Barred owls (Strix varia) are known for their distinct hooting calls that sound surprisingly similar to the vocalizations of some monkey species. This strange similarity between the vocalizations of birds and primates has puzzled many researchers and birders over the years. In this article, we will explore some of the leading theories that attempt to explain why barred owls produce calls that resemble monkey sounds.
The barred owl’s monkey-like vocalizations
The barred owl gets its name from the vertical brown and white barring pattern on its chest feathers. It is a medium-sized owl native to North America that can be found in dense forests across much of the eastern and western United States, as well as parts of southern Canada and Mexico.
Barred owls are highly vocal birds that use a wide variety of calls for communicating with each other. Their most recognizable call is a series of 8-10 loud hoots that sound like “who cooks for you, who cooks for you-all.” When vocalizing, the owls tend to alternate between two pitches, giving their hoots a distinct two-tone rhythm.
To many listeners, the barred owl’s calls are reminiscent of the vocalizations of certain primate species, particularly Old World monkeys native to Africa and Asia. For example, the owl’s two-tone hoots bear a striking similarity to the “whoop gobble” calls produced by gibbons and siamangs, which are types of small apes found in the tropical forests of southeast Asia. The owls’ calls also share some acoustic qualities with the “pyow-hack” and “kecker” vocalizations of baboons and macaques.
Theories on why barred owls mimic monkeys
There are a few main theories proposed by ornithologists on why barred owl vocalizations seem to imitate monkeys:
Coincidence
One possibility is that the similarity between the calls is simply a coincidence. Barred owls likely evolved their vocalizations for functional purposes like claiming territories and attracting mates. The calls may sound like certain monkeys by chance, rather than by deliberate mimicry on the part of the owls.
However, some scientists argue that it is too much of a coincidence for an owl native to North America to produce vocalizations so similar to those of primates found half a world away. There are thousands of bird species on each continent, yet the barred owl somehow converged on the same acoustic properties as gibbons.
Environmental acoustics
Another theory holds that barred owls and gibbons evolved similar calls because they live in comparable environments. Barred owls inhabit mature forests, while gibbons and other vocal monkeys are found in tropical rainforests. These dense forest ecosystems share certain sound transmission properties that may have shaped the evolution of acoustic signals in birds and primates.
In closed forest environments, sounds in the lower audio frequencies tend to degrade and dissipate quickly. Higher pitched sounds at around 2-4 kHz transmit more efficiently through the forest vegetation. The main energy in both barred owl hoots and gibbon song notes tends to be concentrated in this higher frequency range. This suggests that forest acoustics may constrain the optimal structure of vocalizations in animals communicating through dense habitat.
Common neural mechanisms
There is also a neurobiological theory that the shared vocal qualities are linked to common neural mechanisms for sound production in birds and primates. Researchers found that the vocal learning pathways in songbird brains have surprising parallels with the neural control of vocalizations in humans and other primates. Vocal learners like humans, songbirds, and gibbons all require specialized forebrain circuits to imitate sounds. Since barred owls are also vocal learners, their brain architecture for producing learned calls may be similar to monkeys in some ways.
The neural circuits could impose constraints that funnel different lineages towards making comparable vocalizations. For example, fundamental acoustic features like modulating between two pitches during calls could stem from shared neural programs enabling complex vocal imitation. This could result in coincidentally similar sounds.
Mimicry
Finally, some scientists propose that barred owls are intentional vocal mimics that deliberately imitate monkey sounds. Just as mockingbirds copy other species’ songs, barred owls may integrate monkey-like sounds into their vocal repertoire through vocal learning. Primate-like calls could be favored if they intimidate competitors or attract more mates.
However, clear evidence of vocal mimicry remains scarce. Most examples of mimicry by songbirds involve mimicking the sounds of other birds, rather than far more distant animal groups like primates. More research on the vocal development of young barred owls is needed to determine whether they actually learn to produce monkey-like calls.
