However, difficulties in collecting provisioning data beyond video recordings or provisioning patterns at the nest have made this theory hard to test (Savage & Hinde, 2019).įortunately, recent advances in remote monitoring have allowed the study of complex interaction networks between individuals at small spatial and temporal resolution (Krause et al., 2013 Smith & Pinter‐Wollman, 2020). One possible mechanism is that parents forage in proximity to each other, directly monitoring the foraging behaviour of the partner and adjusting their own contribution. This knowledge gap hinders our ability to understand which negotiation rules are used by parents while caring the young, and in turn, how sexual conflict could be ultimately resolved (Griffith, 2019). Therefore, parents are behaviourally responsive to each other, but it is not currently known which behavioural mechanisms are underlying these responses. More recently, it has also been argued that parents alternate their visits at the nest more than expected by chance because they actively monitor and respond to each visit of the partner (Johnstone et al., 2014 Savage et al., 2017 but see Schlicht et al. ( 2016) and Baldan, Hinde, et al. ( 2019). Empirical studies in songbirds have indicated that individuals modify their provisioning rate in response to the partner's experimentally manipulated behaviour, for example, selective playback, handicapping manipulations (reviewed in Harrison et al., 2009). However, it is not yet fully understood how negotiation mediated via direct response to the partner's behaviour occurs in nature. In birds for instance, parents are highly responsive to offspring begging calls (Kilner & Johnstone, 1997) and playback experiments of offspring begging elicit an increase in parental provisioning (Hinde & Kilner, 2007). Negotiation mediated via offspring behaviour has been widely investigated in the field. Because the evolutionary outcome of sexual conflict strictly depends on how parents acquire information and respond to partner's care levels, there is a renewed interest in understanding the negotiation mechanisms that parents adopt when caring for young (Griffith, 2019). Recent models, however, show that if parents directly assess each other's behaviour by coordinating their provisioning activity, such as taking turns of duties, the expected outcome is that parents increase parental care, maximizing both parent and offspring fitness (Johnstone et al., 2014 Johnstone & Savage, 2019). McNamara et al. ( 1999)'s and McNamara et al. ( 2003)'s models do not formally specify the negotiation mechanism through which parents monitor the partner's contribution, but in Lessells and McNamara ( 2012)'s model, parents decide how much to invest based on the current state of the offspring (which, in turn, reflects the cumulative amount of past investment by the two parents). For instance, early models (Lessells & McNamara, 2012 McNamara et al., 1999 McNamara et al., 2003) predict that sexual conflict lowers the amount of parental care and reduces parent and offspring fitness compared to a cooperative situation, that is, each carer withholds part of its potential investment to avoid being exploited by the partner (Lessells & McNamara, 2012). Game theoretical models have shown that the evolutionary outcome of sexual conflict depends on the behavioural (‘negotiation’) rules that parents adopt to assess and respond to each other's level of care over the offspring rearing period (Johnstone et al., 2014 Johnstone & Savage, 2019 Lessells & McNamara, 2012 McNamara et al., 1999 McNamara et al., 2003). A central goal in evolutionary biology is to understand how this sexual conflict is resolved and whether parents can reach a cooperative agreement over how much to care for offspring (Houston & Davies, 1985 Lessells, 2006 Servedio et al., 2019). On the other hand, since parental care is costly for each carer in terms of reduced reproductive opportunities or survival (Williams, 1966), each parent is also expected to exploit the partner and provide a smaller share of the care (hence a ‘sexual conflict’ exists over the evolutionary interests of the two parents Trivers, 1972 Lessells, 2006). While caring for young, parents are expected to cooperate as they both invest in the common goal of successfully raising offspring. Parental care is a source of both cooperation and conflict for parents (Royle et al., 2012).
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