Ions and electrons in heliospheric plasmas commonly possess anisotropic distribution functions, with different temperatures parallel and perpendicular to the local magnetic fields. These temperature anisotropies are important because they affect the dynamics of the plasma and because they embed signatures of the dissipation mechanisms responsible for heating in the corona and solar wind. I will show how statistical studies of solar-wind hydrogen and helium temperature observations collected by the Wind spacecraft permit us to study the mechanisms that regulate anisotropies. In particular, I will present a recent study that offers compelling evidence of solar wind heating by an Alfven-cyclotron dissipation mechanism. Observations are sorted by the rate of Coulomb interactions, or collisional age, in the plasma and the differential flow between the two species. Helium is preferentially heated perpendicular to the magnetic field direction by more than a factor of 6 when the flow between the species is small relative to the Alfven wave speed and collisions are infrequent. These signatures are consistent with predictions of dissipation in the presence of multiple ion species. I will also present an unexpected result: observations of efficient heating of helium parallel to the magnetic field for large differential flow relative to the sound speed.