Behavioral Responses to Predators

With all these animals preying upon primates, it is easy to see how anti-predator behavior was selected. Nevertheless, not all primates avoid predation in the same fashion. Alarm behavior takes on many different forms, some which are used in conjunction or as increments to one another. If we start at the base of anti-predator behavior we find techniques that are passive, such as pelage coloration, which then can advance to more active camouflage, such as hiding and group manipulation. Active behaviors can also manifest themselves as defensive alarm calls to conspecifics or even as anti-predator behaviors such as mobbing.

By being cryptic, primates are able to avoid a predator entirely by evading detection. Many marmosets, for example, have dull coloration which presumably aid them to feed on gums or move through the trees unnoticed. Similarly, nearly all large primates are drably colored in shades of black, grey or brown. Some small splashes of color exist on primates such as the Titi monkeys (genus Callicebus) but mainly, these colored parts are on the hands and ventral surfaces which are easily hidden when the primate is in normal quadrupedal position. (Ferrari 2009)

There is an inverse correlation of body size and the risk of predation, thus one might expect smaller primates to be more cryptically colored (Colquhoun 2006). There are, however, many colorful small monkeys, such as the squirrel monkeys (genus Saimiri), which display contrasting black, green and yellow patterns as well as having white fur over their faces. While these colors may make squirrel monkeys more visible, their agility, coupled with large group sizes, may make their colors less costly to display. Additionally, their troupes tend to travel in lower canopy layers, which are safer from raptors (Ferrari 2009).

In addition to cryptic coloration, primates such as Spectral Tarsiers (Tarsius tarsier) freeze when raptors appear and remain still until it is out of sight. (Gursky 2007) These responses seem to be genetically hard-wired, with anecdotal evidence showing that even early orphaned primates possess this response. This particular case involved an infant dusky titi’s (Callicebus brunneus) response to flying objects or mock predatory noises. (Ferrari 2009) This type of freezing behavior has also been shown in lorises (Gursky & Nekaris 2007). The reasoning for freezing has been agreed upon to be related to raptor visual cues: a lack of movement would allow potential prey to blend into the still background of a forest setting.

Freezing is normally not used against anything other than avian predators, which makes sense because reptile or mammalian predators rely mainly on olfactory cues (Gursky 2007). Some primates, such as the macaques (Macaca fascicularis) will shake branches aggressively, making loud sounds in the process (Stanford 2002). For many primates, branch shaking can sometimes knock arboreal predators off branches entirely. Pottos (Perodicticus potto) have been known to knock African Palm civets (Nandinia binotata) out of trees when they are being pursued. Pottos possess an enhanced circulatory system in their hands and feet which allow them to be nearly impossible to remove from a branch, giving them a huge advantage over many of their predators who lack opposable digits and thus, face a long fall out of a tree (Gursky & Nekaris 2007).

Loud sounds and aggressive motions can warn a predator that it has been detected. Normally noisy Diana monkeys (Cercopithecus diana) are even more so upon detection of a predator. When a leopard is approached by the monkeys, it typically does not attack, but gives up. Since the Diana monkey forages on the ground for food, leopards, which ambush from low tree branches, have no chance of scoring a meal once spotted (Zuberbühler 2007).

Interestingly, individual leopards are highly idiosyncratic in their preferred prey, some preferring monkeys, others hoofed animals, or smaller prey such as pangolins (Jenny 1996). Leopards are also highly territorial (Jenny 1996), which means that a troupe of monkeys in a single area will most likely interact with the same leopard repeatedly. This fact has led some researchers to claim that it is important for a monkey troupe to actively detect and dissuade leopards and other predators from attacking not only for individual gain, but to keep themselves off of the “preferred food” list, thus minimizing their interactions with the leopards in general, and benefitting future generations of monkeys. (Zuberbuhler 2007)

While simply alarm calling or confronting a predator may be enough to deter it, urination and defecation are used in more active deterrence. Often much higher than their predators, many primates simply let loose a mass torrent of urine or feces to deter their predator. This may also serve useful to help track the location of a predator, just as many primates use their urine in marking territory. Thus, “urine-washing” a potential predator can help prevent future attacks by keeping the troupe aware of the predator’s current location and movement by scent. This behavior can be observed in howler monkeys, capuchins and squirrel monkeys, all of whom are notorious for this behavior. (Gursky & Nekaris 2007)

So far, all of these described behaviors have been largely individualized, such as those exhibited by the solitary pottos; however, in the cases of primates such as the tarsiers or capuchins, group living opens up an entirely new set of anti-predator and alarm behaviors which may or may not directly benefit the individual itself, but instead, benefit others. Understanding the evolution of such potentially altruistic behavior is an important problem.

Basic properties of the groups, such as size, can be affected directly by predation. This does not mean that populations in an area are smaller than one would expect, but simply that groups of monkeys have a higher than expected rate of fission. Some preliminary reasoning for this is that smaller groups allow for greater stealth in areas of high predation. This was directly observed in pig-tailed langurs (Simias concolor) where those in the highest areas of human poaching were found in extremely small groups, sometimes consisting of only one male and one female, while those outside the area of hunting could be found in groups of up to eight (Treves 1999). With a group of any size comes the ability to divide up work. Brown capuchin monkeys (Cebus apella) use sentinels. This behavior involves individuals who forgo typical activities, such as feeding, playing or grooming and remain more vigilant for the benefit of the group. This sentinel position often rotates, or simply becomes more focused when a suspicious activity is noted. This behavior was more or less frequency with height in the canopy, with monkeys becoming less vigilant with greater height in the trees. (Hirsch 2002)

Alarm vocalizations play an important part in anti-predator behavior for many primates. In white-faced capuchin monkeys (Cebus capucinus), calls of varying intensity are used to signal intent. Louder, more generalized barks may indicate the use of a typical escape route, while sharper ones may elicit the use of alternative exits. (Digweed et al. 2005) Putty-nosed monkeys (Cercopithecus nictitans martini) use different calls for different predators, illustrating an adaptive ability for the primate to avoid each predator separately. Using a model of an eagle as well as recordings of one, researchers found that alarm calling was strong for each; however, a visual model of a leopard elicited a much higher response than the pre-recorded growls of one. While an audience effect, the phenomena of an elevated reaction only due to the presence of conspecifics, was initially suspected, further insight about the relationship of the monkeys and leopards seemed to indicate otherwise. (Arnold et al. 2008) As mentioned before, leopards are pure ambush predators, often giving up once spotted. (Hart 2007) Since an acoustic recording cannot be readily located by a troupe of monkeys, thus allowing them to drive the offending leopard away, the alarm call response is diminished. (Arnold et al. 2008; Wheeler 2008)