Populations’ reproductive rates compared – how high, how variable?

The ratio of foals to other horses, or foals as a percentage of total population, are comparatively easy to gather and interpret reproductive rates that have a direct relationship to how much a population grows or declines each year (lambda, λ) and the population’s instantaneous rate of increase or decrease (r). Averaging and comparing reproductive rates over several years and how they vary from year to year provides insight about the potential of populations to grow.

Reproductive rates from Jackie’s and Beaty’s Buttes from 1969 to 1980 are amongst the highest and most variable reported. Around 26 foals for every 100 other horses was the average annual rate and it ranged from 15 to 39 foals for every 100 other horses in different years (image source: http://www.blm.gov).

The most extraordinary foaling rates with equally extraordinary variability between years are reported from Jackie’s Butte and Beaty’s Butte, Oregon. Foals to other horse ratios averaged 0.27 and 0.25, respectively, and ranged from 0.19 to 0.39 in Jackie’s Butte, and 0.22 to 0.30 in Beaty’s Butte, from 1969-80 [1]. Thus, foaling rates ranged up to 30% below to 44% above the 12-year average.Similarly high rates are reported from the Red Desert, Wyoming [2], where the foals to other horse ratios were 0.295 and 0.217 in 1978 and 1979, respectively (average 0.256).

Moderate foaling

Foaling rates were not nearly as high in other places but still highly variable from year to year. From 1990 to 1994, in the Elcana Range between the Mohave and Great Basin deserts, at a place better known for being Nevada’s nuclear weapons test site [3], foal to other horse ratios averaged 0.196 and ranged from 0.148 to 0.262, or from 25% below to 34% above the 5-year average [4]. In the Tornquist Park population, Argentina [5], the ratio is not presented but I was able to calculate it for 1995 and 2002 because population counts for those years are also given. Similarly, the foal to other horse ratio was 0.196 and 0.21 for those years.

…even though they had mare-biased sex ratios

Foaling rates on the Pryor Mountain Wild Horse Range from 1976 to 1986 averaged 0.189 and, although reported to be high [6], can be seen here to be moderate. Indeed, the reproductive rate was inflated by

Horses of the Assateague Island National Seashore (1975-79), like the Pryor Mountain herd around the same time, had twice as many mares as stallions but comparatively moderate reproductive rates less than 0.20 foals for every other horse, although a higher ratio of mares to stallions will elevate population-level reproductive rates (image source: tlc.howstuffworks.com).

an adult sex ratio biased towards mares. There were twice as many mares as stallions due to a history of removals that were selective of males. If managers had preserved an even ratio of adult mares and stallions the population would have reproduced itself at a much lower rate.

There were also over twice as many mares as stallions on the Assateague Island National Seashore (2.1 mares per stallion) when Ronald Keiper, Pennsylvania State University, investigated the population dynamics of its introduced horses during the late-1970s. Nevertheless, those rates too were moderate at 0.198 foals per other horse [7].

When comparing reproductive rates it is important to check for distortions in adult sex ratio away from male-female parity (that is equal in number). Stallion-biased sex ratios will suppress, and mare-biased ratios inflate, population-level reproductive rates like those presented here.

Lower foaling rates

Other places had much lower foaling rates. The foaling rate in Stone Cabin Valley, Nevada, during 1975 and 1976 was 0.12 and 0.18 foals for every other horse [8]. Similarly low rates of 0.15 to 0.19 between 1986 and 1990 also occurred on Cumberland Island, Georgia (USA) [9]. Examples of populations with lower reproductive rates appear to be less often written about although they must occur. One would expect a scientific publication bias towards populations with higher reprodcutuve rates where population impact and control, not conservation, has been the motivation for research.

More detailed demographic studies

Unfortunately, most other more detailed demographic studies are more difficult to interpret for comparison because they report the foaling rate as a percentage of adult mares foaling and define the age a mare is adult differently. Some define all females 3 years old and older as adult mares, while others consider 2-year-olds also adults.

This difference in how studies report foaling rate per adult female results because only a few populations report 2-year-olds foaling [10]. In most populations 2-year-olds were not reported to foal and so were excluded from calculations.

Joel Berger's 1986 book - amongst the first to identify comparatively high reproductive rates amongst 2- and 3-year-old mares.

Joel Berger’s 1986 book – amongst the first to identify comparatively high reproductive rates amongst 2- and 3-year-old mares.

Joel Berger reported 37% of 2-year-olds foaling in the Granite Range, Nevada [10], and was the first to find substantial foaling by young mares and so explain why some feral horse populations could grow more rapidly. Because the rate at which 2-year-olds foal may be critical to rapid population growth I will devote a post to two-year-old foaling rates later.

Upper limits to population growth

Studies report, therefore, from 15 to almost 30 foals per 100 other horses and so help define the upper limits of population growth. A population cannot grow 30% or more over several years if each year it averages fewer than 30 foals per 100 other horses each year.

For the moment, reports of the ratio of foals to other horses in populations indicate that their average growth rate should not exceed 27% per year and can vary considerably – by as much as 110%  between years and from 30% below to 44% above the long-term average.


1. Eberhardt, L.L., Majorowicz, A.K., Wilcox, J.A. 1982: Apparent rates of increase for two feral horse herds. Journal of Wildlife Management 1982, 46: 367-374.

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.

