The number of foals when compared to the number of other horses or total population is a useful measure of a population’s reproduction because it is so easily obtained and interpreted. It is also an indication of a population’s potential to grow because it cannot grow faster than it produces foals.
Easy to get
Foals are quickly and reliably distinguished from other horses, even from a light plane or helicopter and especially if the population estimate is made soon after the end of the foaling season. Late-summer through early autumn would be the ideal time for most populations. Most foals are born late-spring and rarely in late-autumn and winter.
Importantly, the measurement does not require that other horses in the population are aged or sexed. More studies, even more detailed demographic studies, should present a ratio of foals to other horses to help make comparisons amongst populations.
Easy to interpret
Conveniently, the proportion of foals in the population (number of foals divided total population size) will be larger than a population’s instantaneous rate of increase – given the label r by population biologists – because there must also be deaths.
For the same reason, the ratio of foals to other horses (number of foals divided by the number of other horses) should be larger than a population’s growth in the same year – that is given the label lambda, λ.
A check for growth potential
Because the proportion of population that are foals and ratio of foals to other horses should be larger than r and λ, respectively, they are a useful guide of a populations potential to grow or a useful retrospective check of the reliability of population growth estimates.
There are some egregious examples published in the scientific literature where extraordinary claims of feral horse population growth were larger than the foals in the population could support (see insert). When an estimate of population size is made, I recommend that a separate count of foals also be made to validate subsequent estimates of population growth.
2. Linklater, W.L., Cameron, E.Z., Stafford, K.J., Minot, E.O. 2001. Estimating Kaimanawa feral horse population size and growth. Science & Research Unit, Science, Technology and Information Services, Department of Conservation.