Polygamy is (not) for the birds

Researchers at Rice University have uncovered new insights into the evolution of bird behavior, revealing why certain mating systems persist while others disappear over time. In a recent paper published in the journal Evolution, Rafael S. Marcondes and Nicolette Douvas reveal that lekking — a mating system where males display for females without forming lasting bonds — is an evolutionarily stable strategy. In contrast, resource-defense polygamy, where one sex — usually but not always the male — guards territories to attract mates, is highly unstable and often reverts to monogamy.

“This research not only examines how mating behaviors influence species survival but also sheds light on larger evolutionary questions,” said Marcondes, the corresponding author and a faculty fellow in ecology and evolutionary biology at Rice. “By studying birds, we’re uncovering principles that may resonate across other species too.”

The study analyzed data from more than 60% of the world’s bird species — approximately 6,620 species — making it one of the most comprehensive analyses of its kind.

Marcondes and Douvas employed a detailed methodology to explore the evolution of bird mating systems. Species sampling and classification were based on data from authoritative sources like the “Handbook of Birds of the World.” Bird species were categorized into three mating systems — monogamy (strong and durable sociosexual bonds), resource-defense polygamy (weak and transient bonds) and lekking (absence of social bonds). The certainty and detail of these classifications were rated on a scale of 1 to 3. Marcondes said the rating scale was an important and novel aspect of the study, because while there is a lot of published information on animal behavior, it is often scattered and reported in a haphazard way.

“We were some of the first researchers to try to systematize that scattered information and use it at a large scale,” Marcondes said.

To analyze evolutionary dynamics, they used large family trees of species and applied stochastic character mapping to address missing data. They further leveraged sophisticated evolutionary models to investigate diversification dynamics and transitions between mating systems. The researchers then modeled diversification rates to uncover correlations between mating systems and extinction/speciation rates and assessed whether transitions between systems followed predictable evolutionary pathways.

The study uncovered several key findings about the evolution of bird mating systems. Lekking, characterized by elaborate male displays and selective female mate choice, was found to be remarkably stable and rarely evolves into other systems, often directly developing from monogamy rather than passing through intermediate stages of polygamy. In contrast, resource-defense polygamy was identified as fragile, frequently reverting to monogamy and associated with higher extinction rates likely due to challenges in optimizing parental care under this system. Monogamy, practiced by most bird species, was shown to be the most evolutionarily resilient system, providing strong sociosexual bonds that support parental investment and species survival.

“This study bridges the gap between ecological and macroevolutionary perspectives,” Marcondes said. “It shows how environmental pressures, mating behaviors and lineage diversification are intertwined over millions of years.”

Douvas, a senior premed student majoring in humanities, reflected on the study’s impact — both on her academic journey at Rice and on the particular field of study.

“Through statistical analysis and evolutionary modeling, we uncovered surprising insights into the stability of lek mating behavior, a topic that had received little prior attention,” Douvas said. “One of the most fascinating discoveries was that lekking behavior originated from monogamy, challenging my initial assumption that it evolved from polygamy. This research not only expanded my understanding of animal behavior but also provided a richer perspective on the intricacies of evolutionary patterns. It has been an incredible opportunity to engage in interdisciplinary research, broadening my academic experience far beyond the traditional focus of premed studies.”