  Modelling animal behavior using active-particle models is a major current
challenge. The unique properties of animals mean that such models require the
use of new types of effective interactions between the particles, and
specifically effective forces that do not obey the usual conservation laws of
Newtonian mechanics. These include nonreciprocal forces that break conservation
of energy and momentum. We demonstrate here two very different animal behavior
systems where such nonreciprocal effective forces naturally arise: the first is
when animals form contests, as many animals do, such as fighting over some
localized resource. The second system is of cohesive swarms, that are
maintained by long-range adaptive attraction. These examples show that
theoretical modelling in terms of active particles, interacting with effective
nonreciprocal forces and potentials, expands the scope of active-particle
research as well as helps to explain complex phenomena in animal behaviour.
