The lowest average reproductive rate is reported from the Elcana Range, Nevada, from 1989 to 1998 when just 36% of mares foaled each year [3, 4].
All other populations have mare reproductive rates that fall between these two extremes. And so it is typical for around half of all mares of reproductive age to foal each year with some variation between populations (see the table below). Rates greater than 65% are comparatively high, and rates less than 50% are comparatively low.
No population is the same…
Large variation, therefore, exists between populations in reproductive rates and their capacity to grow. Most variation between populations is likely driven by the fundamental characters of their locations – their topography and climate – and the influence of those on the quantity and quality of food and shelter, and ease-of-access to water. Some places are just nicer and easier to live and breed.
… even from year to year
It is also evident from the accompanying table, however, that variation in annual reproductive rates from year to year for the same sites are greater than the spread of values between sites, due to annual changes in range conditions.
Range conditions can improve or deteriorate dramatically between years because they are driven by both changes in climate and animal density – food and shelter, and competition for them.
The high reproductive rates on Chinocoteague [1, 2], for example, were probably facilitated by the annual live harvest from the population that occurs to this day and reduces competition amongst horses for grazing.
New Zealand’s wild horses in the Kaimanawa Mountains (highlighted in the Table) mares had comparatively moderate foaling rates averaging 55% during the late 1990s and so sit squarely between the extremes reported . Nevertheless, the substantial population reductions that have occurred since this time are likely to have elevated reproductive rates.
It is likely that average, and especially single annual , rates could be found that are lower than the values shown – extreme climatic events or disease might result in some very poor foaling rates in some years. But it is much less likely that new higher values will be found. Individual annual rates exceeding 81% and average rates of around 75% are probably at or near the largest possible given the inherent constraints on mare reproduction.
These real-world values are a useful insight informing our conversation about population growth and what is and is not possible or claims credible.
The question of 2-year-olds foaling
In this post, I have considered only mares 3 years old and older as adult (reproductive age) because most studies, including those represented in the Table above, report that 2-year-olds did not foal. Nevertheless, in a few populations 2-year-olds do foal with potentially important consequences for population’s capacity to grow.
Does earlier-age foaling elevate a population’s reproductive rate or are younger, still growing mares so exhausted by raising their foal that they are unable to foal again for several years?
I will address reproductive rates of 2-year-old and their adult mare populations next.
Bibliography and notes
1. Keiper RR. 1979. 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; 175-183.
4. I have not used values from studies where only one year of data is available because they are much less likely to represent the population average. For example, Kirkpatrick and Turner (1991)  reported 32.5% of mares foaling in 1990 on Assateague Island National Seashore.
9. Goodloe RB, Warren RJ, Osborn DA, Hall C. 2000. Population characteristics of feral horses on Cumberland Island, Goergia and their management implications. Journal of Wildlife Management 2000, 64:114-121.
10. 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; University of Wyoming, Laramie. Edited by Denniston RH. University of Wyoming, Laramie; 185-204.