How should parents respond to a reduction in litter size: Abandonment or extra care?

Although natural selection may favour the evolution of an optimal brood size, unpredictable environmental factors can intervene to render the brood either considerably larger or smaller than this optimum. The question therefore arises as to how parents should respond to unusually large or small litters. Solutions to this problem which involve abandonment of the offspring or brood reduction, for example by infanticide, have received most theoretical and empirical attention. For small mammalian litters, however, evidence that parents employ these behavioural strategies is sparse. I suggest that, under certain conditions, an alternative to abandonment and infanticide of small litters in mammals is an increase in total parental care above that seen in larger litters. Evidence is presented which supports this possibility. This strategy may allow parents to compensate for the loss of offspring by producing a small number of high quality individuals. It is most likely to occur when opportunities for rapid rebreeding are limited, and when increases in parental care have strong effects on offspring quality. Interest in abandonment and infanticide has diverted attention away from this alternative parental response to small litters. Studies of this response are encourage, not least because they will focus attention on the poorly understood relationship between parental care and offspring quality.