Parental care is common throughout the animal kingdom and among caring species there is a bewildering variation in how parents care for offspring, as well as in the amount of resources parents invest in care. For instance, there is considerable variation in the relative parental investment by the sexes – in some species females invest more, in others males invest more, and in some investment is more or less equally shared. Different hypotheses have been proposed to explain patterns of parental investment between the sexes, as well as among species, and work is still ongoing to develop an overarching hypothesis that can explain the various patterns observed.

Key concepts:

• Among species there is ample variation in how parents care for their offspring, if at all, as well as in how much they invest in offspring.
  
• Differences between the sexes in relative parental investment have been recently proposed to depend on sexual selection and the operational sex ratio, rather than sexual selection patterns resulting from relative parental investment by the sexes, as previously suggested.
  
• Individual decisions on the optimal parental investment can lead to conflict between parents as well as between parents and the offspring.
  
• In some species, care of the offspring is not the exclusive responsibility of the parents but is shared among members of a group, termed cooperative breeding. In other species, adults sometimes take care of unrelated offspring, either willingly (as in adoptions) or unwillingly (as in brood parasitism).
  
• Within species, parents may adjust the amount of investment in offspring based on signals from their partner or from the offspring themselves.