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A foal disappeared recently at the Salt River. There are many theories as to the cause of presumed death on line, including a mountain lion attack, posited by the Salt River Wild Horse Management Group. Despite exhaustive searches, the foal could not be found, suggesting scavenger activity. The following presents the causes of foal mortality in free-roaming horses. Given the overpopulation of the horses, reduced resources, and photographs and first-hand accounts of stallions harassing mares/foals at the river and behind the SRWHMG compound, it is possible the foal died from stallion aggression. Infanticide in wild equids, though relatively rare, is a reproductive strategy rather than an act of aggression without purpose. When a foal is killed, the mare often returns to estrus sooner than she would have if she were nursing. Normally, lactational anestrus, the period when a mare does not cycle while nursing, suppresses ovulation through elevated prolactin and reduced gonadotropin (FSH and LH) levels. Once the foal is lost, these hormonal constraints diminish, and the mare can enter estrus within days to weeks, allowing the infanticidal stallion or another bachelor a mating opportunity that would otherwise have been delayed for months.

The death rate of foals in free-roaming horse populations depends on multiple factors, which include seasonal predator attacks, population density, and fertility management systems. The most noticeable natural cause of death in free-roaming horses occurs through cougar (Puma concolor) attacks. Yet, demographic pressure from excessive population density, food scarcity, and birth control side effects leads to social changes that increase foal mortality rates.

Seasonal Cougar Predation and Sex-Specific Hunting Patterns

The MPWHT located at the California–Nevada border shows that cougars kill 70% of their foal victims before they reach three months of age, while half of all foal deaths happen before July starts. The predator did not target any adult horses during its attacks. The predation period occurred between May and June, which coincided with foal birth, while mule deer became the primary prey during the winter months (Turner JW Jr et al., 1992).

Research on kill sites showed that female cougars who had dependent kittens chose to hunt foals during this specific time, while male cougars occasionally killed adult or subadult horses but mainly focused on foals when they were abundant (Andreasen AM et al., 2021). The study in Alberta showed that female cougars primarily hunted newborns, whereas male cougars killed only adult horses in their nine recorded prey encounters (Knopff KH et al., 2010). According to the Salt River Wild Horse Management Group, a few foals have been attacked/killed by mountain lions. https://www.facebook.com/profile/100064860303576/search/?q=mountain%20lion

The same patterns of predator-prey interactions have been documented in Argentina since pumas returned to their natural habitat. The Ernesto Tornquist Provincial Park shows that 74% of foal deaths occurred during spring through summer, while 54% of the deaths involved foals under six months old and 53% showed evidence of puma predation (Bostal F et al., 2025).

The combination of excessive population numbers with insufficient food resources makes foals more vulnerable to predators.

Behind the SRWHMG’s property along the Bee-Line Highway, horses gather for food (hay and alfalfa) and water. There can be as many as 100+ horses on any given day, several of which may be pregnant or lactating. There are a few water troughs holding 100 gallons of water. Lactating mares have substantially higher water requirements due to the fluid demands of milk production—typically 50–70 litres (13–18 gallons) per day, compared with 25–45 litres (6–12 gallons) for non-lactating horses. The fact that the foal was observed at the river infrequently suggests the dam may have had limited access to water and could have been dehydrated. Insufficient water intake can reduce milk production, compromising the foal’s hydration and nutrition. In herd settings, restricted or hierarchical access to water sources—especially where dominant stallions drink first—may further disadvantage mares with foals, increasing the risk of dehydration and related stress in both dam and offspring.

Additionally, one member of the Management Group was ordered to direct horses away from Rainna, the new foal. This is incredibly dangerous because the dam is potentially in post-partum oestrus. Do not approach wild horses this closely under ANY circumstances, particularly horses that have lost their fear of humans.

Behind Salt River Management Compound along the Beeline Highway, horses have access to several (5+/-) 100-gallon water troughs. The table shows the water requirements for horses, and while they can go to the river a few miles away, most remain.

