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.
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 . 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 , where the foals to other horse ratios were 0.295 and 0.217 in 1978 and 1979, respectively (average 0.256).
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 , 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 . In the Tornquist Park population, Argentina , 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 , can be seen here to be moderate. Indeed, the reproductive rate was inflated by
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 .
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 . Similarly low rates of 0.15 to 0.19 between 1986 and 1990 also occurred on Cumberland Island, Georgia (USA) . 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 . In most populations 2-year-olds were not reported to foal and so were excluded from calculations.
Joel Berger reported 37% of 2-year-olds foaling in the Granite Range, Nevada , 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.
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.
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.