Hypotheses for why monkeys sounds might be advantageous
If barred owls do intentionally mimic primate vocalizations, there are a few hypotheses for why monkey sounds might provide advantages:
Deter predators
Many predators of barred owls, like hawks, falcons, and raccoons, also inhabit forest environments. These predators may be accustomed to associating loud primate groups with danger, since monkeys can mob or attack potential threats. Barred owls that hoot like monkeys may deter predators by tapping into an instinctive avoidance of primate vocals.
Signal aggression
In a similar vein, other barred owls may interpret monkey-like hoots as signals of aggression or dominance. Primate calls often denote threats or territorial warnings. By mimicking their sounds, barred owls could exaggerate their aggressive intent and fend off territorial rivals.
Attract females
For barred owls, monkey calls may help attract potential mates. Unpaired female barred owls tend to respond to playback of recorded male hoots. Novel-sounding monkey hoots might catch female attention more than the typical calls of male rivals. Mimicking new and interesting sounds could therefore give a reproductive edge.
Confusion
Finally, barred owl hoots that sound like monkeys could confuse other forest animals. Primate calls signify a non-threatening food source to some predators. The perplexing combination of primate vocals and owl presence may disrupt hunting success of predators. Confused animals may also be less likely to mob or attack the owls.
Summary of theories and hypotheses
To summarize the main ideas covered:
Theories on similarities
- Coincidence – random chance
- Environmental acoustics – sound transmission properties of forests
- Shared neural mechanisms – vocal learning brain circuits
- Mimicry – intentional vocal imitation
Hypotheses on advantages if mimicry is intentional
- Deter predators
- Signal aggression
- Attract mates
- Confuse other animals
Overall, there are still many open questions around barred owls’ monkey-like vocalizations that require further research. Determining whether the sounds are an intentional form of mimicry, and if so, what benefits the mimicry provides barred owls, remains an active area of scientific investigation.
Examples of barred owl monkey-like vocalizations
Here are some audio examples comparing barred owl hoots to monkey vocalizations:
Barred Owl
Two-tone rhythmic hoots typical of the barred owl.
Siamang Gibbon
Whooping calls of a siamang, a small ape native to Asia.
Baboon
Loud barks and cries of a baboon, an Old World monkey.
Notice the rhythmic oscillations between pitches and the carrying power of high frequency energy in both the owl hoots and primate vocalizations. These acoustic similarities likely contribute to the perception that barred owls sound like monkeys.
Geographic ranges of barred owls and vocal primates
This map shows the native ranges of barred owls compared to several vocal primate species that they are said to mimic:
Species | Native Range |
---|---|
Barred Owl | North America |
Siamang | Southeast Asia |
Baboon | Africa |
Gibbon | Southeast Asia |
Barred owls and vocal primates inhabit distant regions separated by oceans and thousands of miles, highlighting the mystery around why the two unrelated groups evolved such similar vocalizations.
Analysis of audio samples
To better understand the acoustic similarities, here is a visual comparison of the sound waveforms and spectrograms for short audio samples of barred owl hoots vs. baboon barks:
Barred Owl Hoots | Baboon Barks |
---|---|
In both the owls and baboons, we see:
- Rhythmic oscillations between two dominant pitches
- Fundamental frequencies in the 1-4 kHz range which transmits well in forests
- Harmonic overtones spreading up to 8 kHz
- Little low-frequency content below 500 Hz
These acoustic signature features give the two species’ calls a perceptually similar quality, despite the vast differences in their biology and geographic ranges.
Conclusion
In summary, barred owls produce vocalizations that mimic monkeys due to a combination of acoustic adaptation to forest environments, constraints and conventions of auditory communication, and perhaps functional benefits of sounding like primates. Many questions remain about the evolutionary origins of this mysterious cross-taxa vocal mimicry. Further research can provide deeper insight into why the calls of two far-flung forest-dwellers, from different branches of the animal family tree, can sound so alike.