Newsroom: Deciphering Courtship Tactics

A novel statistical approach for analyzing animals’ display behavior was recently published by A. C. Perry, an alumna of the Animal Behavior Graduate Group, along with her colleagues from the Patricelli lab at UC Davis, the Max Planck Institute in Leipzig (Germany), and the University of Florida. This research team used their statistical approach, a custom-built hidden Markov model, to investigate the curious courtship behavior of the Greater Sage-Grouse (Centrocercus urophasianus), a bird native to the sagebrush habitat of western North America. 


A male Greater Sage-Grouse (Centrocercus urophasianus) stands poised, ready to begin his next “strut” display.
[Photo credit: Anna C. Perry]

Biologists have been fascinated by the courtship behavior of the Greater Sage-Grouse since the 1930s. Male sage-grouse gather each year on communal display grounds (i.e., “leks”), where they repeatedly perform an extravagant courtship display (i.e., “strut”) in an attempt to convince females to mate with them [1]. For males, the repetition of this strut display is a feat of endurance. Males’ energetic costs tend to increase with the number of struts that they perform, and many males lose weight after spending several hours on the lek each morning for two-to-three months at a time [2]. During a typical breeding season, a small percentage of sage-grouse males will have the opportunity to mate with dozens of females. However, most males will not mate at all, regardless of all of their time and display effort [1],[2]. What are the courtship tactics that distinguish successfully-mated sage-grouse males from their unfortunate competitors? To investigate this question, the researchers employed a combination of field experiments and advanced statistical modeling.


A female sage-grouse (center bird) evaluates two sage-grouse males (the rightmost male has been caught mid-display, while the leftmost male is shown pausing between consecutive “strut” displays).
[Photo credit: Anna C. Perry]

In the field, sage-grouse males’ display behavior was observed during courtship interactions with a robotic female sage-grouse. This female robot was designed to mimic the posture and body movements of a typical sage-grouse female either “interested” or “uninterested” in an eventual mating [3]. The female robot enabled the researchers to compare the courtship tactics of sage-grouse males under the same social conditions (i.e., their responses to the same set of female behaviors). Without the robot, variation in real females’ behavior (e.g., real females avoiding certain males on the lek) could make it difficult for researchers to determine whether differences in males’ display effort are due to the unique social context of each observation or due to individual differences between males. Robotic animals can be used to address this kind of problem in animal behavior research by allowing scientists to observe animals’ responses to standardized social environments [4]. 


The robotic female sage-grouse, shown above, was used to experimentally manipulate sage-grouse males’ courtship behavior via remote control.
[Photo credit: Anna C. Perry]

In order to decipher males’ courtship tactics from the behavioral data, the researchers developed a customized hidden Markov model (or HMM). This statistical analysis allowed the researchers to determine how males’ display behavior with the robotic female relates to their mating success with real females. The researchers specifically designed this HMM to accommodate the structure of sage-grouse males’ display sequences, where struts are clustered into “display bouts.” During a display bout, multiple strut displays are typically performed in quick succession, and each “bout” is then separated from other bouts by longer intervals of time (or between-bout breaks). Perry and her colleagues found that the HMM performed better than more common statistical techniques that ignore this display bout structure (e.g., simply counting strut displays or averaging the intervals of time separating each strut). 

In their model, a male’s mating success could correlate with the average number of times the male consecutively struts in a bout, with the length of time between struts, or both. Perry and her colleagues found that the males that tended to produce longer display bouts for the robot were the most successful, which indicates that sage-grouse females prefer males that show greater display persistence. Interestingly, the most successful males showed high persistence regardless of the robot’s apparent interest in mating. These results indicate that only the most successful sage-grouse males have sufficient energetic resources or endurance to court “uninterested” females until those females transition to “interested” behaviors (either during the current interaction or during that female’s future lek visits).

Many other animals also display in bouts (e.g., insects, frogs, and other birds) [6], so the statistical approach presented in this study could be useful for investigating the display tactics of other species as well.  

For more information:

Perry, A.C.**, Krakauer, A.H., McElreath, R.*, Harris, D.J., & Patricelli, G.L.*. (2019). Hidden Markov models reveal tactical adjustment of temporally clustered courtship displays in response to the behaviors of a robotic female. The American Naturalist, 194, 1–16. https://doi.org/10.1086/703518

Article Description/Forthcoming Papers/American Naturalist

Science Daily/Press Release/2019/07

** Denotes an ABGG alumna author

* Denotes an ABGG-affiliated author

Dr. Anna C. Perry is an alumna of the Animal Behavior Graduate Group. Her research aims to understand how social context influences animals’ courtship behaviors and mate choice. All of the research described above was conducted under government permits and with IACUC approval.

Other links about Greater Sage-Grouse research in the Patricelli Lab from The Ethogram:

2019/07/10/field-notes-the-highs-and-lows-of-technology

2015/07/01/field-notes-ryanes-first-field-season-at-chicken-camp

References:

[1] Gibson, R. M., & J. W. Bradbury. (1985). Sexual selection in lekking sage grouse: phenotypic correlates of male mating success. Behavioral Ecology and Sociobiology, 18, 117–123.

[2] Vehrencamp, S. L., J. W. Bradbury, & R. M. Gibson. (1989). The energetic cost of display in male sage grouse. Animal Behaviour, 38, 885–896.

[3] Perry, A. C. (2017). Behavioral tactics on the lek: manipulating and modeling the social environment of courtship in greater sage-grouse (Centrocercus urophasianus). Doctoral dissertation, pp. 104–152. University of California, Davis.

[4] Patricelli, G. L. (2010). Robotics in the study of animal behavior. In: Breed, M.D. & Moore J., (eds.) Encyclopedia of Animal Behavior, 3, pp. 91–99. Academic Press.

[5] Gibson, R. M. (1996). Female choice in sage grouse: the roles of attraction and active comparison. Behavioral Ecology and Sociobiology, 39, 55–59.

[6] Gerhardt, H. C., & F. Huber. (2002). Acoustic communication in insects and anurans: common problems and diverse solutions. University of Chicago Press, Chicago.

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