Abstract
In cooperatively breeding species, care provided by helpers may affect the dominant breeders’ investment trade‐offs between current and future reproduction. By negatively compensating for such additional care, breeders can reduce costs of reproduction and improve their own chances of survival. Alternatively, helper care can be additive to that of dominants, increasing the fledging fitness of the current brood. However, the influence helpers have on brood care may be affected by group size and territory quality. Therefore, the impact of helping needs to be disentangled from other factors determining offspring investment before conclusive inferences about the effect of help on additive and compensatory care can be made. We used 20 years of provisioning data to investigate the effect of helping on provisioning rates in the facultative cooperatively breeding Seychelles warbler Acrocephalus sechellensis. Our extensive dataset allowed us to statistically disentangle the effects of helper presence, living in larger groups and different food availability. We show compensatory and additive care (i.e., partial compensation) in response to helper provisioning. Helpers lightened the provisioning load of the dominant male and female and increased total provisioning to nestlings. This was irrespective of group size or territory quality (food availability). Moreover, our results illustrate sex‐specific variation in parental care over the course of the breeding event. We discriminate between temporal variation, group size, and territory quality processes affecting cooperative care and as such, gain further insight into the importance of these factors to the evolutionary maintenance of helping behavior.
Highlights
In cooperative breeding systems, offspring care is often shared between the dominant male and female “breeders,” and a variable number of subordinate helpers (Koenig & Dickinson, 2016; Komdeur et al, 2017; Solomon & French, 1997; Stacey & Koenig, 1990)
When the dominants compensate for the care provided by helpers by reduc‐ ing their amount of care, the total amount of care received by the offspring may remain similar. Such “load lightening” by helpers can reduce the costs of reproduction for the dominants (Bruintjes, Heg‐ Bachar, & Heg, 2013; Dixit, English, & Lukas, 2017; Heinsohn, 2004; Koenig & Walters, 2011; Meade, Nam, Beckerman, & Hatchwell, 2010; Scantlebury, Russell, McIlrath, Speakman, & Clutton‐Brock, 2002; Sharp, English, & Clutton‐Brock, 2013), which can lead to increased dominant survival (Cockburn et al, 2008; Hatchwell & Russell, 1996b; Heinsohn, 1992; Khan & Walters, 2002; Kingma et al, 2010) and increased future reproductive success (Brown & Brown, 1981; Russell, Brotherton, McIlrath, Sharpe, & Clutton‐Brock, 2003; Woxvold & Magrath, 2005; Blackmore & Heinsohn, 2007; but see Meade et al, 2010)
We evaluated if the 95% credibility intervals (95% CrI) of the posterior modes overlapped zero, where a departure from zero was interpreted as a significant effect
Summary
In cooperative breeding systems, offspring care is often shared between the dominant male and female “breeders,” and a variable number of subordinate helpers (Koenig & Dickinson, 2016; Komdeur et al, 2017; Solomon & French, 1997; Stacey & Koenig, 1990). When the dominants compensate for the care provided by helpers by reduc‐ ing their amount of care, the total amount of care received by the offspring may remain similar Such “load lightening” by helpers can reduce the costs of reproduction for the dominants (Bruintjes, Heg‐ Bachar, & Heg, 2013; Dixit, English, & Lukas, 2017; Heinsohn, 2004; Koenig & Walters, 2011; Meade, Nam, Beckerman, & Hatchwell, 2010; Scantlebury, Russell, McIlrath, Speakman, & Clutton‐Brock, 2002; Sharp, English, & Clutton‐Brock, 2013), which can lead to increased dominant survival (Cockburn et al, 2008; Hatchwell & Russell, 1996b; Heinsohn, 1992; Khan & Walters, 2002; Kingma et al, 2010) and increased future reproductive success (Brown & Brown, 1981; Russell, Brotherton, McIlrath, Sharpe, & Clutton‐Brock, 2003; Woxvold & Magrath, 2005; Blackmore & Heinsohn, 2007; but see Meade et al, 2010). We replicate this study using a much larger dataset, and, for the first time in this species, disentangle the impact of help from the effects of group size (including helpers and nonhelpers) and food availability
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