SHORT COMMUNICATIONS 
407 
might also be inferred from the number of animals 
that are known to dodge sideways when faced 
with a predator at close range: song birds (Lima 
1993, Kullberg et al. 2000. Lind el al. 2002, Lind 
et al, 2003), and ostriches, rheas, and gazelles 
(Farina et al. 2005). 
Studies of prey flight paths lend to involve 
terrestrial prey and neglect another 90 degree 
escape path: fleeing straight down from the attack. 
Both saki monkeys in our observations selected 
this flight path when they jumped, as did the 
howler monkey when it swung to the underside of 
the branch. The number of primates that drop or 
rush down to lower levels of the canopy or even 
jump to the ground during last-second flight 
would seem to suggest this flight path also 
reduces raptor success rates (e.g., guenons | Cer- 
copithecus]: Shultz 2001. Corderio 1992; howler 
monkeys [Alouatta]: Eason 1989, this study; 
mangabeys [Lophocebus\: Ariel and Isbell 2009; 
sakis \Pithecia\: de Luna et al. 2010; bearded 
sakis [Chiropotes]; this study; tamarins \Sagui- 
/(«.']: Heymann 1990. Peres 1993). Raptors likely 
will not shift their flight paths down to capture 
prey that jump from horizontal branches not only 
because of the difficulty of tracking prey moving 
at 90 degrees, but also because a downward 
adjustment risks a potentially fatal collision with 
the branch. 
Most primate vocalizations that accompany 
interactions with predators are alarm calls intend¬ 
ed to alert conspecifics or to prevent an attack 
before it begins by discouraging the predator 
(summarized by Wheeler 2008). Primate vocali¬ 
zations that may serve as startle responses, 
however, are rarely discussed. Startle responses 
are abrupt changes in prey behavior meant to 
cause instantaneous responses in predator behav¬ 
ior to interfere with completion of the attack 
(Sargent 1990). These behaviors are used only 
after primary defense mechanisms have failed 
(Robinson 1969a. b; Edmunds 1974) and they are 
novel, rare, conspicuous, anomalous and/or 
threatening” (Sargent 1990; 235) so predators 
are unlikely to habituate to them. The goal is to 
create a moment of hesitation (a startle) that 
provides additional opportunity for escape. The 
saki scream that we heard was a startle response 
because it was given at the moment of attack, it 
was very loud (conspicuous), and it is rare. The 
junior author (dos Reis) has heard this saki call 
only three times in 25 years, twice during the 
attacks reported here and once during a possible 
predation attempt during a previous study with 
605 contact hours with black-bearded saki mon¬ 
keys (Sarah Boyle, pers. comm.). The howler 
vocalization was likely also a startle response as 
the observer (dos Reis) has not heard this 
vocalization outside of this encounter and it also 
meets the other criteria. 
The extent to which startle vocalizations and 
the 90 degree downward flight path aided in 
escape is not clear, but the Harpy Eagle was at 
least required to make multiple attempts in all 
three observations. This suggests these last- 
minute primate anti-raptor defenses are successful 
in exploiting raptor weaknesses to increase the 
probability of survival by extending the predator- 
prey interaction. 
ACKNOWLEDGMENTS 
Wc thank Primate Conservation Inc., Tulane University, 
aiul several generous private individuals for helping support 
the research during which these observations occurred. We 
also thank those whose comments improved this article: E. I. 
Johnson. K. M. Jack. T. M. Sanaiotti. and several anonymous 
reviewers. This is publication Number 570 in the Biological 
Dynamics of Forest Fragments technical series. 
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