Territory exclusivity and intergroup encounters in the indris (Mammalia: Primates: Indridae: Indri indri) upon methodological tuning

Abstract

Territorial, socially monogamous species actively defend their home range against conspecifics to maintain exclusive access to resources such as food or mates. Primates use scent marks and loud calls to signal territory occupancy and limit the risk of intergroup encounters, maximizing their energetic balance. Indri indri is a little-studied territorial, socially monogamous singing primate living in family groups. The groups announce territory occupancy with long-distance calls, and actively defend their territories from conspecific intruders. This work includes data collected in three forests in Madagascar on 16 indri groups over up to 5 years. We aimed (1) to estimate the extent of territories using minimum convex polygon (MCP), implementing minimum sampling effort requirements; (2) to quantify territorial exclusivity, measuring the overlap between territories; and (3) to evaluate the intergroup encounter rate and to quantify the dynamics of group encounters. Our results showed that indris range evenly within exclusive small territories with no or little overlap. Intergroup encounters are rare (0.05 encounters per day), and are located on the periphery of the territories. Disputes were mostly solved with vocal confrontation and only in 13% of the cases ended in physical fights. This frame underlines a cost-benefit explanation of territoriality, favouring a strategy that efficiently limits overlap and avoids costly intergroup encounters. We hypothesize that territorial behaviour in indri is related to mate-guarding strategy and that vocal behaviour plays a fundamental role in regulating intergroup spacing dynamics.

Evidence of genetic monogamy in the lemur Indri (Indri indri)

“…We updated group composition and demographic records year by year, identifying each individual using natural marks. The population density in the study area was 27.7 + 4.0 (mean + SD) individuals and 8.4 + 1.0 (mean + SD) groups per km 2 , comparable to other populations inhabiting different sites (Bonadonna et al, ; Pollock, ).…”

“…Each group exchanges information about individuals’ sex, status, and genetic relatedness throughout the emission of loud songs way beyond the limit of a territory (Torti et al, ). In this way, groups are not isolated units and individuals can communicate with one another without having visual or physical contact (Bonadonna et al, ; Gamba et al, ; Giacoma, Sorrentino, Rabarivola, & Gamba, ). Such a system would allow regulating intergroup dynamics including the possibility of engaging in EPC (Bonadonna et al, ), and therefore providing a certain degree of flexibility in the mating system of this species.…”

“…Group members tend to be cohesive, and usually, the individuals of the reproductive pair stay within a distance between 0 and 13 m from each other (Pollock, ). Each group occupies a stable and exclusive territory and intergroup encounters are very rare (Bonadonna et al, ). The indri is one of the few singing primates and, besides having a function in signaling territory occupancy and defense (Torti, Gamba, Rabemananjara, & Giacoma, ), the song also plays an important role in the mating and social system of this species.…”

Sexually dimorphic phrase organization in the song of the indris (Indri indri)

“…Females may use songs for mate attraction (Rogers et al, 2007), and they may even show a more elaborated song repertoire (Australian magpies; Gymnorhina tibice Brown & Farabaugh, 1991). Like Eastern whip birds and Australian magpies, indris are monogamous (Bonadonna et al, 2019; Torti et al, 2017), form groups that occupy nonoverlapping areas in the forest (Bonadonna et al, 2017), and use the songs to inform neighboring groups about the occupation of a territory and to actively defend resources during group encounters (Torti et al, 2013). Thus, we hypothesized that the female contribution to the song would be structurally different from that of males.…”

Multimodal pair‐bond maintenance: A review of signaling across modalities in pair‐bonded nonhuman primates

“…Multimodal components may be particularly important for pair‐bonded species because a given pair may not be familiar with what other pairs use as their preferred signaling strategies, and because pairs typically try to stay a little further apart from each other such that signals may be more prone to potential environmental degradation. Indri indri uses a combination of auditory (duetting), chemical (scent‐marking), and visual signals (opposite sitting and staring) during intergroup encounters, while the use of overt aggression via tactile signals (biting and scratching) is generally rare (Bonadonna et al, 2017). Thus, pair‐bonded species may rely on multiple modalities during intergroup encounters to overcome potential barriers like within‐pair signal preferences and environmental obfuscation and reduce the likelihood of risky physical altercations.…”