
Settling a Route Debate With Bioenergetics
The question of which pass Hannibal used has been debated for centuries. The Col du Clapier was long considered the most likely candidate based on ancient texts. The Col de la Traversette gained traction more recently from philological analysis, terrain matching, and a 2016 study that identified horse fecal matter at the site dating to roughly 218 BCE, though critics argued medieval mule traffic could account for the find.
Berti and Vollrath’s study adds an entirely new category of evidence: bioenergetics. The finding that Traversette was 11 to 19 percent less energy-costly than the alternatives provides independent quantitative support for the route, and illustrates just how close Hannibal’s army came to the edge of physiological collapse.
Source: Berti, E. & Vollrath, F. “Energy costs of Hannibal’s alpine crossing.” Proceedings of the National Academy of Sciences 123(28) (2026). DOI: 10.1073/pnas.2612764123