Relaxed selection diminishes social memory and expression of host defences against cuckoos

This study demonstrates that in Common Reed Warblers, behavioral defenses against brood parasitism, including mobbing and egg rejection, rapidly degrade under relaxed selection and cannot be restored by social information, even in recently allopatric populations.

The Gist

Imagine a neighborhood where a notorious prankster (the Cuckoo bird) has been dropping off fake eggs in people's nests for generations. The residents (the Reed Warblers) have learned to spot these fakes and throw them out. They've also learned to scream and dive-bomb the prankster whenever they see him, a behavior called "mobbing."

But what happens if the prankster moves away? What happens to the neighborhood's defenses if the threat disappears for a long time?

This study asks exactly that question. The researchers looked at two groups of Reed Warblers:

1. The "Fresh" Group (Finland): These birds moved to a new area only about 100 years ago. They came from a place full of Cuckoos, so they brought their "defense manuals" with them, but they haven't seen a Cuckoo in their new home yet.
2. The "Ancient" Group (Sicily): These birds have lived in an area without Cuckoos for a very long time (hundreds of years).

The scientists wanted to know: If you remind these birds that a Cuckoo is nearby, will they remember how to fight back?

The Experiment: The "Gossip" Test

To test this, the researchers set up a clever experiment using social information (or "gossip").

• The Setup: They placed a fake Cuckoo model near a bird's nest.
• The Gossip: Before showing the fake Cuckoo, they played a recording of other birds screaming and attacking a Cuckoo nearby. This is like the birds hearing their neighbors say, "Hey! There's a bad guy over here! Get ready!"
• The Control: They did the same thing but played recordings of birds attacking a harmless bird (like a Magpie or a duck) to see if the birds just reacted to any noise.

The Results: The "Use It or Lose It" Rule

The results were surprising and tell a story about how quickly we can forget how to protect ourselves.

1. The Finland Birds (The Newcomers):

• The Memory: When the researchers played the "gossip" about a Cuckoo, the birds that already knew how to mob (attack) the fake Cuckoo got even louder and more aggressive. They remembered the drill.
• The Forgetfulness: However, the birds that didn't attack the fake Cuckoo in the first place? They stayed quiet. Even after hearing their neighbors scream about the danger, they didn't learn. They didn't suddenly decide, "Oh, I should attack too!"
• The Egg Test: When the researchers put a fake, non-matching egg in the nest, the birds didn't throw it out. The "gossip" didn't make them suddenly realize, "Wait, that egg looks weird!"

2. The Sicily Birds (The Long-Term Residents):

• The Blank Slate: These birds were even more relaxed. They barely reacted to the fake Cuckoo at all, and the "gossip" from neighbors did absolutely nothing to wake them up. They had completely forgotten how to defend their nests.

The Big Takeaway: Why This Matters

Think of these defenses like a muscle. If you stop going to the gym (relaxed selection), your muscles don't just stay the same; they atrophy and shrink.

• Social Learning Fails: Usually, we think animals can learn from each other quickly. If a neighbor sees a threat, the rest of the group learns. But this study shows that if you haven't practiced your defense for a while, you can't just "catch" the skill from a neighbor. The ability to learn socially seems to fade away along with the defense itself.
• The "Cryptic" Illusion: Scientists used to think that maybe the birds still had the defense hidden inside them, just waiting for the right cue (like the gossip). This study proves that's not true. The defense is actually gone. The "circuitry" is broken.
• The Geographic Mosaic: This explains why nature is a patchwork quilt. In some places, the "arms race" between Cuckoos and Warblers is fierce. In others, the Warblers have laid down their weapons and forgotten how to use them. If the Cuckoos come back to the "forgotten" areas, the Warblers will be sitting ducks.

In a Nutshell

This paper is a warning about complacency. Even if you have a great defense system, if you stop using it and stop teaching your kids how to use it, you will lose the ability to defend yourself. And unfortunately, you can't just "borrow" that skill from a neighbor if you've been out of practice for too long. The "social memory" of the group fades away, leaving the population vulnerable to a surprise attack.

Technical Summary

1. Problem Statement and Research Context

The study addresses a critical gap in evolutionary biology regarding the Geographic Mosaic Theory of Coevolution. While it is well-established that host species evolve defenses against brood parasites (like the Common Cuckoo, Cuculus canorus), it remains unclear how quickly these defenses degrade when populations expand into areas where the parasite is absent (allopatry).

• The Core Question: Do behavioral defenses (mobbing and egg rejection) and the ability to learn from social cues ("social memory") decay rapidly under relaxed selection, or are they retained as "cryptic plasticity" that can be reactivated by environmental cues?
• The Hypothesis: If defenses are retained, providing social information (observing others mobbing) should trigger an upregulation of defenses in allopatric populations. If defenses have decayed, social information will fail to elicit a response.
• The System: The study focuses on the Common Reed Warbler (Acrocephalus scirpaceus) and its interaction with the Common Cuckoo. The researchers compared two allopatric populations:
  1. Finland (Recent Allopatry): Colonized ~100 years ago (approx. 35 generations) from a parasitized range core.
  2. Sicily (Long-term Allopatry): Likely escaped parasitism for centuries/millennia due to geographic segregation (reed warblers in coastal lowlands vs. cuckoos in mountains).

