The songs of the indris (Mammalia: Primates: Indridae): contextual variation in the long-distance calls of a lemur

Abstract

Contextual variation in the loud calls of strepsirhine primates is poorly understood. To understand whether songs given by indris in different contexts represent acoustically distinct variants and have the potential to elicit context-specific behaviours in conspecific listeners, we investigated the acoustic variability of these songs and the distance travelled by vocalizers after their emissions. Songs of 41 individuals were recorded from 16 indri groups in four different forest sites in eastern Madagascar. We collected a total of 270 duets and choruses arising during territorial defence, advertisement and cohesion. We demonstrated that the structure of indri songs conveyed context-specific information through their overall duration, but shared the sequential pattern of harsh units (roars) followed by long notes and, finally, descending phrases. Analysing in detail individual contributions to advertisement songs and cohesion songs, we found that the acoustic structure of units could be classified correctly with a high degree of reliability (96.23% of long notes, 80.16% of the descending phrases, 72.54% of roars). Future investigations using playback stimuli could explore the relationship between acoustic features and the information transmitted by the song.

Why “monogamy” isn't good enough

“…Pairs are cohesive, defend territories together through elaborate duetting, and extra-pair copulations have only been observed once [Bonadonna et al, 2014;Pollock, 1986;Powzyk, 1997;Torti et al, 2013]. While ring-tailed lemurs (Lemur catta) also form male-female bonds, they do not mate monogamously, and live in multi-male multi-female groups [Gould, 1996].…”

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

“…Further support of the cost-benefit explanation of territoriality is given by the rarity of encounters, and by the modality more frequently adopted: most encounters were resolved with the emission of songs (Torti et al 2013) and only in four cases were followed by physical fights. Studies on the sympatric species Propithecus diadema (Powzyk 1997), which lacks conspicuous singing activity, are in line with the "economic" interpretation of the song spacing function ("economics of defendability"; Mitani & Rodman 1979), because diademed sifakas defend their territories by extensive and energetically costly patrolling, scent marks and frequent intergroup fights.…”

“…Group encounters are rare: Pollock (1975) observed eight encounters in 15 months, and Powzyk (1997) none during a year of study. However, when two groups meet, they defend their territory ownership by (1) using long vocal interactions or territorial song (Maretti et al 2010;Torti et al 2013), and eventually (2) starting physical fights (Pollock 1975(Pollock , 1979Powzyk & Mowry 2006). When territorial, monogamous primates live in small groups, are sedentary and invest in far-detectable territorial advertisements (Willems & Van Schaik 2015), they maximize their energetic balance by avoiding territory overlap and physical intergroup encounters.…”

“…As described by previous studies (Pollock 1979;Powzyk 1997), indris actively advertise their presence and occupancy of a territory (Pollock 1979;Powzyk 1997) by chemical and acoustic signals. They emit longdistance advertisement calls throughout the territory, audible up to 2 km away (Pollock 1986;Powzyk 1997;Torti et al 2010Torti et al , 2013Gamba et al 2011;Sorrentino et al 2013). In the indri song, sexes are dimorphic: the overall investment in singing does not differ between males and females , but the tendency to display non-vocal territorial behaviours is more pronounced in adult males (Pollock 1975(Pollock , 1979.…”

OVERLAPPING AND SYNCHRONIZATION IN THE SONG OF THE INDRIS (Indri indri)

“…Group size usually varies between 2 and 6 animals . Loud singing in indris follows a sequential organization (Pollock, 1986), which shows evident species-and sex-specific features Gamba et al, 2011) and may serve conveying different messages (Bonadonna et al, 2013;Torti et al, 2013). We aimed to investigate whether the ability to synchronize calling during group chorusing, which may be a phylogenetic parallel to singing ability in humans, is present in Indri indri, a species who diverged from the human evolutionary path more than 65 million years ago (Steiper & Young, 2006).…”