3. Nuclear explosions at Nevada’s nuclear weapons test site occurred above ground 1951 to July 62, and below ground until 1992.

4. Greger PD, Romney EM. 1999. High foal mortality limits growth of a desert feral horse population in Nevada. Great Basin Naturalist, 59:374-379.

5. Scorolli, A.L. and Cazorla, A.C.L. 2010. Demography of feral horses (Equus caballus): a long-term study in Tornquist Park, Argentina. Wildlife Research 37: 207-214.

6. Garrott R, Taylor L: Dynamics of a feral horse population in Montana. Journal of Wildlife Management 1990, 54:603-612.

7.Keiper RR: Population dynamics of feral ponies. In Symposium on the ecology and behavior of feral and wild equids; University of Wyoming, Laramie. Edited by Denniston RH. University of Wyoming, Laramie; 1979: 175-183.

8. Green, N.; Green, H. In The wild horse population of stone cabin valley, Nevada: A preliminary report, Proceedings of the National Wild Horse Forum, University of Nevada, Reno, 1977; University of Nevada, Reno, pp 59-65.

9. Goodloe, R.B.,Warren, R.J., Osborn, D.A., Hall, C. 2000. Population characteristics of feral horses on Cumberland Island, Goergia and their management implications. Journal of Wildlife Management 64, 114-121.

10. Berger, J. (1986) Wild horses of the Great Basin. University of Chicago Press.

  1. ” If managers had preserved an even ratio of adult mares and stallions the population would have reproduced itself at a much lower rate.”
    That is a bit of important news for the BLM and other wild horse management agencies.
    I’d like to add that while fillies may have their first foal at two, they are often in too poor of condition after foaling and nursing to come back into heat again. As foals often nurse well into their second year without human interference, the dams may be 4 or 5 years old before they have regained enough weight and are in good enough condition to conceive and carry to term again.

    • Thank you for your comment. Indeed, this was also a recommendation of the recent National Academy of Sciences Report when they evaluated the BLM wild horse and burro program. You are correct. Early first-time foaling may also delay subsequent foaling, but how much of a delay will depend on how well resourced the mare is. I will address the trade-off between early foaling and foaling interval in a future post – it is an interesting topic because it draws on ecological and physiological science. Happy New Year!

      • I should have said that my default environment is the high desert here in the southwest, and its seriously limited resources for grazing animals. Your article does make me think that the most effective least disruptive culling would be removing equal numbers of yearling colts and fillies.

      • Interesting. I agree – one shouldn’t remove more males than females if the goal is to reduce population growth rates and reduce the requirement for future management. What makes you think, though, that this would be least disruptive?

      • In my own herd, foals were never injured, but obnoxious adolescents were often thoroughly thumped by their elders until they figured out their new standing in the herd. Some were driven off to the fringes, and I would think in a truly natural setting would have migrated into another herd. I figured the horses had a better sense than I did of what a properly socialized and civil herd member should be, so the cast outs became my sale stock.

      • There is good evidence that aggression, of the sort you describe between dominant and subordinate horses but also between stallions and mares, has more substantial and longer-term effects too. Mares that receive greater harassment from stallions are in poorer condition, are less likely to get pregnant, less likely to foal, and their foals are more vulnerable to injury and death (see citations of relevant article below). Ordinarily one might expect management that induces male-biased population sex ratios (i.e., removing more females) to be a good idea becasue it would further reduce population growth potential. However, it is likely that male-biased sex ratios will also elevate the amount of stallion-mare agression and so their are ethical adn animal welfare impediments to its implementation. Nevertheless, maintaining an even adult sex ratio seems a good place to start – it is the sex ratio that will exisit in the absence of managment. Thanks for your interesting comments – Wayne.
        Linklater WL, Cameron EZ, Minot EO, Stafford KJ: Stallion harassment and the mating system of horses. Animal Behaviour 1999, 58:295-306.
        Cameron E, Linklater W, Stafford K, Minot E: Social grouping and maternal behaviour in feral horses (Equus caballus): the influence of males on maternal protectiveness. Behavioural Ecology and Sociobiology 2003, 53:92-101.

      • The few serious injuries that occurred in my herd were actually adolescent colts that were kicked by the elder mares. One colt ended up with a hairline crack in his stifle because he keep leaping the mares without their consent. That would have been a life-ending injury in the wild. My mature stallions were exceedingly cautious and gracious with the ladies. So my observation was that the aggressive stallions were weeded quickly when they were quite young.The higher the rank of the mare, the kinder and more generous she was. My lead mare often had a foal on each teat and the rest waiting in line. Lower-ranking mares on the social fringe weren’t as caring of their own foals, never mind foals from other mares. I tend to think that issues of inappropriate aggression are usually based in our human mismanagement.

      • The peaceable herd ended up with the elder stallion mentoring the bachelor boys, colts up to about 6 years old who hung out together and followed the old man’s cues on how to interact with the ladies. For the most part, they avoided the mares. Assimilated fillies hung out with their dams and acted as aunties to the new foals. Loner fillies were pushed to the edges of the mares groupings and probably would have ended up in a different herd if that had been an option.

      • Observing herds in need of population control closely and removing those youngsters that are targets for aggression would both honor the existing herd relationships and ease the ethical and animal welfare concerns. That would be best case scenario in my eyes.

  2. Thanks for the link – interesting ideas. I’d love the opportunity to test some of them in other populations.

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