2. Methodology

The researchers employed a rigorous field experiment design to manipulate perceived parasitism risk via social information and measure behavioral and physiological responses.

• Experimental Design:
  • Social Information Manipulation: Focal nests were exposed to "simulated neighbors" (a dummy nest) broadcasting playback of reed warblers mobbing a cuckoo (Cuckoo Treatment) or a control species (Magpie in Finland; Teal in Sicily).
  • Threat Presentation: Model cuckoos and control models were placed near the focal nest at three time points (Pre-Social Info, Post-Social Info, and a control presentation) to measure changes in behavior.
  • Egg Rejection Test: After the social manipulation, a "cuckoo egg" (either mimetic brown-spotted or non-mimetic blue/redstart-gens) was added to the clutch to test rejection rates.
• Metrics Measured:
  • Mobbing Propensity: Whether the birds attacked the model.
  • Mobbing Intensity: The number of vocalizations (calls) and bill snaps.
  • Latency to Approach: The time taken for birds to return to the nest after the model was placed (a measure of wariness).
  • Egg Rejection: Removal or puncture of the foreign egg.
• Statistical Approach: Generalized Linear Mixed Models (GLMMs) and hurdle models were used to analyze changes in behavior across presentations, accounting for nest identity and treatment effects.

3. Key Results

A. Mobbing Defenses

• Initial Response: Only 36% of Finnish pairs and 13% of Sicilian pairs mobbed the cuckoo model initially, significantly lower than rates seen in parasitized range-core populations.
• Effect of Social Information:
  • Finland: Social information did not increase the propensity to mob (i.e., non-mobbers did not start mobbing). However, for the subset of birds that already mobbed, social information significantly increased mobbing intensity (more calls).
  • Sicily: Social information had no effect on either the propensity or intensity of mobbing.
• Latency: Non-mobbing birds in Finland were initially slower to approach the cuckoo than mobbing birds but became faster with repeated exposure, suggesting a lack of specific recognition or habituation rather than active defense.

B. Egg Rejection Defenses

• Mimetic Eggs: Rejection rates were 0% in both Finland and Sicily for mimetic brown-spotted eggs, regardless of social information treatment.
• Non-Mimetic Eggs:
  • Sicily: 0% rejection of highly non-mimetic blue eggs.
  • Finland: Low rejection rates (<10%) of non-mimetic eggs, but social information did not increase this rate.
• Contrast: In parasitized range-core populations, similar social information manipulations typically increase egg rejection rates ten-fold.

C. Social Responsiveness

• The study found that "social memory" (the ability to learn from others) is degraded in allopatry. In parasitized areas, naive birds learn to mob after observing neighbors; in these allopatric populations, the majority of birds failed to acquire this behavior even after observing "neighbors" (via playback) mobbing.

4. Key Contributions

1. Evidence of Rapid Behavioral Decay: The study demonstrates that behavioral defenses can degrade significantly within a relatively short evolutionary timeframe (~100 years or 35 generations in Finland), challenging the assumption that complex behaviors are hard to lose due to genetic constraints.
2. Decoupling of Recognition and Expression: The results suggest that while some level of recognition might remain (evidenced by initial latency differences in Finland), the expression of the defense and the social plasticity required to upregulate it are lost under relaxed selection.
3. Social Memory as a Vulnerability: The paper highlights that "social memory" is not a permanent buffer. If a population loses the ability to learn from others (social responsiveness), it cannot rapidly recover defenses even if re-invaded by a parasite.
4. Methodological Validation: By using consistent protocols with previous studies in parasitized zones, the authors ruled out methodological failure, confirming that the lack of response is a biological reality of relaxed selection.

5. Significance and Implications

• Geographic Mosaic Theory: The findings support the "Geographic Mosaic Theory of Coevolution," suggesting that coevolutionary cycles are driven by the rapid loss of defenses in "cold spots" (allopatric areas) and subsequent re-invasion.
• Conservation and Invasion Biology: As species ranges shift due to climate change, hosts expanding into new areas may lose their defenses against local enemies. If these populations cannot rely on social learning to regain defenses quickly, they may be highly vulnerable to extinction upon re-invasion.
• Evolutionary Speed: The study provides empirical evidence that behavioral traits, often assumed to be stable, can evolve (or devolve) rapidly in response to changing selection pressures, potentially faster than previously thought.

In conclusion, Tolman et al. provide compelling evidence that relaxed selection leads to the rapid decay of both behavioral defenses and the social mechanisms required to maintain them, rendering host populations vulnerable to future parasitic invasions.