Herd size	Stallions	Non-lactating mares	Lactating mares	Foals	Gallons/day (baseline)	100 gal troughs containers needed (baseline)	Extra refills needed	Gallons/day (hot)	100 gal	Extra refills needed beyond 5 (hot)
25	11	12	1	1	253	3	0	375	4	0
30	14	12	2	2	306	4	0	450	5	0
35	16	15	2	2	356	4	0	525	6	1
40	18	18	2	2	406	5	0	600	6	1
45	20	21	2	2	456	5	0	675	7	2
50	22	24	2	2	506	6	1	750	8	3
55	25	24	3	3	559	6	1	825	9	4
60	27	27	3	3	609	7	2	900	9	4
65	29	30	3	3	659	7	2	975	10	5
70	32	30	4	4	712	8	3	1050	11	6
75	34	33	4	4	762	8	3	1125	12	7
80	36	36	4	4	812	9	4	1200	12	7
85	38	39	4	4	862	9	4	1275	13	8
90	40	42	4	4	912	10	5	1350	14	9
95	43	42	5	5	965	10	5	1425	15	10
100	45	45	5	5	1015	11	6	1500	15	10

Perinatal, Disease, and Environmental Mortality

The death of foals occurs through multiple causes, which include dystocia, trauma, failure of passive transfer, septicemia, and environmental exposure (Greger PD and Romney EM, 1999; Roelle JE et al., 2010). The combination of long-distance mare migrations to water sources leads to increased risk of foal abandonment and death from starvation and heat exhaustion. The Nevada herds experienced foal deaths when stallions attacked their young during harem conflicts, demonstrating how population density can drive social aggression that affects reproductive timing.

Bachelor Stallion Aggression and Infanticide

The excess of male horses in the population creates an unbalanced sex ratio, leading to increased aggression among stallions (Berger, 1986; Ransom, J.I., and Cade, 2009). The prolonged battles between bachelor stallions lead to band takeovers and mares experience harassment, which results in foal injuries or deaths through trampling and targeted attacks during harem disruptions (Roelle JE et al., 2010; Greger PD and Romney EM, 1999). The behaviour of killing unrelated foals by new males is an adaptive yet destructive practice observed in other equid species.

Stallions become more aggressive because they have limited access to mares, and their poor nutritional state creates social instability in overpopulated systems. The prolonged stress from harassment causes mares to develop elevated cortisol levels, which leads to reproductive suppression and breaks down social bonds between band members, thus worsening demographic outcomes (Ransom JI et al., 2014).

The video below demonstrates bachelor aggression towards a young foal. It may upset some viewers; however, the foal was fine.
The entire post is here

Fertility Control: PZP Versus GonaCon and Behavioural Implications

The implementation of fertility control measures for non-lethal herd management affects social structures and death rates.

The PZP fertility control method used in free-roaming horse populations stops fertilisation but creates delays in conception and disrupts foaling synchronisation. The extended predation period results from delayed foaling times because PZP contraception leads to more miniature foals who become vulnerable to cougar attacks (Boyce PN and McLoughlin PD, 2021). The GnRH immunocontraceptive GonaCon functions as a reproductive hormone suppressor, affecting both male and female animals to prevent estrus and delay or stop reproductive cycles. The effectiveness of GonaCon in reducing foaling rates leads to different social changes than PZP does. The suppression of reproductive hormones by GonaCon creates stable band behaviour because it eliminates sexual cues, which reduces stallion competition and decreases the chances of aggression and infanticide. The extended period of reproductive behaviour suppression induced by GonaCon treatment might cause stallions to leave their non-receptive mares, as they lose their natural bonding signals.

Research findings on these vaccines remain scarce, but observations indicate that GonaCon-treated horse populations exhibit less aggressive behaviour than PZP-treated populations during the breeding season (Ransom JI et al., 2014). The high population density creates extreme competition between animals, which leads to foal injuries and deaths even when using different contraception methods. The combination of hormonal suppression with GonaCon treatment helps control foal deaths from infanticide and injuries, but does not stop them from occurring when animals face extreme population pressure.

Integrated Interpretation

The combination of seasonal predator attacks, density-related food scarcity, and reproductive social patterns leads to the death of foals in free-ranging horse populations. The Salt River’s overpopulation creates conditions where female cougars can predictably hunt foals. At the same time, male aggression and competition for resources lead to additional foal deaths through social conflicts and physical harm. The selection of fertility control methods affects social dynamics because PZP prolongs foal exposure to danger through delayed births, but GonaCon decreases aggressive and infanticidal conduct by blocking oestrous cycles. Successful population management requires three essential components: reducing population density, selecting fertility control methods based on behavioural effects, and conducting ongoing assessments of both population statistics and animal well-being.

