  Energy saving mechanisms in nature allow following organisms to expend less
energy than leaders. Queues, or ordered rows of individuals, may form when
organisms exploit the available energy saving mechanism while travelling at
near-maximal sustainable metabolic capacities; compact clusters form when group
members travel well below maximal sustainable metabolic capacities. The group
size range, given here as the ratio of the difference between the size of the
largest and smallest group members, and the size of the largest member (as a
per cent), has been hypothesized to correspond proportionately to the energy
saving quantity because weaker, smaller, individuals sustain speeds of
stronger, larger, individuals by exploiting the energy saving mechanism (as a
per cent). During migration, small individuals outside this range may perish,
or form sub-groups, or simply not participate in migratory behavior. We
approximate drag forces for leading and following individuals in queues of the
late Devonian (approx. 370 Ma) trilobite Trimerocephalus chopini. Applying data
from literature of R. herculea, a living crustacean, we approximate the
hypothetical walking speed and maximal speeds for T. chopini. Findings
reasonably support the hypothesis: among the population of fossilized queues of
T. chopini reported by Kin and Blazejowski (2013), trilobite size range was 75%
while the size range within queues, was 63%; this corresponds reasonably with
drag reductions in following positions that permit approx. 61.5% energy saving
for trilobites following others in optimal low-drag positions. We model
collective trilobite behavior associated with hydrodynamic drafting.
