Brief Report Received: April 5, 2001 Accepted: May 24, 2001
Folia Primatol 2001;72:245–247
Carriage of Infants by a Silverback Mountain Gorilla Ymke Warren a Elizabeth A. Williamson b, c a School
of Life Sciences, University of Surrey Roehampton, London, UK; Research Centre, Ruhengeri, Rwanda, and c Department of Psychology, University of Stirling, UK
b Karisoke
Key Words Mountain gorilla W Infant carriage W Agonistic buffering W Male care
Introduction
A previously unreported behaviour in mountain gorillas (Gorilla gorilla beringei) studied at the Karisoke Research Centre, Rwanda, is described in this paper. The silverback, Pablo, was seen carrying an infant in front of another silverback, Cantsbee. There is great interspecific and intraspecific variation in male primate interactions with immatures [1, 2]. Amongst the great apes, silverback mountain gorillas [3, 4] and adult male orangutans [5] rarely interact with infants, adult male chimpanzees transport and play with infants to varying degrees [6]. Two hypotheses are proposed to explain Pablo’s behaviour: agonistic buffering [7] and paternal investment [8].
Methods Data come from ad libitum observations (Sept. 1995–Jan 1997) and 825 h of focal follows (May 1995–Feb. 1997) on three habituated gorilla groups in the Volcanoes National Park, Rwanda [9].
Results
Two of 12 infants in one group were carried eight times by one silverback, Pablo (table 1).
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E.A. Williamson Department of Psychology University of Stirling Stirling FK9 4LA (UK) E-Mail
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Table 1. Summary of infant handling incidents by the silverback Pablo
Infant/age, months
Distance Activity between Pablo period and Cantsbee
Ovulating/ cycling females in group
Context1
Mahirwe 17
!5 m
Forage
One
After exchange of screams between Pablo and adult female Infant whimpered
Mahirwe 18
3m
Forage
None
Right-of-way conflict Aggressive calls Infant whimpered
Mahirwe 18
1m
Forage
None
Right-of-way conflict
Mahirwe 24
5m
Rest
One
Competition over cycling female Infant whimpered
Mahirwe 25
!5 m
Rest
One
Competition over cycling female Aggressive calls Infant whimpered
Mahirwe 25
!5 m
Rest
None
Aggression between silverbacks or protection of infant by Pablo Aggressive calls
Mahirwe 26
3m
Forage
None
Forage competition Aggressive calls
Rukundo 25
2m
Forage
None
Cantsbee displayed at Pablo Forage competition
1
All conflicts, competition and aggression were between Pablo and Cantsbee.
Discussion
Idiosyncratic interactions between silverbacks and immatures have been recorded [9], but infant carriage has not been described as part of the silverback-immature interaction repertoire [3, 4]. With such a small sample size it was not possible to make definitive functional explanations for Pablo’s infant carrying, nevertheless, we propose two possibilities. Paternal investment: Males expend energy increasing the survival and, consequently, reproductive success of their offspring, at the cost of investment in future infants [8]. No evidence was found for extensive caretaking relationships, but it is possible that male care or paternal investment could account for some of Pablo’s infant carrying as generally silverbacks appeared to be ready to protect infants from aggression [9]. Even if Pablo was not the father, it is probable that he was related to the infants via his kinship with Cantsbee [10]. Agonistic buffering hypothesis: Males use infants to manipulate their social situation and prevent aggression from other males [7]. It was noted in previous studies that
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infant gorillas are not used as buffers [4, 11], but deterring aggression towards himself seemed to prompt Pablo’s infant handling. The context of triadic interactions is often tension between males [8, 12]. Rates of aggression while feeding are higher than during rest [13], and five incidents occurred during forage periods. The presence of receptive females might increase tensions between the males, and three incidents of infant carrying occurred while females were ovulating/cycling and both males mated, so there was reproductive competition. Additionally, in all but one of the incidents the males were within 5 m of each other and close spatial proximity creates tension [11]. It is also arguable that the two silverbacks’ dominance relationship was unstable, because it was relatively recently that Cantsbee gained dominance over Pablo [9]. Interestingly, in a captive colony of sooty mangabeys (Cercocebus atys), a recently-deposed alpha male carried an infant in the presence of the new dominant male. It was concluded that triadic interactions might be associated with periods of instability or changes in dominance relationships [14]. Thus, if Pablo perceived that a situation could become aggressive, he may have picked up an infant to deter a potential attack by Cantsbee and/or to protect the infant. We propose that infant handling was agonistic buffering, but do not exclude male care.
Acknowledgments We thank ORTPN, DFGF International and Europe, the Karisoke trackers, D. Greer, C. Key, F. Maisels, C. Ross and V. Sommer. This is Karisoke publication No. 147.
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