In the ice age, dozens of species of saber-toothed cats roamed North America. They were as big as lions, uncannily strong and robust, with massive front legs, in addition to their iconic, enlarged canines.

Most cats are running animals, so their limbs get longer as they age. Saber-toothed cats (sometimes colloquially called “saber-toothed tigers”) have much more robust limbs than other, similar cats. Until now, paleontologists believed that their limbs must have become thicker as the animals aged.

But as it turns out, saber-toothed kittens were born with strong bones.

“We were expecting them to show that they grew stronger with age,” said Donald Prothero, a paleontologist with the Natural History of Los Angeles County and Cal Poly Pomona. But, as it turns out, “they started out with the most robust limbs of any cat ever known.”

In a study published today in PLOS One, scientists describe their discovery that saber-toothed cat bones were wider and stronger than other large cat bones, even as kittens.

To investigate the heartiness of these extinct baby mammals, the researchers turned to the La Brea Tar Pits and Museum in Los Angeles and consulted its immense collection of bones, which has more than one million ice age fossils, a result of the area’s history. Thousands of years ago, in what is now downtown Los Angeles, animals became trapped in the sticky tar. An animal stuck in the tar was an easy meal for predators. But when these animals came to the pits for a snack, they in turn became stuck. This cycle of animals becoming trapped and dying in the tar left a vast fossil record, with 90 percent from carnivores.

Unfortunate as this was for the animals, the La Brea Tar Pits became a vast reservoir of preserved remains for scientists to study in the present. Paleontologists have recovered an enormous collection of mammoths, mastodons, dire wolves, birds and insects to study. So it was easy for scientists to find multiple samples of limb bones from saber-toothed cats of all ages and to graph their girths based on their age.

By charting the size of bones from Smilodon fatalis, one of the more common saber-toothed cat species, and comparing it to the extinct Panthera atrox (American lion), the paleontologists found that the lengths of their limbs followed similar growth patterns. The saber-toothed cats started short and grew longer with age, much like that of Panthera tigris, or the modern tiger. But the Smilodon kitten bones started off with much more bulk.

Thick leg bones means that animals usually can’t run as fast as those with light, long bones. However, strength is useful for saber-toothed cats taking down large prey and holding them still while puncturing their throat or stomach with long teeth. The strong bones of saber-toothed kittens lead scientists to suspect that Smilodon wasn’t a pursuit predator that ran across the prairie for its prey; rather, its specialty was ambush.

In spite of its impressive strength and teeth, all saber-toothed cats died at the end of the ice age, leaving nothing but their hulky bones in the ground for us to learn from.