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Comparative studies on urgency gradient alarm systems

Alarm systems that convey urgency should be beneficial in primates if the species has only one anti-predation response. If this system has evolved because the primates can only respond in one way, but do not always need to waste their energy in responding to a non-urgent threat, then a similar system should have developed in other species with limited strategies. The California ground squirrel has two distinct alarm calls, a whistle and a chatter, that were originally believed to signal either large raptors or terrestrial predators (Owings&Virginia 1978; Owings&Leger 1980). However, the California ground squirrel does not have multiple anti-predator strategies, and its alarm calls were eventually found to denote the degree of urgency, instead of the specific type of predator (Owings&Hennessy 1984). Belding’s ground squirrels are another species that use alarm calls; again, these squirrels have only one way of dealing with predators, and thus their alarm calls vary in frequency and duration depending on the risk a predator presents (Robinson 1981; Robinson 1980). Even though only one response is possible, it is advantageous to know how quickly to respond to a predator. If the risk is not great, perhaps the squirrel should continue using its energy to forage or find mates. These ground squirrels have developed alarm systems similar to those of the chacma baboon, which affirms the idea that limited anti-predator responses will result in the evolution of a risk based alarm system.

Alarm calls can discourage predation

An additional hypothesis for the evolution of alarm calls asserts that primates may actually communicate to the predator (Zuberbühler 1999). An alarm call directed at a predator would let the predator know he had been spotted; this strategy could be adaptive if the predator relied on surprise for successful hunting and could be expected to give up once spotted. A predator that relies on ambush might “realize” that to catch this monkey would take more time and energy than attacking a group of unsuspecting monkeys. Six different monkey species in the Tai forest on the Ivory Coast are subject to predation by leopards, which hunt by surprise (Hoppe-Dominik 1984), and all give different alarm calls in response to a leopard versus a chimpanzee predator (Zuberbühler 1999). The alarm calls given in response to leopards are much more conspicuous than those given to chimpanzees, who are not as dependent on surprise and hunt using the acoustic cues of their prey (Boesch&Boesch 1989; Zuberbühler 1999). This conspicuous signal seemed to be directed at the leopard, as it was repeated often and affected the leopards’ hunting behavior – the leopard gave up the hunt after the alarm calls more often than can be attributed to chance (Zuberbühler 1999).

a white-faced Capuchin monkey
A White-faced Capuchin monkey keeps an eye out for any potential predators.
(Picture obtained with permission from (External Link) )

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Interspecies understanding

Recent studies suggest that primates may understand and respond to the alarm calls used by other species. Diana monkeys are often observed to form mixed species groups, notably with the red colobus monkey and the campbell’s monkey (Höner et al, 1997; Zuberbuhler 2000). Originally, these associations were thought to be due to improved foraging efficiency, but this hypothesis does not stand as the primates have different diets (Noë et al., 1997). Instead, its been proposed that group intermingling provides increased protection from predation; this view is supported by the red colobus monkeys’ increase in association with Diana monkeys during their main predator’s, the chimpanzee, hunting season (Noë et al., 1997). These two primate species also decrease their vigilance and increase their exposure when associated with another group, further supporting the idea that the pressure from predation is mitigated (Bshary et al., 1997). The decreased risk is most likely due to the Diana monkeys’ skill as ground sentinels accompanied by the red colobus monkeys much better effectiveness for aerial predators. In fact, red colobus monkeys were significantly more exposed from below and descended to the ground more often when associated with Diana monkeys (Bshary et al., 1997). However, this hypothesis requires proof that the monkeys are actually communicating between each other about the presence of predators using alarm calls. A study on Diana monkeys and Campbell’s monkeys, who also form associations, has suggested that this is the case. Campbell’s monkeys produce two distinct alarm calls in reference to crowned eagles and leopards, and Diana monkeys respond by producing their own distinct alarm call (Zuberbuhler 2000). The Diana monkeys always give their leopard call in response to the Campbell monkey’s leopard call, and their eagle call in response to the Campbell monkey’s eagle call (Zuberbuhler 2000). Additional species need to be studied, but it appears that primates may have evolved to understand the meanings in other species alarm calls.

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Source:  OpenStax, Mockingbird tales: readings in animal behavior. OpenStax CNX. Jan 12, 2011 Download for free at http://cnx.org/content/col11211/1.5
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