Egg components and offspring survival vary with group size and laying order in a cooperative breeder

Abstract

Prenatal resource allocation to offspring can be influenced by maternal environment and offspring value, and affect offspring survival. An important pathway for flexible maternal allocation is via egg components such as nutrients and hormones. In cooperative breeders, females with helpers may increase resource allocation to eggs—‘differential allocation’—or reduce it—‘load-lightening’. Yet, helper effects on egg composition have been poorly studied. Moreover, it is unknown how helpers’ presence modulates laying order effects on egg content and survival. Here, we investigated how maternal allocation varied with group size and laying order in the cooperatively breeding sociable weaver (Philetairus socius). We estimated interactive effects of helpers and laying order on allocation to egg mass, yolk nutrients—yolk mass, proteins, lipids, carotenoids, vitamin A and vitamin E—and hormones—testosterone, androstenedione, and corticosterone. Results concurred with the ‘differential allocation’ predictions. Females with more helpers produced later-laid eggs with heavier yolks and more lipids, and laid eggs overall richer in lipids. Proteins, antioxidants, and hormones were not found to vary with helper number. We then analyzed how helper number modulated laying order effects on survival. Females with more helpers did not specifically produce later-laid eggs with higher survival, but eggs laid by females with more helpers were overall more likely to fledge. These findings show that some egg components (yolk mass, lipids) can positively vary according to females’ breeding group size, which may improve offspring fitness.

Interplay of cooperative breeding and predation risk on egg allocation and reproductive output

“…Instead, we found that the predator treatment similarly affected females regardless of their group size, which suggests that females with more helpers may not be at an advantage particularly under high predation risk environments. This corroborates previous results in this species suggesting lack of helper effects on egg mass and clutch size, either in general or in interaction with climatic conditions and nest predation risk ( Fortuna et al 2021 ; but see helper effects on egg content in Fortuna et al 2023 ). In sociable weavers, it has never been investigated whether predation risk decreases in larger breeding groups, and this would help clarify whether group size confers protection or other advantages to individuals before and during rearing ( Sorato et al 2012 ).…”

“…Hence, we cannot determine whether this adjustment was similar for the remaining eggs or exclusive to later-laid ones, and previous investigations in this population suggest only ca. 20–25% repeatability in yolk mass within clutches ( Fortuna et al 2023 ). Nevertheless, a decrease in yolk mass—generally or specifically in later-laid eggs—might represent a reduced female investment in reproduction, which may lead to brood reduction.…”

“…For the models analyzing clutch size, yolk mass, and yolk corticosterone, there was only one value per clutch; hence, we only included clutch identity and colony identity as random terms. Additionally, in the yolk mass model, we accounted for egg mass (mean ± SD = 2.57 ± 0.21) as a predictor of yolk mass but also tested if results were the same when removing this variable (i.e., absolute changes in yolk mass; Paquet et al 2013 ; Fortuna et al 2023 ). Similarly, effects on corticosterone concentration were tested when including egg mass and yolk mass (mean ± SD = 0.51 ± 0.06) in the model, thus testing for an absolute change in corticosterone levels.…”

Mothers in a cooperatively breeding bird increase investment per offspring at the pre-natal stage when they will have more help with post-natal care

“…While we cannot formally assess the relative importance for offspring fitness of this potentially "indirect" helper effect on maternal pre-natal investment and the direct causal positive effect of helpers on post-natal provisioning (demonstrated in [35]), the direct post-natal helper effect may still dominate, as the within-mother effect of female helper number on egg volume is modest compared to the within-mother effect of female helper number on the overall rates of post-natal provisioning (Fig 1 in [35]). That said, the effect size for the change in egg volume could underestimate the fitness consequences for offspring of the maternal pre-natal response to helpers, as any accompanying helper-induced changes in egg composition [7,40] could yield fitness effects that differ in magnitude from the observed change in egg volume. Second, while helper-induced reductions in maternal post-natal workloads are typically thought to benefit mothers (e.g., by improving maternal survival; [5,7]), our findings highlight that associated changes in egg investment could pass these benefits, in part or whole, to the offspring being reared.…”

“…We also confirm that our findings are not complicated by parallel maternal plasticity in clutch size according to helper numbers, by verifying that within-mother variation in female and male helper numbers does not predict clutch size. As egg traits often vary across the laying sequence [38,39] and associations between helper numbers and egg composition have also been found to vary across the laying sequence [40], our analysis of egg volume also controls for effects of egg position within the laying sequence and allows for interactive effects of helper numbers and egg position. We then investigate whether within-mother variation in female and male helper numbers predict variation in the mother's nestling feeding rate (again utilizing a large longitudinal data set; 124 broods being fed by 50 mothers in 34 social groups; 1 to 7 broods [median = 2] per mother).…”