L11- Mutualisms

Mutualism:

• action is beneficial for recipient and itself:

  

• there needs to be an immediate mutual benefit, not much opportunity to defect

  • however, each individual wants to maximise own fitness→ others fitness is not in their best interest

  

Semantics:

• Mutualism- interspecific

• Mutual benefit- intraspecific

• but both can be used interchangeably, have the same kind of interactions and stabilising mechanisms

Examples of Mutualisms:

Improved foraging efficiency:

• can perform tasks that would have been impossible individually e.g. affects which prey lions can tackle:

  • mostly single individual chase warthogs

  • as prey get larger, more likely to hunt together

    

Territoriality e.g. lions:

• infanticide occurs when males take over a pride

• hypothesis→ females live in prides to reduce rate of turnover of male owners of pride

• advantage of living in large groups for lionesses-

  • defend a larger area, fewer neighbours, higher reproductive success

  • important for them to reduce male neighbours→ more male neighbours increases takeovers and therefore female mortality rate and wounding rate

    

  → living in large female prides allows them to defend a larger area and reduce density of lions in orbit

Sentinels:

• sentinels are posted to take turns to watch for predators

  

• traditional interpretation was altruistic kin selection→ dangerous for sentinels but protecting kin BUT see sentinels when there are no/many non-kin→ doesn’t work

• new view→ sentinels are actually in the safest place- can see predator, know where to go→ is mutualism

  
  

e.g. Meerkats:

• hypothesis: take it in turns to be sentinels when fully fed

• evidence for this:

  • search for invertebrates, dig into soil to find scorpions- very vulnerable to predation

  • post sentinels, usually on raised porches

    

  • experiment→ tested if sentinels are well fed

  • method→ fed some, left others unfed

  • results→ highly significant difference between time spent as a sentinel if you are fed vs unfed

    

  → sentinel behaviour is the optimal activity once their stomach is full and no other animal is on guard→ is mutualistic

e.g. Pied Babblers and Drongos:

• PB forage on invertebrates deep underground→ highly vulnerable to predators

• Post own sentinels or post Drongo sentinels

• Drongos benefit→ are highly specialised for catching any insects flushed whilst babblers are digging

• PB benefit→ Drongos alarm call

  • presence of drongo reduces need for babbler sentinels so each individual spends more time foraging:

  

  
  

• BUT as group size increases, babblers became less responsive to alarm calls of drongos and there are more attacks on the drongos by the babblers:

  

  • kleptoparasitism→ Drongos give false alarm calls to young, inexperienced babblers who flee and then Drongos take their food, are more successful at this when babblers are in smaller groups:

    

  → mutualism where both get benefits but Drongo tries to maximise own fitness through kleptoparasitism→ in response babblers resist mutualism

• general problem of mutualism→ are always liable to be exploited by one group or the other→ parasitism

Solutions to exploitation of mutualisms:

1. Facultative mutualism

2. Punishment/enforcement

3. Image-scoring/reputation

4. Cultural coevolution

Facultative mutualism-

• engage in mutualism when it is mutually beneficial but default back when it is not e.g. Pied Babblers and Drongos

Punishment/enforcement-

e.g. interspecific mutualism in cleaners and clients:

• seen in lots of taxonomic groups

  

• often lapses into exploitation e.g. ox peckers on the back of a wildebeest eating from the open wound, cleaner fish also feed on fish mucus/scales rather than ectoparasites

  

• e.g. blue-streaked cleaner wrasse fish-

  • eat 1200 parasites every day, inspect 2300 fish a day, some clients visit every 5 minutes

  • are tempted to cheat but why can they not?

  • hypothesis→ punishment:

  • get eaten→ not likely, most clients aren’t fish eaters

  • client will leave or chase cleaner:

    • experiment→ offered cleaner fish plates with fish flakes and prawns

    • initial preference for each cleaner fish was prawns

    • control- allowed fish to do what they wanted

    • fleeing- as soon as cleaner fish fed on prawns, they removed plate

    • punishment- every time cleaner fish fed on prawns, it was poked with a glass rod

    • results→ when there was fleeing or punishment, there was more feeding on flakes (equivalent of feeding on ectoparasites), 6 learning trials- very fast learning

      

• e.g. coffee room in university room-

  • contributions in an honesty box for coffee

  • put an image above the box each week for 10 weeks- flowers or eyes

  • results→ payment was 3x greater when images of eyes were added

    

→ punishment or even the threat of punishment is enough to enforce cooperation

Image-scoring/reputation-

• e.g. blue-streaked cleaner wrasse fish-

  • field study in the red sea→ observed whether they had cheated (taken a scale/mucus) or when clients were ignored

  • Q→ were clients more likely to return if their last interaction was positive or negative (cheated or were ignored)?

    • return rate→ 60% if last interaction was positive, 5% if negative

      

    → being more inclined to cheat would build a negative reputation

  • lab experiment→ fish had a choice of interacting with a cooperative or non-cooperative cleaner fish

    

    • results→ over two years, both had a significant preference for cooperative cleaners

      

  • another experiment in handout→ cleaner fish were more likely to feed on fish flakes than prawns when being observed

    → conscious of being image-scored

Cultural coevolution-

• e.g. honeyguides (birds) and honey hunters-

  • 2 groups of honey hunters→ Hadza (Tanzania) and Yao (Mozambique)

  • benefit from interacting with honeyguides→ know where bee colonies are, offer calls to each other, honeyguides lead them to a bee’s nest, extract honey, always leave food for honeyguide

  • each honey hunter has different signals in different tribes- Hadza whistle whilst Yao trill-gruny

• measured probability of being guided by honeyguide:

  • honeyguides in Tanzania were more responsive to whistle than to trill-grunt, vice-versa found in Mozambique

    

• honeyguides are parasitic- lay nest in beeeaters, cannot learn calls from parents, each honeyguide has to learn call across life

  → learned cultural mechanism that maintains the mutualism cooperations

Summary-

• Mutualistic interactions require no special explanation because of immediate benefit to both partners (usually)

• Mutualisms widespread within and between species

• Mutualists may be tempted to cheat, but costs may be reduced by facultative association, and cooperation can be maintained by punishment and need for a good reputation