  Recent observations and theories have presented a strong challenge to the
universality of the stellar initial mass function (IMF) in extreme
environments. A notable example has been found for starburst conditions, where
evidence favours a top-heavy IMF, i.e. there is a bias toward massive stars
compared to the IMF that is responsible for the stellar mass function and
elemental abundances observed in the Milky Way. Local starburst galaxies have
star-formation rates similar to those in high-redshift main-sequence galaxies,
which appear to dominate the stellar mass budget at early epochs. However, the
IMF of high-redshift main-sequence galaxies is yet to be probed. Since
$^{13}$CO and C$^{18}$O isotopologues are sensitive to the IMF, we have
observed these lines towards four strongly-lensed high-redshift main-sequence
galaxies using the Atacama Large Millimeter/sub-millimeter Array. Of our four
targets, SDSS J0901+1814, at $z \approx 2.26$, is seen clearly in $^{13}$CO and
C$^{18}$O, the first detection of CO isotopologues in the high-redshift
main-sequence galaxy population. The observed $^{13}$C/$^{18}$O ratio, $2.4 \pm
0.8$, is significantly lower than that of local main-sequence galaxies. We
estimate the isotope ratio, oxygen abundance and stellar mass using a series of
chemical evolution models with varying star-formation histories and IMFs. All
models favour an IMF that is more top-heavy than that of the Milky Way. Thus,
as with starburst galaxies, main-sequence galaxies in the high-redshift
Universe have a greater fraction of massive stars than a Milky-Way IMF would
imply.
