6.1 Primate alarm calls  (Page 4/6)

Other semi-arboreal and semi-terrestrial primates, such as tamarins and the red-tailed sportive lemurs, seem to have developed predator-specific alarm systems for the same reason (Kirchhof et al, 2006; Fichtel 2007). Kirchhof observed that mustached and saddleback tamarins’ alarm calls do correspond with certain predators by measuring the time spent searching in a certain direction by the listening tamarins. He found tamarins spent a significantly longer amount of time looking up in response to the aerial alarm call but a much longer time looking down in response to the terrestrial alarm call (Kirchhof et al, 2006). Putty-nosed monkeys are another species that uses two distinct calls, pyows and hacks (Arnold et al, 2006). The monkeys combine these two calls in various ways; however, eagles always elicit hacks first and more often. The Putty-nosed monkeys use the different sequences of the distinct calls to determine whether an aerial or terrestrial predator is attacking (Arnold et al, 2008). These alarm calls are predator specific which is advantageous to species with multiple escape strategies.

Alarm calls that indicate urgency are also adaptive

In species with only one escape strategy, identifying the predator has less value. Instead, it might be more helpful to evolve a communication system that conveys the degree of risk. A primate that can only escape to the trees for safety would prefer to know whether the predator is fifty feet away or in the bush next to him, but not necessarily which type of monkey-eater he must avoid. Chacma baboons face a wide range of predators but do not have multiple escape strategies. The baboons use the same calls, which range from tonal clear barks to harsher alarm barks for all types of predators, but vary the calls to convey the degree of risk (Fischer et al, 2001; Cheney et al, 2003). The baboons respond differently based on the duration and frequency of the calls, simply ignoring any intermediate calls that are not urgent alarm calls (Fischer et al, 2001). One of the first clues that this system may be based on risk determination is the increase in alarm calls when the group is more widely dispersed (Rendall et al, 2000). This is expected; a greater area of dispersal increases the danger to each individual, representing heightened danger. In addition, chacma baboons may give a few scattered alarm barks when confronted with relatively low risk predators such as hyenas or wild dogs (Cheney et al, 2003). Lions, however, always elicit frequent harsh alarm barks from more than one baboon because they pose a greater risk (Cheney et al, 2003). The most frequent alarm calling occurs when baboons cross water infested with crocodiles. This is a high risk situation for the chacma baboons, thus it makes sense that a call system based on the degree of the threat would cause the baboons to bark most frequently in response to their riskiest predator (Cheney et al, 2003). The baboons even barked in response to harmless objects that could potentially be crocodiles, such as floating elephant dung or submerged hippos (Cheney et al, 2003). The heightened sensitivity and increase in call frequency in response to a more threatening predator confirms that their system is meant to demonstrate the relative urgency of the threat, not the particular predator (Fischer et al, 2001).