Nancy Segal

One theory posits that females develop quicker than males in utero, which makes them hardier.

In fact, Nancy Segal, PhD, in her paper Twin Types, More Than Just Two, claims that a newborn female has the physical maturity of a 4-6 week old male.

If we consider that all mammals begin female in the womb, and maleness only develops after the second month of gestation, then this makes some sense.

A female embryo would continue growing on course while a male embryo would have to spend time developing… maleness.