In humans, children tend to be brought up by their mother and father, but in the rest of the animal kingdom that is not always the case. In some animals, only the female looks after the offspring and in other cases only the male. But why does this happen? What advantage does a parent have in leaving their offspring in the care of another? Why not stay and protect your young?
More often than not parental care is maternal. Why? The common explanation is that the female has already invested so much into the offspring that it is worthwhile to ensure the survival of their offspring. This investment includes the production of large gametes and the period of gestation. But this hypothesis conflicts with the Concorde fallacy. The Concorde fallacy suggests only future costs are relevant to an investment decision.
Another common argument is that anisogamy (the union of gametes of a different size) produces a male based operational sex ratio (OSR). This means there are more males than females and as a result, males have to compete for females to mate with. If males are competing, paternal care is less likely to evolve as it would take up resources that could instead be used to compete. However, this argument falls flat considering under the Fisher condition whichever sex faces the most competition should exhibit the most parental care. So why isn’t daddy there? No argument has yet been proved, and it is still a conundrum for evolutionary biologists.
As is often the case in biology there are exceptions to this rule, when it is the father that looks after the offspring. So why is daddy there? There are many examples of paternal care in fish. Male fish still produce vast quantities of sperm, and could still have many more offspring if they skipped out their parental care responsibilities, so why waste time and resources caring for their young?
Sticklebacks are known for their paternal-only care. They guard a nest with a clutch of fertilised eggs and protect them from predators. These males are also attractive to females and often females lay their eggs in a male’s nest. The male fertilises the eggs and will protect them with his other clutches. A male can brood up to 10 clutches of eggs during the 2 week gestation period whereas a female could only brood 7 clutches (and would not have time to defend her young or forage for food for her).
It is obvious the case of parental, particularly paternal care, is not clear cut. The care exhibited by parents seems to be based on the circumstances (ecological and evolutionary) of the species in question. The evolution of parental care is also coupled to the mating system used by the species (not covered here). It is apparent parental care is a complex issue to understand. It is difficult to uncover the “real” ultimate causes of the behaviour in question, but parental care is unlikely to be fully understood until these causes are defined.