For the past nine months the Marine Corps has been experimenting with a gender-integrated task force at both Twentynine Palms and Camp Lejeune, to see how women would fare in various combat roles in the USMC.
The integrated Ground Combat Element Task Force contained a variety of separate unit types (infantry, artillery, mechanized reconnaissance and tanks) that all function together - just as they would during actual combat. Women were scattered throughout the units in expected ratios, with a clear majority of the soldiers (about 90%) male.
The exercise as a whole was broken into two large parts - the first four months consisted of a training work-up at Camp Lejeune, which was followed by a five month "deployment" to Twentynine Palms. During each phase Marines were hooked up to equipment that monitored both their health and their marksmanship. This information could be used to request an exception to the Jan. 1, 2016 deadline for gender integration, and could also be used to help establish gender-neutral baselines for Marine Corps combat jobs that do open to women in the future.
According to an article in the Washington Post, the attrition rate for the women was quite high - only two of the two dozen were able to overcome the physical and mental stress of the simulated combat and survive until the end of the nine month exercise. Although the dropout rate for women was quite high, even the male Marines who expressed concerns about women in combat roles during and after the experiment all found female Marines that impressed them over the nine-month exercise.
A MarineCorpsTimes.com report mentions that many of the women came to this experiment from non-combat units or they were directly out of boot camp and follow-on training, meaning they had significantly less experience and were potentially less physically and mentally prepared for the arduous and repetitive tasks. This may partially account for the large number of women dropping out, although it should be noted that many men voluntarily withdrew as well - although more often for personal rather than health reasons.
The final determination isn't expected for a few more months, but after the Army opened up Ranger School to women earlier this month, who knows?