Leah Carpenter

BIOL 708W

April 9, 2009

Article Summary 4

Sarno, R.N., Bank, M.S., Stern, H.S., Franklin, W.L. (2006) Effects of age, sex, season and social dynamics in juvenile guanaco subordinate behavior. J. Mammal. 87(1):41-47.

In their paper, Sarno et al (2006) investigated the subordinate displays of juvenile guanacos, a relative of the llama. Juvenile guanacos perform a highly stereotyped crouch, similar to the position assumed during suckling when threatened by dominant males. This crouch is thought to be a reversion of behavior to an earlier developmental stage. The authors studied the appearance, ontogeny, and body posture of guanacos from birth to 9 months of age. It was shown in earlier work by this group that aggression from adult males was the proximate cause of juvenile male dispersal and here they tested whether this subordinate behavior was related to the time of dispersal. The authors predicted that juveniles that displayed crouches earlier will remain longer in family groups; juveniles in their first season will exhibit the lowest rates and durations of crouches and these measures will increase with age to 9 months of age; aggression from territorial males will be lowest early in life and steadily increase throughout the first year; juvenile males, who receive more aggression from territorial males, will perform submissive behavior more often and with longer duration than females, and the position of these gestures will be more exaggerated in males than females. Through three years of observation the authors found that some of their predictions were supported, while others were not. There was no difference between the sexes in time of first crouch, nor was there a correlation between first crouch and date of dispersal. Males did show a higher frequency of crouches than females, but did not show a difference in duration of crouch. Juveniles received the most aggression in the autumn, and males showed a higher frequency and more extreme body positions in crouches than females during this time. Increased cohort size was associated with a decrease in likelihood of crouching, suggesting a dilution effect of cohort size. Crouching frequency was also different across the three cohorts (one for each year). Cohorts also differed in rates of aggression received from adult males. Cohort differences were attributed to differences in density of the guanaco population for their respective years. The cohorts in the most dense population saw the highest frequency of aggression, earliest expulsion date and the highest rate of crouching. These density-related effects were attributed to higher resource competition.

The most relevant aspect of this research to our discussions is the type of behavior which the authors chose to study. They picked a highly ritualized, very stereotyped display that also conveys the animal’s intentions and motivational state. The stereotypical nature of the display, the consistency of its form and function, allows it to be easily and quickly interpreted by the receiver of the signal, in this case territorial males. The intentions and motivational state of the sender, a juvenile, can be inferred by the receiver in the display’s reversion to a behavior from an earlier developmental stage. Essentially this body position sends the message “I am not a threat, look, I am still a baby”. It conveys the juvenile’s intention to submit and the individual’s low aggression and high fear state.