By Carolyn Bulin
Spring has arrived in Yellowstone! For the past several years, spring has meant watching the Junction Butte wolf pack tend to puppies at their dens near Slough Creek. Unfortunately, this year that is not the case, and if you – like so many others – are wondering why, please read on.
In mid-April, a subordinate female in the Junction Butte pack appeared to be denning at the traditional Slough Creek dens. She spent a lot of time within and on the porch of the Natal Den as her pack mates attentively delivered food to her. The lead female, 1385F, also spent some time in and near the Natal Den. Perhaps she was simply checking in on the subordinate female’s litter, or perhaps 1385F also whelped a small litter (she was never visibly pregnant) within that same den.
A couple of weeks after localizing around the den, and before we ever saw pups emerge from the den, the entire pack including the mother(s) abandoned the den. On several occasions over separate days, we observed the entire pack of 15 adults and yearlings traveling together and away from any potential den areas, negating the possibility that the pack simply moved the pups. Somehow, all of the pups apparently perished before emerging from the den.
One possible cause of the pup mortality is Canine Distemper Virus (CDV), and you will see this bandied about on the Internet as the certain explanation. However, one of the beauties of Yellowstone lies in the mysterious: While Yellowstone is the very best place in the world to observe and study wild wolves, there is still much that we don’t know about their lives – including with certainty how these pups died.
You may recall that last summer the Wapiti Lake pack whelped at least 11 pups. Ten of them died before midsummer, and the final one eventually perished as well. Other packs in Yellowstone’s Interior met similar fates: No pups survived in any Interior packs in 2025. Circumstantial evidence (the timing and scale of pup mortality, the presence of CDV outside Yellowstone in Wyoming, some observations of possible CDV symptoms in adult wolves in the Interior) supports the possibility that this mortality may have been caused by CDV. The Yellowstone Wolf Project currently is awaiting lab results that would either confirm or disprove this speculation.
While it seems possible that CDV may have caused the Junction Butte pup mortality this year, there are myriad other possibilities: For example, nontraditional social dynamics within the Junction Butte pack have caused significant pup mortality in the past, and other predators such as grizzlies can also pose a threat to young wolf pups. The pending lab tests from Interior packs, future lab tests, or additional observations may shed more light on the situation – or we may never have certain answers, and that is part of the beauty of learning about wild wolves in Yellowstone!
FAQ: What is Canine Distemper Virus?
A: Canine Distemper Virus (CDV) causes high fever, malformation of tooth enamel, secondary infections, and attacks the central nervous system. Because it mutates often (like the flu virus), it is difficult for wolves to build immunity to it. It is an old disease that likely originated in Europe, has been in North America since at least the 1800s, and was first documented in the Greater Yellowstone Ecosystem by Adolph Murie in coyotes in the 1940s. It shows up in the wolf population here occasionally. In years when it is not present in wolves, it likely persists in other carnivores (especially members of the weasel family) within the ecosystem. Some wolves who have survived CDV outbreaks in the past have lived with no teeth or with observable central nervous system issues for the rest of their lives.
FAQ: What about Parvovirus?
A: When mass wolf pup mortality was first documented in Yellowstone, the leading hypothesis was Parvovirus. However, blood tests showed that Parvo is present in almost 100% of Yellowstone wolves almost every year, while presence of CDV correlated with years of low pup production/survival. While Parvo causes high mortality in other ecosystems (e.g., Isle Royale), it is enzootic (always present with little-to-no mortality) here in the Greater Yellowstone Ecosystem. Why that is is one of the lingering mysteries of wolf biology.
Want to learn more about wolves and other Yellowstone Wildlife? Check out our upcoming Multi-Day Field Excursions:
Trail of the Wolf: Hiking the Landscapes of Yellowstone’s Wolves: May 30-June 2, 2027 (registration to open soon!)
Wolves, Bears, and Cougars of Yellowstone: September 22-25
Yellowstone Yoga & Mindfulness: A Naturalist Retreat: September 9-12
Wolves of Lamar Valley: February 1-4, 2027; and March 1-4, 2027
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