Testosterone-induced Polygyny in Normally Monogamous Birds
Imagine a devoted bird father, diligently helping to feed his chicks, suddenly abandoning his family to pursue other females. What could trigger such a drastic change in behavior? For decades, scientists have been piecing together this puzzle, and one crucial piece appears to be testosterone, a powerful sex hormone.
This article explores the fascinating science behind how testosterone can potentially rewrite the rules of avian relationships, transforming monogamous species into polygynous ones. The implications extend far beyond birds, offering insights into the complex interplay between hormones, evolution, and behavior across the animal kingdom.
A cornerstone of our understanding is the "Challenge Hypothesis," first proposed by Wingfield and colleagues in 1990. This theory posits a fundamental trade-off between mating effort (competing for and attracting partners) and parental effort (caring for offspring) 4 .
Males provide little paternal care and maintain high testosterone levels throughout the breeding season to support sustained aggression and courtship 6 .
In the real world, bird mating systems are often more complex than they appear. A bird can be socially monogamous (forming a pair bond) but genetically promiscuous. This is where extra-pair paternity (EPP)—where a male sires offspring with a female outside his pair bond—becomes crucial.
Comparative studies have found a significantly positive relationship between testosterone levels and the rate of extra-pair paternity 1 . This suggests that even in socially monogamous species, testosterone drives mating competition beyond the pair bond.
| Mating System | Paternal Care | Predicted Testosterone Pattern | Example Species |
|---|---|---|---|
| Polygynous | None or minimal | High, sustained throughout breeding season | Pectoral Sandpiper 6 |
| Monogamous | Biparental care | Sharp peak during mating, then declines for parenting | Semipalmated Sandpiper 6 , Downy Woodpecker 7 |
| Classically Polyandrous | Male-only care | High in competitive females; low in caring males | Black Coucal 8 |
To move beyond correlation and prove causation, researchers have turned to elegant hormonal manipulation experiments. One of the most revealing was conducted on the dark-eyed junco, a socially monogamous songbird.
The experiment was designed to test what happens when the hormonal profile of a monogamous bird is artificially altered to resemble that of a polygynous one .
Wild male dark-eyed juncos were divided into two groups: an experimental group and a control group.
The experimental group (T-males) received subcutaneous implants that steadily released testosterone, maintaining elevated levels typical of more polygynous species. The control group received empty or placebo implants.
Researchers then meticulously tracked key outcomes for both groups over the breeding season, including apparent reproductive success and genetic reproductive success determined through DNA fingerprinting.
The findings were striking and revealed a complex story of trade-offs.
| Metric | Control Males | Testosterone-Treated Males (T-males) | Biological Interpretation |
|---|---|---|---|
| Parental Care | High nestling provisioning | Reduced provisioning | High testosterone shifts effort from parenting to mating. |
| Apparent RS (offspring in own nest) | Higher | Lower | Due to reduced care, more offspring in their own nest died. |
| Losses from EPFs (young in nest not his) | Moderate | Lower | T-males were better at guarding their mates from rival males. |
| Gains from EPFs (young sired in other nests) | Moderate | Higher | T-males were more successful at securing extra-pair matings. |
| Final Genetic RS (total young sired) | Similar | Similar | Losses and gains balanced out, so net fitness was equal. |
The analysis showed that testosterone caused a direct trade-off between mating effort and parental effort. While T-males were less successful parents, they became more successful competitors. Their elevated testosterone made them more aggressive and attractive, allowing them to better guard their own mates from intruders and to sire more offspring with the mates of other males .
To conduct such detailed research, scientists rely on a suite of specialized tools and methods.
| Tool / Reagent | Function | Example in Context |
|---|---|---|
| Radioimmunoassay (RIA) | Precisely measures hormone concentrations in small plasma samples. | Used to confirm elevated T levels in polygynous Pectoral Sandpipers vs. monogamous Semipalmated Sandpipers 6 . |
| Testosterone/GnRH Challenge | Injects GnRH to stimulate the body's maximum testosterone production capacity. | Used to show that female Black Coucals have a higher capacity for T production 8 . |
| Hormone Implants | Slow-release devices filled with hormones to experimentally manipulate levels. | Critical for the dark-eyed junco experiment to establish causation . |
| DNA Fingerprinting | Uses molecular markers to determine paternity and genetic relatedness. | Allowed researchers to calculate the true genetic reproductive success in the junco study . |
| Behavioral Observation | Systematic recording of behaviors such as song rate and aggression. | Used to link high T in European stonechats to increased territorial aggression 4 . |
Precise measurement of hormone levels using advanced biochemical techniques.
DNA techniques to determine parentage and genetic relationships.
Systematic recording of behavior in natural habitats.
The question of whether testosterone can induce polygyny in monogamous birds has a nuanced answer. Evidence clearly shows that testosterone is a powerful driver of mating competition and can suppress the parental care side of the equation. In the dark-eyed junco, artificially elevating testosterone created a behavioral shift towards a polygynous strategy, with males investing more in seeking additional mates and less in raising their own young .
However, this shift is not without its costs. The junco experiment revealed that the trade-off is often a zero-sum game; the gains from extra-pair matings were balanced by the losses from reduced parental care . This suggests that the "monogamous" hormonal pattern is an evolutionary adaptation that optimizes lifetime reproductive success in certain environments.
Ultimately, testosterone is not a simple on-off switch for polygyny. It is more like a master regulator in a complex feedback loop, interacting with ecology, brain sensitivity, and evolutionary history to produce the breathtaking diversity of avian family lives we observe in nature.