= Solution
\b[Relatedness is not required when responding to others yields a sufficient direct benefit, and coordination need not begin as costly altruism.] <Coordinated behaviour> means that individuals adjust the timing, position or type of their actions relative to others. <Cooperation> concerns benefits provided through an interaction, while evolutionary <altruism> specifically imposes a direct fitness cost on the actor. Confusing these categories makes unrelated coordination appear harder to explain than it is.
Consider animals travelling together. Matching a neighbour's direction can reduce an individual's predation risk or help it reach food. Both the responder and the neighbour may benefit immediately; no deferred repayment or kin recognition is necessary. Likewise, complementary hunters can each increase their own chance of capture by responding to another's actions. The relevant condition is that the expected direct return to the response exceeds its costs, relative to available alternatives. That condition depends on ecology, not merely on whether companions share genes.
A clear interspecific example is joint hunting by <groupers> and <giant moray eels>, which cannot be close genetic relatives. Field observations documented nonrandom associations, grouper recruitment signals and benefits to both partners. Their different hunting modes expose prey that could evade either hunter alone. Each captured prey item is swallowed by its captor, limiting disputes over division of a carcass. The study supports a direct-benefit mechanism; it does not require alternating favours or prove that every recruitment signal evolved specifically in that association.<Interspecific communicative and coordinated hunting between groupers and giant moray eels in the Red Sea by Redouan Bshary and others>[2006, associations, recruitment and complementary hunting modes]{ref}
\b[Simple response rules can make individually advantageous actions fit together.] Attraction, alignment, avoidance and following informed individuals can generate collective movement without central control. A <quorum response> makes commitment more likely after enough others act, potentially preventing one mistaken individual from drawing the whole group into danger. Such proximate rules still need an evolutionary explanation: heritable differences in responsiveness must affect expected survival or reproduction in the environments where the rule is expressed.
Experiments with <three-spined sticklebacks> used moving replica fish to manipulate apparent social information. Larger groups under perceived risk were less readily led by a single replica than solitary fish, and nonlinear response models explained these decisions better than simple proportional copying. Simulations linked these rules to improved decisions under specified assumptions. The experiment establishes a plausible coordination mechanism; actual lifetime fitness gains and the historical selection of its parameters remain separate questions.<Quorum decision-making facilitates information transfer in fish shoals by Ashley J. W. Ward and others>[2008, replica-fish experiments and decision models]{ref}
The strategic problem also depends on the <payoff matrix>. In a <coordination game>, joining a hunt can be a best response when others join, while acting alone is better when nobody else participates. Shared cues, recruitment signals and learned conventions can move a group toward the mutually beneficial equilibrium. Here the difficulty is reaching or maintaining compatible choices. In a <prisoner's dilemma>, by contrast, withholding a contribution pays better against either partner action in a one-shot encounter. An account of coordinated hunting must establish which incentives actually apply rather than assume every group activity is a public-goods dilemma.
When cooperation does carry an immediate cost, <direct reciprocity> can make future consequences matter. In a deliberately simple repeated donation game, giving costs $c$ and benefits a partner by $b>c$. If encounters continue with probability $w$, perpetual mutual cooperation yields $(b-c)/(1-w)$. Defecting once against a cooperator yields $b$ if it then triggers permanent noncooperation. Cooperation is preferable against that deviation when
$$
\frac{b-c}{1-w}\ge b\qquad\Longleftrightarrow\qquad w\ge\frac cb.
$$
This is a conditional comparison under recognition, reliable observation and the specified retaliation rule. It is not a universal threshold for animal cooperation, nor does it show that a rare cooperative strategy can always invade an uncooperative population. Primary repeated-game modelling and computer tournaments established how such contingent interactions can sustain cooperation under suitable conditions.<The evolution of cooperation by Robert Axelrod and William D. Hamilton>[1981, theoretical model and strategy tournament]{ref}
Other mechanisms need not involve repayment by the same individual. <Partner choice> can reward useful collaborators through access to future interactions; leaving an unproductive association can impose an opportunity cost on a defector. <Reputation> can alter future access to different partners. <Partner fidelity feedback> occurs when helping a partner increases the actor's future benefits because that partner survives or becomes more productive. These mechanisms require appropriate encounter structure, information and opportunities to choose; naming them is not evidence that a particular species uses them.
<Evolutionary policing> or <partner sanctions> can change the returns to exploitation. As a mechanistic example involving unrelated partners, soybean experiments prevented a normally fixing rhizobial strain from fixing nitrogen by changing its atmospheric supply. Reduced bacterial reproductive success and oxygen-supply measurements supported a host-sanction interpretation. This shows how conditional resource allocation can favour service provision without kinship, although it is not by itself an example of synchronized animal movement or proof of the historical origin of the interaction.<Host sanctions and the legume-rhizobium mutualism by E. Toby Kiers and others>[2003, experimental prevention of nitrogen fixation and symbiont performance]{ref}
Coordination can consequently evolve through several routes: immediate mutual benefit, stable contingent exchange, partner assortment, or changed incentives imposed by others. Learning can implement or transmit successful rules, while genetic evolution shapes learning capacities and costs. These explanations are compatible with multilevel descriptions of selection, but a claim that groups benefit is insufficient unless the process maintaining the relevant individual strategies is specified. Effective groups of unrelated individuals emerge when responding and contributing are viable strategies in their actual ecological and social setting.
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