Abstract: Much evidence now suggests that the postnatal killing of young in primates and carnivores, and induced abortions in some rodents, are evolved traits exerting strong selective pressures on adult male and female behaviour. Among ungulates it is perplexing that either no species have developed convergent tactics or that these behaviours are not reported, especially as ungulates have social systems similar to those of members of the above groups. Only in captive horses (Equus caballus) has infant killing been reported. It has been estimated that 40,000 wild horses live in remote areas of the Great Basin Desert of North America (US Department of Interior (Bureau of Land Management), unpublished report), where they occur in harems (females and young) defended by males. Here I present evidence that, rather than killing infants directly, invading males induce abortions in females unprotected by their resident stallions and these females are then inseminated by the new males.

Abstract: Blood was collected every 3 days for 13 months from 4 captured [female][female] of proven fertility kept adjacent to a teaser stallion. Basal plasma LH level was greater during Apr.-July (8.1+or-0.5 ng/ml) than during Nov.-Jan. (2.2+or-0.2). A total for 21 LH peaks occurred between 13 Apr. and 31 Aug. among the 4 [female][female]; many peaks exceeded 20 times the basal level, and there was a trend to a higher LH level with each succeeding peak. On all occasions except one, LH peaks were associated with progesterone levels of 0.5 ng/ml and with increases of oestrogen (peak average 43.1+or-12.1 pg/ml). Basal progesterone level during Apr.-July (1.5+or-1.2 ng/ml) did not differ significantly from that during Oct.-Jan. (1.1+or-0.7), nor did basal oestrogen level differ significantly between those 2 periods (8.4+or-3.2 and 12.9+or-4.6 pg/ml resp.). Behavioural oestrus always occurred with LH and oestrogen peaks during Apr.-July. However, behavioural oestrus was occasionally observed during Aug.-Oct., when LH peaks no longer occurred.