The greater sage-grouse (Centrocercus urophasianus) is a large ground-dwelling bird found in western North America. Male greater sage-grouse are known for their elaborate mating displays that take place on leks during the breeding season. A key feature of these displays are the large, inflatable air sacs on their chests, which are often referred to as “balls.” In this article, we’ll take a closer look at the anatomy and purpose of these fascinating structures.
What are the “balls” on male sage-grouse?
The balls on male greater sage-grouse are paired air sacs located on their breast. When inflated, these sacs resemble large, round balls protruding from the chest. The air sacs are covered with white feathers and are usually about 2.5-3 inches in diameter when fully inflated.
Each air sac is connected to an air passage leading from the lungs and trachea. The air sacs inflate and deflate as the bird breathes, allowing it to rapidly inflate the sacs during its mating display. When not inflated, the sacs lay flat against the breast and are barely noticeable.
Anatomy of the air sacs
The greater sage-grouse has a complex respiratory system that allows it to inflate its chest air sacs:
Lungs
Like other birds, sage-grouse have a pair of lungs that connect to air passages called bronchi. When the bird inhales, air travels from the trachea into the lungs.
Interclavicular air sacs
Two large, membranous sacs called interclavicular air sacs are located in the breast region, positioned between the clavicles. These are the main sacs that inflate into the prominent “balls.”
Patagial air sacs
There is also a pair of smaller air sacs called the patagial air sacs located under the wings. These connect to the interclavicular sacs.
Thoracic air sacs
In addition, sage-grouse have several thoracic air sacs distributed throughout the chest cavity which act as reservoirs of air.
Air passages
Air passages (bronchi and tracheae) connect the lungs to the interclavicular, patagial, and thoracic air sacs. When the bird inhales, air travels from the lungs into these various sacs for storage.
During courtship displays, contractions of specialized muscles surrounding the air sacs force air from the thoracic and patagial sacs into the interclavicular sacs. This allows the prominent breast air sacs to fully inflate within just a fraction of a second.
Purpose of the air sacs
Male sage-grouse inflate their air sacs for the purpose of visual displays during the breeding season. The displays take place on communal breeding areas called leks.
Researchers have proposed several evolutionary theories about how these elaborate rituals could benefit the birds:
Female mate choice
The visual prominence of the inflated air sacs allows females to better judge males. Females likely prefer males with the largest, most completely inflated sacs as this may indicate overall health, fitness, and genetic quality.
Acoustic amplifier
The inflated sacs may also amplify the popping and whistling sounds made by males during the displays. This could allow females to better judge a male’s acoustic qualities.
Intrasexual competition
Displays with fully inflated air sacs signal a male’s dominance over other males. The size and symmetry of the air sacs indicates better health and fitness compared to subordinates.
Semaphore signaling
The contrasting white feathers on the inflated air sacs are highly visible against the male’s darker plumage. This may allow better communication among males regarding territorial boundaries on the lek.
Air sac display process
So how exactly do sage-grouse inflate their air sacs during these elaborate mating rituals? Here’s a look at the full process:
Inflation muscles contract
Special breast muscles surrounding the air sacs rapidly contract. This forces air from the thoracic and patagial sacs into the interclavicular sacs.
Air sacs inflate
Within fractions of a second, the paired interclavicular air sacs fully inflate into prominent round shapes protruding from the breast.
Tail fanning
At the same time, the male rapidly fans his tail feathers into a near-vertical display. The fanned tail forms a wispy background that contrasts with the inflated white air sacs.
Strutting and posturing
With airs sacs fully inflated, the male struts around with wings drooped. Head feathers may be raised into a “V” shape. These postures showcase the inflated air sacs.
Vocalizations
Males may incorporate vocalizations like pops, wheezes, or whistles. The air sacs may acoustically amplify these sounds.
Deflation
After displaying, the air passages close off the interclavicular sacs, allowing them to deflate. The cycle then repeats multiple times as the male continues displaying for females.
Notable features of inflated air sacs
When inflated, the greater sage-grouse’s chest air sacs have some notable features:
Large size
Each air sac inflates to about 2.5-3 inches in diameter. They may expand to over 1 liter in total volume.
Round, smooth shape
The fully inflated sacs are quite round and smooth in contour. Irregular shapes may indicate illness or deficiencies.
Bright white color
The air sac feathers are brilliant white. This contrasts against the grayish-brown breast plumage.
Symmetry
The right and left air sacs inflate in a symmetrical fashion. Asymmetry could reflect developmental issues.
Prominent display
When both air sacs are fully inflated, they protrude prominently from the breast and are highly visible.
Rapid inflation/deflation
Air sacs can inflate fully in just 0.3 seconds and deflate in 0.5 seconds. This allows for rapid repetition.
Conclusion
The greater sage-grouse’s inflatable air sacs play an important role in the species’ intricate mating rituals. These prominent round features allow males to visually display their fitness. Research suggests the air sacs help attract females, communicate with other males, and amplify vocalizations. The air sacs inflate due to an intricate respiratory system that allows for rapid inflation and deflation. When properly inflated, the air sacs are large, round, bright white, symmetrical, and prominently displayed. The greater sage-grouse provides a fascinating example of how specialized anatomy can evolve for elaborate visual courtship displays.