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Summary of Relative Mortality Causes (Most to Least Significant)

1. Predation by cougars, concentrated in late spring–early summer, accounts for the majority of documented foal deaths (<3 months).
2. Nutritional limitation and dehydration from overpopulation, drought, and poor forage quality, leading to starvation and higher predation vulnerability.
3. Social aggression and infanticide, amplified by overpopulation and foal heat estrus, with added risk under PZP contraception.
4. Perinatal complications, including dystocia, birth trauma, and maternal exhaustion.
5. Environmental exposure, accidental injury, and infectious disease are often secondary to other stressors.

Management Implications:

• Reduce herd density toward ecological carrying capacity (100–200 animals for Salt River).
• Prioritise GonaCon over PZP in high-density, aggression-prone herds to suppress foal heat and reduce infanticide.
• Coordinate fertility control with habitat restoration, water management, and behavioural monitoring.
• Recognise that effective management must balance population control, social stability, and foal welfare across ecological and behavioural dimensions.

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In summary:
Foal mortality in overpopulated free-roaming herds arises primarily from predation, followed by nutritional stress, social aggression and foal heat–related infanticide, perinatal complications, and environmental or infectious factors. Overpopulation magnifies all mortality pathways. Management strategies emphasising density reduction, behaviorally informed fertility control (GonaCon), and habitat restoration offer the best prospects for improving foal survival and ecological balance.

The Salt River herd experiences higher foal death rates because of its dense population, which creates multiple factors that lead to increased mortality. The competition for resources between horses and their environment leads to poor mare body condition, which weakens foal health and raises their risk of dying early. Research on free-roaming horses shows that population density and restricted habitats lead to lower foal survival rates.

The Salt River Wild Horse Management Group reports that Salt River foal survival rates reach only 70% during their first year of life. Salt River Wild Horse Management Group. The Salt River Wild Horse Management Group observes that foal deaths occur because of social conflicts between bachelor stallions, which become more severe when band numbers increase in densely populated areas. (https://saltriverwildhorsemanagementgroup.org/on-this-foal-friday-we-have-good-news-and-bad-news/?)

The combination of high animal numbers at water sources creates conditions that increase foal dehydration and trauma risks, according to research on feral horses and ungulates, which shows density affects juvenile survival through maternal health, resource competition, and environmental stress.
The current evidence indicates that Salt River herd overpopulation leads to higher foal mortality rates, making density management essential to protect foal survival and maintain herd health.

References

1. Turner JW Jr, Wolfe ML, Kirkpatrick JF. Seasonal mountain lion predation on a feral horse population. Can J Zool. 1992;70:929-934.
2. Andreasen AM, Stewart KM, Longland WS, Beckmann JP. Prey specialization by cougars on feral horses in a desert environment. J Wildl Manag. 2021;85:1104-1120.
3. Knopff KH, Knopff AA, Kortello A, Boyce MS. Cougar kill rate and prey composition in a multiprey system. J Wildl Manag. 2010;74:1435-1447.
4. Bostal F, Scorolli AL, Zalba SM. The comeback of a top predator and its effects on a population of feral horses. Perspect Ecol Conserv. 2025;23:121-129.
5. Arizona Department of Agriculture. Salt River Horse Management Plan. Phoenix, AZ: AZDA; 2025.
6. Greger PD, Romney EM. High foal mortality limits growth of a desert feral horse population in Nevada. Great Basin Nat. 1999;59:374-379.
7. Roelle JE, Singer FJ, Zeigenfuss LC, Ransom JI, Coates-Markle L, Schoenecker KA. Demography of the Pryor Mountain Wild Horses, 1993–2007. US Geol Surv Sci Invest Rep. 2010-5125.
8. Berger J. Wild Horses of the Great Basin: Social Competition and Population Size. Chicago, IL: University of Chicago Press; 1986.
9. Ransom JI, Cade BS. Quantifying equid social behavior: a review of current research and future directions. Appl Anim Behav Sci. 2009;120:1-11.
10. Ransom JI, Kaczensky P, Lubow BC, et al. Influence of demography and social stress on feral horse reproduction. J Wildl Manag. 2014;78:916-925.
11. Kirkpatrick JF, Turner JW Jr. Achieving population goals in wild horses with fertility control: science and social context. Wildl Soc Bull. 2011;35:102-110.

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