Romulus and Remus, the gene-edited wolves created by Colossal Biosciences, have turned two, and the company says they have reached the size researchers predicted from dire wolf fossil records. Along with their younger sister, Khaleesi, the brothers are healthy and thriving under human care, giving scientists a rare opportunity to study how genetically engineered animals develop over time. The trio was introduced to the public in April 2025, when Colossal announced that it had edited gray wolf DNA to recreate selected traits of the extinct dire wolf, which disappeared more than 10,000 years ago. Although the animals resemble their ancient relatives, they are genetically edited gray wolves, not exact genetic copies of the extinct species.
These gene-edited wolves were created to bring back traits of a species extinct for over 10,000 years
According to a health report obtained by PEOPLE in October 2026, Romulus now weighs 61 kg (134 pounds), while Remus weighs 60.25 kg (about 132 pounds). Both are in ideal body condition, and their growth plates have closed, indicating that they have reached their full size. Their measurements match estimates from a 2006 study of dire wolf fossils discovered at the Rancho La Brea fossil site in Los Angeles. The brothers also weigh roughly one-third more than an average gray wolf. Colossal's chief science officer, Beth Shapiro, told PEOPLE that the genetic edits were intended to give the animals larger, more muscular bodies and the white coats associated with dire wolves. Now that the brothers have reached adulthood, the team says their size matches its expectations.
Now two years old, Romulus and Remus have reached the size scientists predicted from ancient fossils
Matt James, Colossal's chief animal officer, says the team has conducted extensive examinations over the past two years, including CT imaging, scans, and blood tests. According to the company, the results have been encouraging, with both brothers and their sister showing good overall health. Their size is particularly striking in person: James, who stands 6 feet 4 inches tall, says their shoulders reach just below his hips. Shapiro also remembers meeting Khaleesi when she was around six months old, when the young wolf ran up to her and placed her large paws on her shoulders.
Khaleesi is now 20 months old and weighs 46.35 kg (102 pounds). She measures approximately 94 centimeters (37 inches) at the shoulder and 201 centimeters (about 6 feet 6 inches) from nose to tail. Unlike her brothers, she is still growing and is expected to remain around 20% smaller than them, which James says is normal for a female. Khaleesi also has an unusual genetic characteristic: all three wolves were cloned from the same male gray wolf donor and carry XY chromosomes, but the gene that typically triggers male development did not activate in her. According to the report, similar cases have been documented in dogs and a wild gray wolf in Alaska. Shapiro told PEOPLE that Khaleesi is healthy despite this finding.
All three wolves were cloned from the same male gray wolf, but their DNA was edited to give them dire wolf traits
The team recently spayed Khaleesi and used the procedure to examine her reproductive tract. According to the health report, she received anesthesia through a hand injection at the enclosure fence while being petted, without being restrained, and was eating again within hours of surgery. The three wolves will not be bred together because they share the same genetic line. Instead, James says Colossal plans to expand the pack by introducing wolves from different genetic backgrounds to increase genetic diversity before relying on natural breeding in future generations.
Their younger sister, Khaleesi, has XY chromosomes but developed as a female because a key gene never activated
The latest examinations have revealed few health concerns. Imaging found no masses, stones, or structural disease, while examinations of the wolves' hips, spines, and teeth, along with their bloodwork, returned normal results. Their only reported illness over the past two years was intestinal coccidiosis, which they developed as juveniles and have since recovered from. The veterinary team continues to monitor one minor finding: all three wolves have slightly wider-than-expected collecting spaces in their kidneys. The company believes this variation may have been inherited from their genetic donor.
The trio now hunts together, catching rabbits and pursuing deer on their protected preserve
The wolves have also developed distinct personalities. Although Romulus is slightly larger than his brother, Remus appears to lead the pack, with both Romulus and Khaleesi deferring to him. "He's got a little Napoleon in him," James joked. Khaleesi is the most comfortable around people and often seeks attention from her keepers, while her brothers tend to keep their distance. The trio has also begun hunting together on the preserve where they live, catching rabbits and pursuing deer. They will remain in this protected environment rather than being released into the wild.
They won't be released into the wild, but scientists hope what they learn from the wolves can help protect endangered species
For Colossal, the wolves' development offers more than an opportunity to study animals with traits resembling those of an extinct species. The company says the experience is helping its team understand how to care for and monitor gene-edited animals over time. That knowledge could inform other conservation projects, including efforts involving species threatened in their natural habitats. One example is Colossal Australia's work with quolls, small spotted marsupials that can die after eating invasive cane toads. The company says it created a quoll with a single DNA change intended to make it resistant to the toads' toxin.
Shapiro believes gene editing could help conservationists respond to environmental changes that happen faster than some species can naturally adapt. Rather than waiting for a beneficial genetic change to spread through a population over many generations, researchers may be able to introduce certain traits more quickly. However, the approach remains one tool among many in conservation, and its long-term effects need careful assessment. As Shapiro explained to PEOPLE, gene editing could speed up evolution when habitat changes outpace natural selection.