Mate-finding dispersal reduces local mate limitation and sex bias in dispersal

1. Sex-biased dispersal (SBD) often skews the local sex ratio in a
population. This can result in a shortage of mates for individuals of the
less-dispersive sex. Such mate limitation can lead to Allee effects in
populations that are small or undergoing range expansion, consequently
affecting their survival, growth, stability and invasion speed. 2. Theory
predicts that mate shortage can lead to either an increase or a decrease
in the dispersal of the less-dispersive sex. However, neither of these
predictions have been empirically validated. 3. To investigate how
SBD-induced mate limitation affects dispersal of the less-dispersive sex,
we used Drosophila melanogaster populations with varying dispersal
propensities. To rule out any mate-independent density effects, we
examined the behavioral plasticity of dispersal in presence of mates as
well as same-sex individuals with differential dispersal capabilities. 4.
In the presence of high-dispersive mates, the dispersal of both male and
female individuals was significantly increased. However, the magnitude of
this increase was much larger in males than in females, indicating that
the former show greater mate-finding dispersal. Moreover, the dispersal of
either sex did not change when dispersing alongside high- or
low-dispersive individuals of the same sex. This suggested that the
observed plasticity in dispersal was indeed due to mate-finding dispersal,
and not mate-independent density effects. 5. Strong mate-finding dispersal
can diminish the magnitude of sex bias in dispersal. This can modulate the
evolutionary processes that shape range expansions and invasions,
depending on the population size. In small populations, mate-finding
dispersal can ameliorate Allee effects. However, in large populations, it
can dilute the effects of spatial sorting. 28-May-2020