Male social dominance affects access to mates but not female mate choice in a livebearing fish

In the pygmy halfbeak *Dermogenys colletei*, male social dominance successfully secures mating access through monopolization and increased courtship, yet it does not influence female mate choice, suggesting that intense male-male competition can limit the scope for pre-copulatory female preference.

The Gist

Imagine a high school cafeteria, but instead of students, it's filled with tiny, colorful fish called Pygmy Halfbeaks. In this underwater world, there are two main ways a male fish can get a date:

1. The "Bully" Strategy: Fighting other guys to clear the room so you can be the only one left to talk to the girls.
2. The "Charmer" Strategy: Being so cool, handsome, or talented that the girls choose you over everyone else, even if you didn't fight for the spot.

Scientists often assume these two strategies work together. They think: "If a guy is tough enough to win a fight, the girls will naturally want to date him because he's clearly the 'best' guy."

But a new study on these little fish suggests that this assumption might be wrong. Here is what the researchers found, broken down simply:

The Experiment: The "Room" and the "Window"

The scientists set up two different scenarios to see how the fish behaved:

Scenario A: The Open Floor (The Fight)
They put two male fish and two female fish in a tank. The males could see each other, swim around, and even get into a scuffle.

• The Result: The "Alpha" male (the one who won the fight) immediately took over. He swam around the females constantly, trying to impress them, while the "Beta" male (the loser) was pushed to the corner. The Alpha got all the attention, not because the girls asked for him, but because he physically blocked the other guy from getting close. It was like a bouncer at a club who only lets the VIP in.

Scenario B: The Glass Wall (The No-Fight)
Next, they put the same setup in a tank, but this time, they put a clear glass wall between the two males. They could see each other, but they couldn't touch or fight.

• The Result: Surprisingly, the "Alpha" male didn't act any differently than the "Beta" male. Without the ability to physically push the other guy away, the "tough guy" didn't get any more attention. He wasn't automatically more attractive just because he could have won a fight.

Scenario C: The "Eavesdropping" Test
Finally, they put a female fish in a main room with two males in separate side rooms.

1. First, she looked at them without them fighting.
2. Then, she watched the two males fight each other through a window.
3. Finally, she got to choose who she wanted to hang out with.

• The Result: The female fish did not care who won the fight. She didn't suddenly prefer the winner. She was just as likely to hang out with the loser as the winner. To her, the fight was just background noise, not a sign of a "better boyfriend."

The Big Takeaway

In many animals (and maybe even in human dating movies), we think that winning a fight = being a good partner. We assume the girls are watching the fight and thinking, "Wow, he's strong! I'll pick him!"

But for these Pygmy Halfbeaks, the reality is different:

• Dominance gets you access, not affection. The "Alpha" male gets to mate more often simply because he is strong enough to block the other guys from getting close. He monopolizes the girls by force, not by charm.
• The girls are indifferent. The females don't seem to care who won the fight. They aren't using the fight as a clue to pick a "high-quality" mate.

The Analogy: The Concert Ticket

Think of it like trying to get into a concert:

• The "Bully" Strategy: A big guy pushes everyone out of the way and stands right in front of the stage. He gets the best view, not because the band likes him, but because he physically prevented everyone else from standing there.
• The "Charmer" Strategy: The band actually chooses to play a special song for a fan in the front row because they like that fan's energy.

This study shows that for these fish, the "Big Guy" gets the best view purely because he pushed everyone else away. The band (the females) didn't choose him; they just couldn't get to anyone else.

Why Does This Matter?

This changes how we understand animal relationships. It suggests that sometimes, competition creates the illusion of choice. Just because a male gets to mate a lot, it doesn't mean the females wanted him. It might just mean he was too strong to be ignored.

In the world of Pygmy Halfbeaks, being the "King of the Hill" is great for your dating life, but the girls aren't falling in love with you because of it—they're just stuck in the room with you.

Technical Summary

1. Problem Statement

In sexual selection theory, intra-sexual selection (male-male competition) and inter-sexual selection (female mate choice) are often assumed to act in concert. The prevailing hypothesis suggests that traits conferring social dominance (e.g., aggression, size) serve as honest signals of male quality, leading females to prefer dominant males. However, empirical evidence disentangling these two mechanisms is limited. It remains unclear whether dominant males secure mating opportunities because females actively choose them or because dominant males physically monopolize access to females, thereby constraining female choice. This study investigates this interaction in the pygmy halfbeak (Dermogenys collettei), a small freshwater fish where males use elongated lower jaws (beaks) for agonistic interactions.

2. Methodology

The researchers conducted two distinct experiments using wild-caught and lab-reared D. collettei to separate the effects of male competition from female choice.

