  The transverse momentum dependence of the anisotropic flow $v_2$ for $\pi$,
$K$, nucleon, $\Lambda$, $\Xi$ and $\Omega$ is studied for Au+Au collisions at
$\sqrt{s_{\rm NN}} = 200$ GeV within two independent string-hadron transport
approaches (RQMD and UrQMD). Although both models reach only 60% of the
absolute magnitude of the measured $v_2$, they both predict the particle type
dependence of $v_2$, as observed by the RHIC experiments: $v_2$ exhibits a
hadron-mass hierarchy (HMH) in the low $p_T$ region and a
number-of-constituent-quark (NCQ) dependence in the intermediate $p_T$ region.
The failure of the hadronic models to reproduce the absolute magnitude of the
observed $v_2$ indicates that transport calculations of heavy ion collisions at
RHIC must incorporate interactions among quarks and gluons in the early, hot
and dense phase. The presence of an NCQ scaling in the string-hadron model
results suggests that the particle-type dependencies observed in heavy-ion
collisions at intermediate $p_T$ might be related to the hadronic cross
sections in vacuum rather than to the hadronization process itself.
