This is a fascinating question but has not been answered well enough yet. From observations in the wild, it seems that other cats besides lions have manes. This means that something else must influence their growth because they live in groups and reproduce sexually. The only difference between them is that they do not have a mane. Several big cats live in groups and can reproduce sexually, but they do not present a mane even though they are the same size as other large male cats, such as tigers and leopards, who have one. A possible reason would be that some other genes influence the growth of the mane in these animals during their development to maturity. These genes exist because all individuals, regardless of whether or not they have a mane follow the same pattern of hair growth before and after sexual maturity. Even though it has not been proved yet, if we think about this hypothesis, it would make sense to say that in addition to the genes related to sex determination and mane growth. There is also a gene or some other genetic factor that determines whether an individual will have a mane or not.