Lecture 15: Wild Horse and Burro

Horses evolved in North America

Late Pleistocene in southern Nevada

Went extinct in North America at the Pleistocene-Holocene transition

Horses evolved in North America 30+ million years ago:

• Eohippus (Greek: Dawn Horses) lived in horses

• The genus Equus, including all horses, evolved during the Pliocene (~4-5 million years ago)

• Spread across Beringia to the Old World by ~2.5 million years ago

• Three divergent lineages of equids existed during the Pleistocene

• Horses were present in North America until at least 12K years ago

Horses spread across Beringia to the Old World by?

~2.5 million years ago

Three divergent lineages of equids existed during the Pleistocene

Large stout-legged horse (Equus scotti)

Small stout-legged horse (Equus sp.)

Small stilt-legged horse (Haringtonhippus)

Horses were present in North America until?

at least 12K years ago

Feral: represents surrogates for ancestors that went extinct

Feral horses (Equus ferus caballus)

Feral burros (Equus africanus asinus)

What did some have argue about feral horses (Equus ferus caballus) and burros (Equus africanus asinus)?

they represent surrogates for ancestors that went extinct in North America at the Pleistocene- Holocene transition

What is a Przewalski’s horse?

a rare species from the Asian Steppe that looks like most depictions of extinct Equus scotti, once found in Southern Nevada

Arrived on the North American continent (in Florida and Mexico) with the Spanish in the early 1500s

Hernan Cortes, 1519

How did wild horses and burros arrive back in North America?

In the 1600s-1700s, the British, French, and others brought additional horse breeds to the east and west coasts

Strays eventually started feral populations:

‘Spanish mustangs’

Iberian breeds

Native peoples began to master and spread horses at least by early 1600s:

horse cultures appear to have developed slowly after that

Westward expansion in 1800s brought saddle and draft horses, large number of burros

• Following industrial changes in transport, mining, military, and ranching

  • many of these animals were abandoned

• Genetic studies indicate current populations mostly reflect these later influxes of large numbers of animals

Following industrial changes in transport, mining, military, and ranching:

further genetic admixing of horse breeds, likely establishment of burro populations in the Southwest

What do genetic studies indicate current populations mostly reflect?

later influxes of large numbers of animals

By the 1930s, commercial harvesting became increasingly prevalent - ‘Mustangers"‘

In the 1950s, “Wild Horse Annie” (Annie Johnson)

By early 1970s, populations had declined substantially from human encroachment and commercial harvesting

Who was Annie Johnson?

an advocate against commercial harvest

Wild Free-Roaming Horses and Burros Act of 1971

Federal management of feral horses and burros on western federal lands

BLM and USFS have responsibility

In areas where wild horses and burros were found roaming in 1971

To maintain wild horses and burros in good condition and protect the health of our public lands

Who was the Wild Free-Roaming Horses and Burros Act of 1971 signed by?

Richard Nixon

What did the Wild Free-Roaming Horses and Burros Act of 1971 declare?

wild horses and burros were to be “living symbols of the historic and pioneer spirits of the West”

What do stipulates suggest that BLM and USFS have responsibility of?

to maintain wild horses and burros in good condition and protect the health of our public lands in areas where wild horses and burros were found roaming in 1971

Today, what are the results of the Wild Free-Roaming Horses and Burros Act of 1971?

Free-roaming horses in 10 Western States

Free-roaming burros in 5 Western States

177 Herd Management Areas

Current situation in 2026

BLM estimates >61000 horses, >23000 burros on BLM lands, but these are likely underestimates

The current 85,466 animals are 3x the ‘Appropriate Management Level’

Horse and burro populations can grow at 15-20% per year

Overpopulated, overgrazing

How many horses and burros does BLM estimates?

>61000 horses, >23000 burros on BLM lands, but these are likely underestimates

The current 85,466 animals are 3x the ‘Appropriate Management Level’; what was the original goal set by BLM as the appropriate management level across HMAs?

27K

Recent scientific reviews indicate that many areas are overpopulated and habitats (plants, soil, water) have been degraded due to?

overgrazing

Some herds are vulnerable to climate fluctuations, which leads to?

starvation

Current BLM Management

Gather and removal: animals are moved to short-term holding pens and to contracted (long-term) off-range ranches

• >63k animals held off-range in 2026

Animals are placed for non-consumptive adoption

177 designated HMA

>85k animals is 3 times the goal of 27k

What are some Fertility-control treatments?

PZP 1-2 years; Gonacon 4-5 years; Sterilization?

Plan for 2026

Removed at least 14k animals

Treated 1064 with fertility control (5500 mares in 2021-2025)

Placed 8080 animals in adoptions in 2025

What is the result of at least 14k animals removed?

Problems aren’t actually solved, as it’s only 22% of the population, not effective

Reproduction rates of horses and burros outperform these efforts

Cost of BLM Horse and Burro Management

$153 M expenditure in 2024

$142 M in 2025 is the budgeted amount, expenditure expected to be higher