Thus, different species do not compete reproductively among themselves and so variation in reproductive pattern is common and widespread throughout the animal kingdom. One of the goals of sociobiology is to explain this variation. Since time and energy are limited and both of these components require the expenditure of time and energy, we can expect a trade-off between efforts directed toward child bearing and child rearing. This trade-off is especially important for iteroparous species which reproduce more than once in their lifetime because resources devoted to child caring are not available for child rearing and what matters in intrapopulational competition for reproductive success is total lifetime reproductive output. Since the production of surviving offspring is the means whereby parents realize their fitness, we might expect that parents would invest maximally in caring for their young, but this is not universally true.
Some species exhibit no parental care at all and in others only one sex cares for the young.
The reproductive interest of family members is not the same and this difference results in conflict between parents in providing parental care, between parents and offspring over the amount of parental care given and between siblings in the distribution among offspring of the care provided by the parents. We will confine our discussion in this topic to the conflicts generated between parents and offspring and between siblings. Parents are willing to invest in parental care if by so doing they can maximize their fitness. As noted before, however, there are costs associated with parental investment and parents will be selected to cease giving parental care when the costs exceed the benefits. The costs increase with the age of the offspring and as the offspring reach the point of self-sufficiency the costs are greater than the benefits.