Figure 1. Central to the question how much care a parent should provide is the trade-off between current and future reproduction. In general, if a male has the opportunity to decrease parental care (a), this may constrain the possibility for his partner to do so, and the female may even compensate for the lack of care by the male. By decreasing paternal care, the male may enhance his own reproductive output by acquiring multiple mates, or by enhancing his own longevity, thus enhancing his future reproductive output. If the female, however, decreases parental care, this will negatively affect the male's reproductive output. If the female has the opportunity to lower parental care (b), the effects are a mirror image. An increase in parental care by either parent is expected to reduce its future reproductive output. The focal sex (a: male, b: female) is in italics in both diagrams (see Szentirmai et al., 2007).

Figure 2. The Eurasian penduline tit, Remiz pendulinus, has an extremely variable breeding system among birds: incubation and feeding of nestlings is carried out by either the male or the female, whereas about 30–40% of nests are deserted by both parents. This diverse breeding system appears to be driven by intense sexual conflict over care. Photograph R.E. van Dijk.