Saturday, December 26, 2015

New research on vocalizations, grooming, and social bonds

Lemur catta Photo: author
Both grooming and vocalizing are linked to social bonds and relationships. Increases in the vocal repertoire of primates is correlated with group size and amount of time spent grooming (McComb and Semple, 2005). Previous research has shown that pairs of baboons (Papio cynocephalus) spend more time grooming one another (allo-grooming) and allo-groom more frequently if they are bonded socially (Silk et al., 2006). Dunbar first suggested that vocalizations may serve to act as a way of socially bonding when larger group sizes make grooming every member more difficult (1993; 2003; 2004).

A study published this December provides further insight into the reasons why small talk may have evolved and how it relates to social relationships. Studying ring-tailed lemurs (Lemur catta), Kulahci and colleagues (2015) examined vocalizations and grooming.  Four troops of free-ranging and semi-freeranging lemurs at the Duke Lemur Center and on St. Catherine's Island were studied. Vocalization and grooming network outdegrees were calculated to determine who each individual initiated vocalizations and grooming towards. The grooming network outdegree was simply the number of individuals the individual in question groomed. The vocalization network outdegree was   the number of individuals that the individual in question produced a vocal response towards upon hearing a call.

Kulahci and colleagues (2015) found that ring-tailed lemurs were more selective in who they vocalized to than who they chose to groom: as troop size increased, lemurs groomed more individuals but they did not vocalize directly towards more individuals. Regardless of troop size, vocalization network outdegrees were lower than grooming network outdegrees, showing that lemur are picky about who they vocalize to but less so about who they groom. Individuals responded to the vocalizations of lemurs they groomed more frequently than they did lemurs they groomed less often.

When audio playbacks were used, the same selectivity in vocalization responses was displayed. Thus, this selectivity in vocalizations is not due to olfactory or visual cues.

While this study agrees with previous work highlighting the connection between grooming and vocalizations, it does not align with Dunbar's hypothesis (1993; 2003; 2004) that vocalizations allow an individual to maintain more social bonds than grooming would. Whereas we would have expected to see vocalizations increase with group size, the opposite was true for this study. Rather than vocalize with more troop members as group size increases, L. catta selectively vocalize and groom all members. Thus, vocalizations may be a better indicator of social bonds than grooming, at least in L. catta.

Links of possible interest:
 Social grooming in primates
How the size of the neocortex and the size of grooming clusters relate
Derived vocal complexity of geladas


Dunbar, R. I. (1993). Coevolution of neocortical size, group size and language in humans. Behavioral and brain sciences, 16(04), 681-694.

Dunbar, R. I. (2003). The social brain: mind, language, and society in evolutionary perspective. Annual Review of Anthropology, 163-181.
Dunbar, R. I. (2004). Gossip in evolutionary perspective. Review of general psychology, 8(2), 100.
Kulahci, I. G., Rubenstein, D. I., & Ghazanfar, A. A. (2015). Lemurs groom-at-a-distance through vocal networks. Animal Behaviour, 110, 179-186.
Silk, J. B., Altmann, J., & Alberts, S. C. (2006). Social relationships among adult female baboons (Papio cynocephalus) I. Variation in the strength of social bonds. Behavioral Ecology and Sociobiology, 61(2), 183-195.

Monday, December 21, 2015

Another primate "sleeps" away hard times

Hibernation allows an organism to lower its metabolism, heart rate, body temperature, and breathing in order to expend less energy during what may be a difficult time, such as a cold winter.  Hibernation is a tactic commonly associated with rodents and polar bears but primates do it too. The fat-tailed dwarf lemur (Cheirogaleus medius) on the island of Madagascar hibernates during the dry season, using the fat in its tail to survive the winter. The Crossley's dwarf lemur (Cheirogaleus crossleyi) and the Sibree's dwarf lemur (Cheirogaleus sibreei) also hibernate. Some primates (Microcebus murinus, Allocebus trichotis, Galago moholi, and others) will enter what is called torpor, during which body temperature and metabolism is lowered but for a period less than 24 hours. Lemurs were thought to be the only primate that hibernated, until a close cousin changed the game.

Pygmy slow loris. Photo: David Haring at Duke Lemur Center
It has recently been reported that the pygmy slow loris (Nycticebus pygmaeus), a nocturnal primate living in Cambodia, China, Laos, and Vietnam does in fact hibernate (Ruf et al., 2015). The authors behind this study set out to find a hibernating primate outside of Madagascar and they succeeded.

Ruf and colleagues found it hard to believe that hibernation in primates would be restricted to only Madagascar, so they started thinking about what environmental conditions and physical characteristics would make an animal likely to hibernate. They hypothesized that a hibernating primate would be small, as most hibernating animals are (Ruf and Geiser, 2015), that the animal would live in an environment that is distinctly seasonal in temperatures and/or precipitation, and in an environment with seasonal changes in food availability. Given the primates known to use torpor, primates falling into the suborder Strepsirrhines seemed like their best bet.

The pygmy slow loris fit their criteria. It was logical that a small primate facing cold temperatures in winter and low food availability would adapt using hibernation. Thus lowering energy requirements during a period of limited resources. While previous descriptions of the pygmy slow loris were in accord with hibernation (Ratajszczak, 1998; Streicher, 2005), no measurements had been taken. Thus, Ruf and colleagues decided to take some.

After measuring temperature (but not metabolic rate) in five adults over 769 days total, animals hibernated for several days during midwinter interspersed with periods of activity and or torpor. On average, animals hibernated for 43 hours (± 3 hours with a range of 25.9-62.6 hours).

Thus, evolutionary mechanisms have not limited hibernation in primates to Madagascar. As more research is done, it is possible that hibernation will be found in other primate species inhabiting seasonal environments.

Links of possible interest:
Discovery of hibernation in fat-tailed dwarf lemur
Primate hibernation more common than previously thought
The costs and benefits of hibernation


Ratajszczak, R. (1998). Taxonomy, distribution and status of the lesser slow loris Nycticebus pygmaeus and their implications for captive management. Folia Primatologica, 69(Suppl. 1), 171-174.

Ruf, T., Streicher, U., Stalder, G. L., Nadler, T., & Walzer, C. (2015). Hibernation in the pygmy slow loris (Nycticebus pygmaeus): multiday torpor in primates is not restricted to Madagascar. Scientific reports, 5.

& Daily torpor and hibernation in birds and mammals. Biol. Rev. 90, 891–926, doi: 10.1111/brv.12137 (2015).

Streicher, U. (2005). Seasonal body weight changes in pygmy lorises Nycticebus pygmaeus. Verhandlungsber. Zootierkrk, 42, 144-145.