Kidnapping intergroup young: an alternative strategy to maintain group size in the group-living pied babbler (Turdoides bicolor)

Abstract

Both inter- and intragroup interactions can be important influences on behaviour, yet to date most research focuses on intragroup interactions. Here, we describe a hitherto relatively unknown behaviour that results from intergroup interaction in the cooperative breeding pied babbler: kidnapping. Kidnapping can result in the permanent removal of young from their natal group. Since raising young requires energetic investment and abductees are usually unrelated to their kidnappers, there appears no apparent evolutionary advantage to kidnapping. However, kidnapping may be beneficial in species where group size is a critically limiting factor (e.g. for reproductive success or territory defence). We found kidnapping was a highly predictable event in pied babblers: primarily groups that fail to raise their own young kidnap the young of others, and we show this to be the theoretical expectation in a model that predicts kidnapping to be facultative, only occurring in those cases where an additional group member has sufficient positive impact on group survival to compensate for the increase in reproductive competition. In babblers, groups that failed to raise young were also more likely to accept extragroup adults (hereafter rovers). Groups that fail to breed may either (i) kidnap intergroup young or (ii) accept rovers as an alternative strategy to maintain or increase group size. This article is part of the theme issue ‘Intergroup conflict across taxa’.

Group augmentation underlies the evolution of complex sociality in the face of environmental instability

“…Such need-based group size adjustments have previously been demonstrated in cooperatively breeding fish where group augmentation benefits similarly drive the formation of mixed-kin groups in an environment with high predation pressure ( 44 , 88 ), but not in terrestrial species where climatic variability influences fitness. Interestingly, however, two other cooperatively breeding bird species living in harsh environments have been observed actively “kidnapping” unrelated young from other groups to augment group size and mitigate the risk of group extinction ( 89 , 90 ), indicating that species may have different solutions to the same demographic and environmental challenges. By adjusting their reproductive strategies to alter group demography, social animals may therefore maximize fitness directly via group augmentation to ensure the persistence of their long-term associations in the face of environmental uncertainty.…”

Intergroup conflict: origins, dynamics and consequences across taxa

“…Mathew [46] provides extensive ethnographic evidence for the factors underlying partner choice and alliance formation for intergroup raids and warfare among the Turkana. Ridley et al [27] document an intriguing and hitherto poorly understood form of partner choice and alliance formation in pied babblers (Turdoides bicolor): kidnapping young from rivalling groups and raising them as one's own. They reason that although raising kidnapped young requires energetic investment and abductees are usually unrelated to their kidnappers, kidnapping may be beneficial in species where group size is critically a limiting factor on territory defence or reproductive fitness, for instance.…”

“…It provides, for the first time, to our knowledge, a side-by-side treatment of intergroup conflict across taxa, detailing the origins and consequences of intergroup conflict. Each in their own way, the contributions provide insight on intergroup conflict for social insects [32,33], social fishes [40], group-living birds [26,27], non-primate mammals such as banded mongoose [24], lions and wolves [17], several monkey species [41][42][43], chimpanzees [18,44] and humans [45][46][47].…”

“…Papio cynocephalus ), engage in violent conflict with neighbouring groups [ 13 – 18 ]. Intergroup conflict has also been observed in spider monkeys ( Ateles paniscus ) [ 19 ], hyaenas ( Crocuta crocuta ) [ 20 ], wolves ( Canis lupus ) [ 21 ], meerkats ( Suricata suricatta ) [ 22 ], banded mongooses ( Mungos mungo ) [ 23 , 24 ], in various group-living birds [ 25 – 27 ] and social fishes [ 28 ]. Social insects raid neighbouring colonies and kill enemy conspecifics [ 29 – 33 ].…”

Fitness consequences of outgroup conflict

“…Contest-related deaths of adults and offspring, infanticide by incoming breeding males, female transfers, and dispersal or eviction following a change in breeder (details above) can all lead to a reduction in group size. Conversely, group-size increases can arise when a breeding individual is usurped by a same-sex coalition ( Bygott et al, 1979 ; Ridley, 2011 ), when there is kidnapping of young from rival groups as seen in white-winged choughs ( Corcorax melanorhamphos ), banded mongooses and pied babblers ( Heinsohn, 1991 ; Müller and Bell, 2009 ; Ridley et al, 2022 ), and when intraspecific slave-raiding occurs in insects such as the honey ant Myrmecocystus mimicus and the fire ant Solenopsis invicta ( Bartz and Hlldobler, 1982 ; Tschinkel and Howard, 1983 ). Changes in group size can have a variety of fitness consequences, with the positive effects of increased group size the reverse of the negative ones arising from a reduced group size that we describe here.…”