For many years during the early, vigorous debates about how fast wild horse populations could grow, the ability of younger mares to foal, especially 2-year-olds, was unknown. The first few studies found that 2-year-olds did not foal and so it was believed unlikely.
Even in populations like Chincoteague (1975-82), with the highest foaling rates reported, no 2-year-old mares foaled.
Pregnancy testing wild mares
The early development of pregnancy testing by radioimmunoassay and its application to wild mares mustered from their range also found no evidence for 2-year-old foaling.
From three places near Salmon, Idaho, during October, 1980, musters removed about half the horses – 300 horses all together including 137 mares – but found no evidence of pregnancy in 1-year-olds or lactation by 2-year-olds .
First-foaling as a 3-year-old was the norm
During the early years of the debate, however, high population growth rates were being calculated from sequences of population counts.
Counts from Beaty’s Butte and Jackie’s Butte during the 1970s indicated a 20-22% average annual increase .*
Similar, but probably less reliable count sequences, led to reports such as:
“During the past 3 years it has been estimated that wild horse numbers are increasing at an estimated rate of 20-30% each year”  – C. Wayne Cook: Rangeman’s Journal, 1975
To achieve population growth of this magnitude, high foaling rates, especially by younger mares, were expected but not being supported by demographic evidence from the better studied populations.
From Assateague Islands and Chincoteague (1975-82) [1, 6], and the deserts of Wyoming (1978-79)  and Nevada (1980-82) , mares foaled for the first time only as 3-year-olds and most in all populations did not foal until they were 4 or 5 years old. Two-year-olds did not foal.
But then came Joel Berger’s landmark fieldwork in the Granite Range of Nevada during the early 1980s . It changed our minds entirely.
Teen pregnancy, Nevada
From 1979 to 1983 Joel Berger followed a population that was 58 at the beginning of his study.
He discovered extraordinary foaling rates, especially in younger mares foaling for the first time.
About 37% of 2-year-olds and 40% of 3-year-olds foaled. Given that 2-year-olds that foaled are unlikely to have foal again as 3-year-olds, these figures indicate that about 80% of mares foaled for the first time as 3-year-olds or younger. Suddenly, the capacity for rapid population growth was understood.
During the five years of the study, the Granite Range population grew by 91 horses to 149 – an instantaneous rate of increase (r) of 0.188 or a finite rate of increase (lambda, λ) of 1.21. In other words, the population had an average annual increase of 21% **. Clearly, two-year-old foaling in large numbers was possible.
Young mare foaling and eruptive growth
Joel’s work explains how some populations of horses can undergo, what is called, eruptive growth – short periods of maximum reproduction and survival leading to rapid population growth.
It is clear that for short periods of time, such as 4-5 years, extraordinarily high rates of first time foaling by 2- and 3-year-olds can translate into population growth that just exceeds 20% per annum. Importantly however, the population growth that Joel observed did not approach the earlier alarmist reports of growth rates up to up to 30% per annum .
Joel’s work linking first-time foaling rates and rapid population growth is exceptional and a useful insight into the largest growth rates possible for wild horse populations. Nevertheless, many subsequent studies would continue to demonstrate that in other places 2-year-old foaling was uncommon.
In similar North American studies on Pryor Mountain (1976-86)  and Cumberland Island (1986-90) , in the mountains of New Zealand (1994-98) , and grasslands of Argentina (1995-2002) , 2-year-old foaling either did not occur or was rare.
If 2-year-old foaling is possible and can occur at high rates, then why wasn’t it more commonly found?
Or, if 2-year-old foaling is more common than studies have detected, why is rapid population growth not more common?
– topics for subsequent posts.
* Eberhardt, Majorowicz & Wilcox (1982) has been repeatedly used as supporting evidence of high population growth rates in wild horse populations but the authors were careful to write “We do not propose that these herds are necessarily typical of feral horses in general”. It should not be assumed that the high growth rates reported by Eberhardt, Majorowicz & Wilcox are ubiquitous, only possible.
** In Joel Berger’s book (P76) the average finite rate of increase is written as 31%. This is incorrect and probably a printing error that should have read 21%. A 31% rate of increase would have generated 225 horses from 58 over the study period, not the 149 horses that were alive at the end of the study.
2. Boyd, L. (1979) The mare-foal demography of feral horses in Wyoming’s Red Desert. In Symposium on the ecology and behavior of wild and feral equids (Denniston, R.H., ed), pp. 185-204, University of Wyoming, Laramie.
5. Cook, C.W. (1975) Wild horses and burros: a new management problem. Rangeman’s Journal 2, 19-21.
8. Berger, J. (1986) Wild horses of the Great Basin. University of Chicago Press.