If, however, the two curves cross above the threshold ( Figure 4d), there will be conflict between the parents. When neither parent can do all the work alone there is no ESS, because each will be able to exploit the other by slacking off, therefore obliging the partner to fully compensate. There will, however, be limits to the “laziness” of each partner. These limits function to maintain biparental care. Neither parent will want to do so little work that their partner is pushed over its abandonment threshold, thereby ruining the breeding attempt for both parents. This means there is a threshold for each parent beyond which exploiting their partner does not pay, as they would forfeit the current breeding attempt when the partner abandoned ( Figure 5). The ideal level of investment for each individual will be just more than his or her laziness threshold. Unless the abandonment threshold is above the intersection of the two curves (when both parents abandon) or it goes through the intersection (when both parents agree on startegy), the ideal investment levels are different for the male (?m) and for the female (?f). This means that there will be a region of conflict between the laziness thresholds. Within this region of conflict, there is no ESS, and we would predict that observed strategies should drift between these limits.
The model with a pair in similar body condition. The female’s survival curve is denoted by the thinner curve, and the male’s by the thicker curve. Dashed vertical lines denote the thresholds of laziness for the male and the female, the least amount of work that the individual can do without its partner abandoning. The arrows point to the preferred incubation proportion of the female (?f) and the male (?m) and the area of conflict between male and female is shaded.
Application of the model
Reducing the condition of one or both parents narrows the possible strategies that they may follow ( Figure 6). This makes the pair less flexible in their choice of possible strategies while, at the same time, bringing their ideal strategies closer together, thereby lessening the conflict between them. This suggests that there should be less conflict over parental care under poor environmental conditions or when parents have been negatively manipulated. Several workers have noted that seabirds operating a more synchronized and equitable system of incubation are more successful ( Bukacinska et al., 1996; Burger, 1987; Morris, 1987; Ratcliffe and Furness, 1999), so less conflict between the birds may improve breeding success and compensate to some extent for poor breeding conditions.
The females are denoted by the broken curves and the males by the thick curve. In controls (solid line), where both parents are in good condition, the whole shaded area shows the region of conflict. In manipulated pairs, where the female is in poorer condition (dotted curve), the darkly shaded area denotes the region of conflict.
Many empirical studies have tested whether parents partially compensate for changes in parental care by their partner. Results have not been consistent between studies, with some finding full compensation ( Hunt et al., 1999; Mrowka, 1982; Sanz et al., 2000), which would be expected to make biparental care unstable. We have demonstrated, using a modification of Houston and Davies’ ( 1985) model, that full compensation for decreased parental effort by one partner should occur under certain conditions, specifically, when a small decrease in parental effort results in a large decrease in breeding success. For different ecological situations, Ratneiks ( 1996) showed that biparental care becomes unstable when future fitness is a decelerating function of expenditure, and Houston and Davies ( 1985) show that biparental care is an ESS only under a partial compensation strategy.