Understanding Foal Loss at the Salt River

 Predators, Patterns, and the Cost of Predictability 

Recent reports that foals are being lost to mountain lions have sparked strong reactions and widespread speculation. Unfortunately, much of what’s circulating online mixes guesswork, misinformation, and a limited understanding of both prey and predator behavior. This issue deserves a careful, evidence-based look.

1. What the Research Actually Shows

First, research confirms that mountain lions prey on free-roaming horse foals, with most of what we know coming from long-term studies in areas such as the Great Basin and Montgomery Pass along the Nevada–California border. These regions have been the focus of detailed fieldwork, but similar predator–prey dynamics are expected wherever cougars and free-roaming horses overlap.

Studies using GPS-collared cougars show that adult males target foals primarily in spring and early summer, coinciding with the peak foaling season, while females increase horse predation during periods when they are provisioning kittens, typically late spring through early autumn. In areas with large horse populations, horses can account for up to 60 percent of recorded cougar kills by biomass, though even then, the overall predation rate—typically 0.5 to 2 foals per cougar per year—remains low.

 In more managed herds, however, the use of PZP contraception—which extends the foaling season well beyond its natural spring peak—has demonstrably altered the seasonal pattern of predator behavior, giving cougars access to vulnerable foals over a longer span of months and potentially sustaining predation pressure later into the year. Because deer fawning ends by midsummer, late-born foals emerging in fall may become the primary remaining young, vulnerable prey, effectively extending the hunting window for mountain lions beyond what occurs under natural conditions. 

2. Environmental Shifts and Predator Behavior

Second, environmental conditions in the Salt River region of Arizona’s Tonto National Forest appear to have shifted predator behavior through prey scarcity and habitat compression. Mountain lions are dietary generalists but typically depend on mule deer as their primary prey. When deer numbers fall—as recent Arizona Game and Fish surveys suggest, by roughly 15–20 percent in several central units—cougars exhibit prey-switching behavior, targeting alternative species such as foals, javelina, or bighorn sheep. Prolonged periods without rain further compress habitat by forcing wildlife toward the same limited riparian zones and artificial water points.

This spatial overlap elevates encounter rates and improves ambush success, since cougars rely on cover, proximity, and predictable movement patterns rather than pursuit speed to capture prey.  Recent changes in prey distribution and habitat use in the Salt River region are consistent with conditions known to shift predator behavior in other arid ecosystems. These studies show that reduced prey diversity and water scarcity concentrate both predator and prey near perennial sources, effectively shrinking home ranges and intensifying kill-site clustering.   

3. How Management Practices May Be Increasing Risk

Finally, management practices may have unintentionally increased predation risk by altering horse movement patterns in ways that favor ambush predators. Many Salt River horses now return predictably to feeding zones or water points at roughly the same times each day, forming stable clusters. In contrast, free-ranging bands in unmanaged systems roam fluidly across large home ranges, shifting routes with forage availability, weather, and perceived threats. This irregularity is not random—it is a key anti-predator strategy, making it difficult for ambush hunters like mountain lions to anticipate where prey will appear.

Research on feral horse ecology in the Great Basin (U.S.) and central Australia demonstrates that supplemental feeding or permanent water points can reduce home range size by 40–70 percent, increasing site fidelity and decreasing movement variability. These shifts create predictable traffic corridors between feeding and watering locations—ideal for ambush setups. In the Salt River area, where emergency feeding and artificial water access have become routine, horses show similar behavioral compression: tighter spatial clustering, narrower movement loops, and more consistent daily patterns —conditions that favor ambush predators. 

Conclusion

Taken together, these three points suggest a clear pattern. Mountain lion predation on foals is a documented ecological behavior, not a new phenomenon; however, environmental stress—such as declining deer populations, reduced forage availability, and lowered moisture in the Salt River horse management area—has likely increased local predation pressure. The use of PZP, which extends the foaling season beyond its natural peak, has also prolonged the period when foals are most vulnerable.

At the same time, human management practices have changed herd movement dynamics, reducing unpredictability and creating more favorable conditions for ambush predators. Predation remains largely compensatory, removing weaker or slower foals rather than threatening herd stability. Yet the emerging evidence indicates that the current management model—based on fixed feeding and watering points—has unintentionally increased vulnerability.

This underscores a need for a revised strategy that prioritizes restoring natural movement patterns, dispersing resources to reduce clustering, and encouraging broader range use. Such adjustments would better align management practices with ecological principles and support both the long-term health of the herd and the natural balance of the river ecosystem.

Final note: More study is needed to fully understand what’s happening along the Salt River right now. Patterns of predation can shift with water levels, forage, and herd movement, and any single season can look different from the next. What’s written here is an initial read of the available research in light of recent events—a starting point for clearer, evidence-based discussion, not the final word. 

References: 

Turner Jr, John & Wolfe, Michael & Kirkpatrick, Jay. (2011). Seasonal mountain lion predation on a feral horse population. Canadian Journal of Zoology. 70. 929-934..

Hampson, B. A., de Laat, M. A., Mills, P. C., & Pollitt, C. C. (2010). Distances travelled by feral horses in ‘outback’ Australia. Equine Veterinary Journal, 42(S38), 582–586

Nuñez, C. M. V., Adelman, J. S., & Rubenstein, D. I. (2010). Immunocontraception in wild horses (Equus caballus) extends reproductive cycling beyond the normal breeding season. PLOS ONE, 5(10), e13635.

Smith, J. A., Donadio, E., Bidder, O. R., Pauli, J. N., Sheriff, M. J., Perrig, P. L., & Middleton, A. D. (2020). Where and when to hunt? Decomposing predation success of an ambush carnivore. Ecology, 101(12), e03172.