  The average number of muons in air showers and its connection with shower
development has been studied extensively in the past. With the upcoming
detector upgrades, UHECR observatories will be able to probe higher moments of
the distribution of the number of muons. Here a study of the physics of the
fluctuations of the muon content is presented. In addition to proving that the
fluctuations must be dominated by the first interactions, we show that
low-$N_{\mu}$ tail of the shower-to-shower distribution of the number of muons
is determined by the high-$x_{\rm L}$ region of the production cross-section of
neutral pions in the first interaction.
