The few that manage to reproduce end up exhausted, and that is the sum total of their contribution as fathers. They go off, leaving all the work to the females. And this system perpetuates itself. This female right whale can’t complain that her son is like his father, because she herself chose him: attractive, enormous, capable of making enormous leaps, but a complete loss when it comes to contributing to the family. This female and the male that copulated with her each contributed 50% of the genetic makeup of the calf. Then, the male hands her the bill for all the time and effort he had invested in fighting and overcoming his rivals. In accordance with this biological pact, she must now pay the price for the genes of a winner, assuming full responsibility for the care and education of their mutual child. The southern or South Atlantic right whale gives birth to a calf every three years. Here, in the protected waters of the Valdés Peninsula, in Argentina, the young whales learn different techniques from their mothers, before they both have to return to the open sea, to find food. Until the calf reaches the age of six months, the mother will eat nothing, because these shallow waters do not contain the plankton she needs. But the fact that some female mammals spend so much time taking care of their young means that during this period they do not mate with the males. And some of them may lose patience and give into the temptation to kill the young, so the mother will again come into heat. Not only does the female grizzly bear have to bring up her children all alone, she also has to be careful to avoid all contact with the adult males prowling around the area. And in this part of Alaska there are a lot of them. The mother will teach her young where and how to find food, who their enemies are, and how a grizzly bear lives. Meanwhile, the large, solitary males, weighing up to 600 kilos, are desperate to find females willing to mate. And if they aren’t, then he knows what to do to make them. Though it might not seem so, the life of these cubs will not be at all easy if they are male. From the moment their mother stops protecting them, they will go through hell. They will then have to compete against the big boys, and victory will go only to the strongest. In the meantime, they can enjoy one of the happiest childhoods in the animal kingdom; bears love playing. The biological differences between male and female mammals can very clearly be seen in one of the most common reproductive strategies: the harem. This male impala is the only one authorised to copulate with all the females of his herd, but during the time he is the leader, his life will be one of constant stress. All of the little ones are his children, and one day one of them will take his place. He produces millions of sperm every day, and is capable of making all the females pregnant in very little time, whereas the does can only have one child at a time. The inequality stems from the very nature of their reproductive cells. On the other hand, all the females copulate and procreate, whereas only a few males manage to do so, and at an extremely high price. Being male is not as easy as it may at first seem. These Australian emus go one step further. As they are birds, the females do not have to carry the embryo inside their bodies as the impalas do. They can leave their future offspring, well protected inside the egg, in the care of their fathers. He built the nest, and will incubate the eggs, without eating or drinking for 56 days. Many females go off to copulate with other males, while a few remain behind to defend them while they are incubating. When the chicks hatch, the male will be with them for the next seven months. Responsible fatherhood, without a doubt. But the greatest example of the liberated female is this animal: the Australian brushturkey It is related to the turkeys and capercaillies, and the males build nests of up to four tonnes in weight, piling vegetable matter up to form a heap up to one metre high. The females approach the enormous nest and, after mating with the owner, bury their eggs in it, then simply march off, never to return. Several females lay their eggs in the same nest, and the male takes care of them all. The most curious thing is that the decomposition of this heap of dead leaves produces sufficient heat to incubate the eggs. The male adds or removes vegetable matter to maintain a constant temperature of between 30 and 35 degrees centigrade. Sticking his head inside, his beak and tongue act as a thermometer, telling him what he should do. The females, meanwhile, happily feed with never a thought for the future of their offspring. Dawn is breaking in this village in the north of Laos, very close to the Chinese border. The Ekkor are a people who live from subsistence agriculture, like so many others in the world. From the early hours of the morning, the only people working are the women. They carry the water from miles away, grind the rice, take care of the children, and still manage to dress elegantly, in their traditional costumes, as they go about their daily tasks. The majority of the human population of the world is rural and poor, but they have another thing in common: the women not only play the role of mothers, but also carry out the hardest work. When did this inequality begin? Why is it accepted virtually everywhere in the world? Men and women, it is true, are of different size and physical strength, though biologically women are stronger. The evolutionary reason for the strength of the male is defence and competition. Defending the group is a dangerous job, and it would seem logical that evolution has reserved it for the men, saving the women, who have a more important mission. But both hunting and defence are no longer necessary in the majority of human societies. Wild animals no longer pose a threat to the children, and cattle-rearing has made hunting by and large obsolete. The males have seen how their duties have diminished, while the females took on more and more tasks. The pact has been broken, the balance is no longer fair. And what is more, women are shut away inside the home, unable to take part in public life and adult culture. Hope lies in the new generations.