Collective behavior of animal groups and cell collectives

5 problems

• Physical limits of gradient sensing by cell collectives

  Partially resolved

  Single cells can be poor at sensing a weak chemical gradient, but clusters of cells sometimes follow gradients better than any single member. How much a group can gain, and what limits it, is unsettled.

• How groups reach consensus led by a few informed individuals

  Partially resolved

  In fish schools and bee swarms, a small fraction of informed members can steer the whole group to a target. How the needed fraction depends on group size, and how conflicting preferences are resolved, has had limited quantitative tests in natural groups.

• Topological or metric interactions in animal groups

  Partially resolved

  A bird could respond to all neighbors within a fixed distance, or to a fixed number of nearest neighbors whatever their distance. Field data favor the second for starlings, but why the number is about seven, and whether this holds across species, is open.

• Why speed correlations in starling flocks are scale-free

  Contested

  In starling flocks, a change in one bird's flight is correlated with birds across the whole flock, however large it is. Whether flocks are tuned to a special critical state to achieve this, and how, is debated.

• Dynamic critical exponent of natural insect swarms

  Partially resolved

  In midge swarms, the time over which fluctuations die out grows with the correlation length according to a power set by one number, the dynamic exponent. Its measured value, about 1.4, differs from standard theories of moving groups.