Reproductive consequences of a matrilineal overthrow in rhesus monkeys

Matrilineal overthrows in macaque societies are rare but devastating events, often resulting in severe morbidity, mortality, and loss of individual and group fitness. The handful of documented macaque overthrows provides scant evidence to reveal the severity or longevity of reproductive consequences resulting from such violent events. We analyzed archival records from semi-free ranging rhesus monkeys, Macaca mulatta, across 6 years (55 ≤ N ≤ 107, from 2007 to 2012) during which time a matrilineal overthrow occurred (in 2009) to test the hypothesis that extremely violent interactions such as a matrilineal overthrow would significantly reduce reproductive fitness for the involved matrilines and for the troop collectively. The matrilineal overthrow resulted in a significant increase in infant loss for the population from the previous year (χ(2)  = 8.117, df = 1, P = 0.004), as evidenced by the fact that in 2009, but not in other years, the proportion of infants lost was greater than the proportion of viable infants (χ(2)  = 4.55, df = 1, P = 0.03). Moreover, the deposed matriline suffered 100% infant loss in 2009, a significant change from the previous year (χ(2)  = 7.87, df = 1, P = 0.005) while the attacking matriline suffered 50% infant loss (also a significant change from the previous year; χ(2)  = 4.44, df = 1, P = 0.035), with the uninvolved, lowest-ranking matriline showing no change in infant loss from the previous year (χ(2)  = 0.008, df = 1, P = 0.93). The deposed matriline did not produce viable offspring again until 3 years later. We further found that rates of severe fighting (as indicated by the number of fight wounds requiring medical treatment) were positively correlated with infant loss across the 6 years of the study (r[s]  = 0.943, P = 0.005). Our data indicate that extreme periods of intra-group conflict, such as the matrilineal overthrow, have marked short-term consequences for individual fitness, and may be extreme examples of the long-term influences that group violence exerts on the mean fitness within a primate troop.