Female counterstrategies against male infanticide
Extra-unit mating In total, 4,697 copulation events were
observed. Only 317 of these copulation events were extra-unit matings
(6.7%), and these involved almost exclusively males in other one-male
units (309 events) rather than males in the all-male unit (8 events)
(χ2 = 182.00, p < 0.001).
Seven cases of extra-unit paternity, which involved resident males in
other OMUs, were identified in 99 births (6.25%). All new males who
took over OMUs came from AMUs, and accordingly were not the fathers of
any offspring in the OMUs they entered.
Of the nine mothers with unweaned infants who left their original OMU
and transferred to another OMU following male replacement, two mothers
(Ln in Case #6, Yb in case #16) had previously been observed mating
with the resident male of the destination OMU into which they
transferred, but no extra-unit paternities were identified for these two
cases.
Joint defense with
other femalesIn
cases of male attacks on unweaned infants, most females of the same OMU
(83.1%, 59/71) were observed to joint with other females against the
new male (Table S1 ). The lactating female whose infant was
under attack was the initiator of 66.7% (10/15) of coalitions as she,
after firstly seizing her infant, started shouting at the perpetrator.
Then, other females of the same unit (and sometimes also juveniles) ran
to the mother and subsequently started vocalizing loudly at the male. In
the remaining instances the mother was still seizing her infant while
other female already started shouting at the male before being joined by
others (including the mother). In a few cases, non-lactating female
collectively chased the perpetrator and keep it away. In response to
females’ facing and shouting at the male, he usually retreated quickly
and stayed away from the females for some time.
The number of adult females involved in collective defense was 3.38 ±
0.89 (mean ± SD). The defensive behavior of females resulted in 85.0%
(17/20) of the initial male attacks being unsuccessful, and since in the
majority of cases mothers whose infants were attacked subsequently
transferred to another OMU, most of these infants (16/17, 94.1%)
survived to weaning. The effectiveness of female joint defense appears
to depend on the size of OMU (the number of individuals except infants),
as OMU size was smaller in OMUs in which attacks had been observed than
in OMUs in which no attacks were observed (Fig 3a , t =
4.57, df = 26, p < 0.001), and the size of OMUs
in which infants died from infanticide was also significantly smaller
than those in which males never killed infants (Fig 3b ,t = 2.53, df = 29, p < 0.05).
.
F emales’
transfer with infants Nearly a third of mothers (11/31) transferred to
other OMUs or returned to stay with the old (ousted) male in response to
the unsuccessful attacks on their offspring or another offspring in the
OMU (Fig 1 ). The contexts of the nine mothers with dependent
offspring who successfully immigrated into an already existing OMU
included (i) five females who had at least one close female relative
(mother-daughter, sister-sister) in the new unit they joined, (ii) two
females who had been observed mating with the resident male of
destination unit, and (iii) two sets of two females who collectively
joined another OMU as a clique.