Teenage pregnancy in horses…

…or, why is foaling by 2-year-olds not more common.

(image source: http://www.thehorse.com)

Young, still growing, mares face greater challenges to succesfully foaling (image source: http://www.thehorse.com)

Our transition from child to adult is a fraught with physical and mental challenges, made worse by our inexperience. We are less able, when our life’s challenges are greatest. Its tough being a teen.

For horses the transition to adulthood occurs from about just before their first birthday through their second year and, for some, into the beginning of their 3rd year. Compared to people, horses (and most other animals), mature quickly.

A demanding, vulnerable time

Young mares are naive and still investing into growing their bodies. Their inexperience at competing for resources, like food and shelter, and protecting themselves from risks, like aggressors, competitors and predators, makes them vulnerable.

They have not yet formed protective relationships with other mares [1] and a stallion [2] in a breeding and social group called a band. They will, at some time during their teens and probably around their first oestrus, leave their mothers band for another – called natal dispersal [3].

During their teens they may move often between bands – called social dispersal [3] – where they will be more attacked and harassed by other mares, stallions, and bachelor males than at any other time in their lives. It is a high-risk period and the consequence is reproductive failure.

Reproductive delay

The challenges of maturing and growing can delay a mares first foaling and makes rates of reproductive failure higher in younger mares such that foaling rates rise as mares age into their middle years.

Mares are capable of conceiving as soon as they reach sexual maturity around 1 year and, therefore, foaling as a 2-year-old but more commonly they will foal for the first time as 3-, 4-, or 5-year-olds because the physical, cognitive (mental), and social demands of maturing place great demands on them.

Young mare foaling rates measured

The age that mares first breed and foaling rates of young mares have been measured in only a handful of studies where frequent observations have meant that mare birth dates were known and their subsequent foals found.

On Assateague Island National Seashore no 2-year-olds foaled but 23% of 3-year-olds did and from there foaling rates increased up to 69% in 6-year-olds [4]. Similar values with mare age are reported from Nevada, and Cumberland Island, Georgia [5, 6] (see accompanying graphic).

young mare foalingStallions impact on young mare breeding?

The pattern of climbing foaling success was very different in the Pryor Mountain population [7] because, although 2-year-olds did not foal and older mares had comparative low foaling rate, 3-year-olds had a substantial foaling rate – almost identical to the extraordinary foaling rate of 3-year-olds in Joel Berger’s erupting population in the Granite Range, Nevada [8].

I suspect that this difference reflects the extremely mare-biased adult sex ratio due to stallion removals from that population. Where there are substantially fewer stallions, the costs of social dispersal and sexual harassment for 1- and 2-year-old mares breeding for the first time might be substantially reduced. Sexual and social competition have consequences, especially during dispersal for first-time breeders.

Teen pregnancy in Nevada

The magnitude of the eruptive reproductive rates that Joel Berger observed in the Granite Range, Nevada (1979-93) are best revealed when plotted for comparison against other populations [8]. Not only were 2 year olds foaling, but all mares 2 years old and older were foaling at the highest rates ever reported. A remarkable 86% of mares foaled in their sixth year.

The extraordinary foaling rates are because a suite of historical events, particularly the removal of cattle – a competitor of horses – combined to improve the amount and quality of food (grass) available to the Granite Range population.

Kaimanawa mare foaling unremarkable

Although many claimed that Kaimanawa population was undergoing eruptive growth like the populations described in the Granite Range, and Jackie’s and Beaty’s Butte, Oregon, the data on foaling rates by mares did not support those claims [9].

The graphic serves to show how unremarkable foaling rates by young mares in the Kaimanawa Ranges were compared to other populations. Foaling rates by 3 and 4-year-olds were year olds were low compared to all except the Cumberland Island population, Georgia [6].

Interestingly though, in 1998 a single two-year-old foaled – the first and only one to do so. Was this an indication of improved range conditions because musters had reduced horse densities?


1. Cameron, E. Z., Setsaas, T. & Linklater, W. L. Social bonds between unrelated females increase reproductive success in feral horses. Proceedings of the National Academy of Sciences of the United States of America 106, 13850-13853 (2009).

2. Linklater, W. L., Cameron, E. Z., Minot, E. O. & Stafford, K. J. Stallion harassment and the mating system of horses. Animal Behaviour 58, 295-306 (1999).

3. Linklater, W. & Cameron, E. Social dispersal but with philopatry reveals incest avoidance in a polygynous ungulate. Animal Behaviour 77, 1085-1093 (2009).

4. Keiper, R. & Houpt, K. Reproduction in feral horses: An eight year study. American Journal of Veterinary Research 45, 991-995 (1984).

5. Siniff, D. B., Tester, J. R. & McMahon, G. L. Foaling rate and survival of feral horses in Western Nevada. Journal of Range Management 39, 296-297 (1986).

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

7. Garrott, R. & Taylor, L. Dynamics of a feral horse population in Montana. Journal of Wildlife Management 54, 603-612 (1990).

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

9. Linklater, W. L., Cameron, E. Z., Minot, E. O. & Stafford, K. J. Feral horse demography and population growth in the Kaimanawa Ranges, New Zealand. Wildlife Research 31, 119-128 (2004).

  1. Wayne, since the Kaimanawa horse population has been reduced to just 300 horses, we’re now finding foaling as 2 year olds to be quite common. We definitely speculate that it’s due to less competition for resources & fitter, healthier animals with lesser parasite burdens than previous.

    • Thank you Simone for your message. I would be very interested to compare Kaimanawa reproduction and survival rates now with those in the past. Do you know what sort of information is currently being recorded?
      – Wayne

      • No research, just anecdotal evidence from those who receive young mares from musters. They’ve had them aged by dentists to confirm but that’s as formal as the evidence gets. We’re desperate to have more research done on the population! There has been so many changes since your work was done. Now that there are only 300 horses, we should probably know exactly what we’re dealing with so we know how to best manage them.

      • Thank you Simone. I would very much like to see new research done too and support some post-graduate researchers working with the population again. Would ‘Kaimanawa Heritage Horses’ like to help support further research? Perhaps we should discuss in greater detail what might be useful and what opportunities exist for research, or what could be done to facilitate new research?
        – Wayne

      • That would be a resounding YES on all counts! 😀

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