In 1981, New Mexico moved bighorn sheep into the Peloncillo Mountains; after contact with Arizona sheep, every transplanted Red Rock animal died of pneumonia

Sometimes a wildlife project ends in a way nobody expected. In 1981 and 1982, New Mexico moved bighorn sheep into the Peloncillo Mountains. Amy S. Fisher, New Mexico Department of Game and Fish, reported in a 1991 report published in the Desert Bighorn Council Transactions that, after contact with Arizona sheep, every Red Rock sheep moved there died of bronchopneumonia. The report is short, but there is a clear lesson about how fragile wildlife recovery can be. It also shows how careful follow-up can answer difficult questions even years later.A move that ended in tragedy Fisher’s report lists 38 sheep transplanted into the Peloncillo Mountains in 1981 and 1982. Red Rock sheep were released there together with bighorns from Arizona. Red Rock is a captive herd which, by 1990, numbered 80 individuals. However, the Peloncillo story went very differently. According to Fisher, all the Red Rock sheep released to the range died from bronchopneumonia, an infectious lung disease, after coming into contact with the Arizona sheep. There is no information provided as to how the disease spread. The remaining herd never grew. Between 1981 and 1990, it was below 30 animals, and pneumonia was suspected to limit its growth, says Fisher. A survey in the fall of 1990 found only 14 sheep, but biologists estimated the actual number to be 30.Testing if the danger had passedIn January 1991, biologists set up an experiment. They released six radio-collared rams from Red Rock into the Peloncillos and monitored them until 30 June. The main objective was to see whether the resident lambs and the new rams contracted clinical pneumonia from the Arizona sheep already present in the flock. The researchers predicted that rams would die within four months if pneumonia was still active, based on the deaths of the early 1980s. They didn't. Except for one ram, all met the existing herd of 38 sheep, a count higher than the fall 1990 estimate of 30. No clinical signs of pneumonia were observed in any of the sheep, including lambs, and all the released rams were alive at the end. Fisher wrote that the herd was presently considered clinically free of pneumonia. She also said the herd was still not increasing because of low lamb survival in the southern Peloncillos, with the ratio of 35 lambs to 100 ewes. This suggested that there could be other problems.What later research addsMore recent studies have provided insight into why contact between sheep is so dangerous. According to the review, ‘Pneumonia in Bighorn Sheep: Risk and Resilience,’ by E. Frances Cassirer and team, in the Journal of Wildlife Management, reports of pneumonia outbreaks in wild bighorns due to contact with domestic sheep have been supported through captive tests. It says that a bacterium known as Mycoplasma ovipneumoniae, which is regularly found in healthy domestic sheep and goats, seems to be the main source of the disease. The review also notes that once a bacterium enters a bighorn population, it can remain there for decades. Fisher's report does not say that domestic sheep were involved in the 1981 deaths, so this research is background on how the disease works, not an explanation of what went down in the Peloncillos.A more recent study by Colton J. Padilla and colleagues published in Ecosphere in 2025 shows that from 2007 to 2022 the team had sampled 466 bighorn sheep from 19 New Mexico populations. The ratio of young sheep to adult females fell by 20% to 69% in the seven populations where it declined, and the drop was seen in both desert and Rocky Mountain populations after exposure to the bacterium. The authors suggested that these herds may be limited by lamb survival. The germ that caused the 1981 deaths isn't named in Fisher's report, so we can't tell if it was the same bacterium. Later studies showed that contact followed by illness is a known risk, and that it can have lingering effects in a herd.You use AI every day. Now get your AI Quotient. Take the AIQ test.