Experiment 1: Male Access to Mates

• Objective: To determine if male social dominance influences access to females and courtship effort, and if this effect depends on the presence of direct male-male competition.
• Design:
  • Setup: Two size-matched unfamiliar males and two females were placed in a tank.
  • Phase 1 (Competition): Males were allowed to interact physically for 30 minutes to establish a dominance hierarchy. Dominance was quantified using a Dominance Index (DIndex) based on displacement behaviors (aggressor displacing the rival). Males with a DIndex > 0.7 were classified as dominant.
  • Phase 2 (Interaction):
    • Treatment A (With Competition): Both males and females were allowed to interact freely for 30 minutes.
    • Treatment B (Without Competition): Only one focal male interacted with the females; the other male was separated by a transparent partition (allowing visual/chemical cues but preventing physical interaction).
• Metrics: Researchers recorded the duration of "swimming under" (a pre-copulatory behavior) and the total count of courtship behaviors (circling, swimming under, mating attempts).
• Controls: Phenotypic traits (body length, beak length, red fin coloration) were quantified to control for morphological variables.

Experiment 2: Female Mate Choice

• Objective: To test if females prefer dominant males and if observing male-male competition alters this preference.
• Design: A dichotomous choice assay using a central chamber for a focal virgin female and two stimulus chambers for unfamiliar males.
  • Baseline Phase: Females observed both males through opaque dividers (no interaction) for 1 hour to establish baseline preference.
  • Eavesdropping Phase: Opaque dividers were removed, allowing males to interact physically for 1 hour while the female observed.
  • Post-Interaction Phase: Males were separated again, and female preference was re-assessed for 1 hour.
• Metrics: Strength of Preference (SOP) was calculated as the difference in time spent in the association zone of the dominant male versus the subordinate male.
• Sample Size: 46 valid trials (after excluding trials with unclear hierarchies or incomplete exploration).

Statistical Analysis

• Linear Mixed Effects Models (LMMs) and Generalized Linear Mixed Effects Models (GLMMs) were used.
• Response variables included courtship duration, courtship counts, and SOP.
• Fixed effects included dominance status, trial type, and morphological traits; random effects included male/female identity and tank ID.

3. Key Results

Experiment 1: Dominance Drives Access

• Overall Effect: Dominant males spent significantly more time swimming under females and performed significantly more courtship behaviors than subordinate males.
• Context Dependence:
  • With Male-Male Competition: Dominant males monopolized access, courting females significantly more than subordinates.
  • Without Male-Male Competition: When physical interaction was prevented, no significant difference was found between dominant and subordinate males in courtship effort or time spent with females.
• Female Traits: Female body size and gravid spot size did not significantly influence male courtship effort.

Experiment 2: No Female Preference for Dominance

• Baseline vs. Post-Interaction: Females showed no significant preference for dominant males either before or after observing male-male competition.
• Preference Shift: The change in preference ($\Delta$SOP) between the baseline and post-interaction phases was not significantly different from zero ($p = 0.90$).
• Morphology: Female preference was not influenced by male body length, beak length, or red coloration. Females exhibited an unbiased mating preference regarding social status and the measured morphological traits.

4. Key Contributions

1. Disentanglement of Mechanisms: The study provides clear experimental evidence that in D. collettei, male mating success is driven by male-male competition (access monopolization) rather than female choice.
2. Contextual Necessity of Competition: The study demonstrates that the reproductive advantage of dominant males is contingent upon the ability to physically displace rivals. Without direct competition, dominance status alone does not confer a mating advantage.
3. Challenge to "Honest Signal" Theory: The findings contradict the assumption that dominance traits are universally used by females as honest signals of quality. In this species, females appear indifferent to social dominance even after "eavesdropping" on contests.

5. Significance and Implications

• Evolutionary Dynamics: The results suggest that in species with intense male-male competition, pre-copulatory female choice may be relaxed or rendered unnecessary because dominant males effectively "pre-select" mates by excluding subordinates.
• Sexual Selection Theory: This study highlights that male-male competition and female choice can act independently rather than synergistically. It challenges the generalization that females universally prefer dominant males, suggesting that the fitness benefits of dominance (e.g., genetic quality) may not be the primary driver of female choice in all species.
• Future Directions: The authors note that while dominance drives access in small groups (dyads), the dynamics in larger, natural shoals (6–120 individuals) may differ, as larger groups might make mate monopolization more difficult for dominant males.

Conclusion: In the pygmy halfbeak, social dominance is a critical determinant of reproductive success, but this success is achieved through the physical exclusion of rivals rather than through active female preference for dominant males.