Conditional fetal and infant killing by male baboons

Abstract

Sexually selected feticide-the death of infants in utero as a result of male behaviour-has only rarely been described or analysed, although it is presumed to be favoured by the same selective pressures that favour sexually selected infanticide. To test this hypothesis, we measured the frequency of feticide and infanticide by male baboons of the Amboseli basin in Kenya, and examined which characteristics of a male and his environment made him more likely to commit feticide and/or infanticide. We found a dramatic increase in fetal and infant death rates, but no increase in death rates of 1-to 2-year-old individuals, following the immigration of males who stood to benefit from feticide and infanticide. Specifically, fetal and infant death rates were highest following immigrations in which: (i) the immigrant male rapidly attained high rank, (ii) that male remained consistently resident in the group for at least three months, (iii) food availability and social group range overlap was relatively low and (iv) relatively many pregnant females and/or dependent infants were present. Together, these results provide strong evidence for the existence of both sexually selected feticide and infanticide in our population, and they indicate that feticide and infanticide are conditional male behavioural strategies employed under particular circumstances.

Male‐mediated prenatal loss: Functions and mechanisms

“…Like the Bruce effect, feticide may also occur any time after implantation, just as severe stressors and/or physical injury can affect the viability of a pregnancy at any time throughout gestation . Because the literature does not distinguish between the Bruce effect and feticide during the peri‐implantation period, we restrict the remainder of our review of feticide to documented cases of fetal loss after implantation has occurred (Supporting Information Table S1; e.g., langurs, Semnopithecus entellus and yellow baboons, Papio cynocephalus ).…”

“…Given this obvious point, perhaps a more interesting question when considering male‐mediated prenatal loss is how does it compare to infanticide as a sexually coercive strategy? Because pregnant females are more temporally distant from a return to cycling than are lactating females, killing a fetus at any given time may increase a male's mating opportunities to a greater extent than would killing an infant . In other words, the cost of waiting for males that do not commit feticide are much greater than the cost of waiting for males that do not commit infanticide .…”

“…To test this claim, we used R to conduct computer simulations of the behavioral strategies of male yellow baboons (a species in which feticide has been reported). Specifically, we assessed the number of reproductive opportunities that immigrant males encountered in new social groups under different behavioral strategies: infanticide, feticide, random aggression toward females, and inaction (Box ).…”

“…If the Bruce effect is an adaptive female response to infanticidal males, why do we not observe the Bruce effect in all species with feticidal and/or infanticidal males? First, if feticide/infanticide occurs at very low frequencies (e.g., in yellow baboons), then the Bruce effect will confer more costs than benefits on pregnant females. Second, if feticide/infanticide occurs at high frequencies, but the rate of immigration by nonsire males is high relative to the rate of female reproduction, then—again—the Bruce effect will have higher costs than benefits for pregnant females.…”

“…In this review, we provide an overview of male‐mediated prenatal loss, a phenomenon by which inseminated or pregnant females abort reproduction following exposure to a nonsire male, either through implantation failure or pregnancy termination. Two related types of male‐mediated prenatal loss have been identified (Box ): (a) Sexually selected feticide occurs when direct aggression from a nonsire male causes a female to terminate an established pregnancy, either by causing direct damage to the fetus and/or by inducing a generalized stress response in the mother . (b) The Bruce effect occurs when females terminate a conceptive event (i.e., implantation failure or pregnancy termination) after exposure to nonsire males without the need for male aggression (although male aggression may occur, aggression is not a necessary component).…”