Long‐term research and hierarchical models reveal consistent fitness costs of being the last egg in a clutch

Abstract

Maintenance of phenotypic heterogeneity in the face of strong selection is an important component of evolutionary ecology, as are the consequences of such heterogeneity. Organisms may experience diminishing returns of increased reproductive allocation as clutch or litter size increases, affecting current and residual reproductive success. Given existing uncertainty regarding trade‐offs between the quantity and quality of offspring, we sought to examine the potential for diminishing returns on increased reproductive allocation in a long‐lived species of goose, with a particular emphasis on the effect of position in the laying sequence on offspring quality. To better understand the effects of maternal allocation on offspring survival and growth, we estimated the effects of egg size, timing of breeding, inter‐ and intra‐annual variation, and position in the laying sequence on gosling survival and growth rates of black brant Branta bernicla nigricans breeding in western Alaska from 1987 to 2007. We found that gosling growth rates and survival decreased with position in the laying sequence, regardless of clutch size. Mean egg volume of the clutch a gosling originated from had a positive effect on gosling survival ( β = 0.095, 95% CRI: 0.024, 0.165) and gosling growth rates ( β = 0.626, 95% CRI: 0.469, 0.738). Gosling survival ( β = −0.146, 95% CRI: −0.214, −0.079) and growth rates ( β = −1.286, 95% CRI: −1.435, −1.132) were negatively related to hatching date. These findings indicate substantial heterogeneity in offspring quality associated with their position in the laying sequence. They also potentially suggest a trade‐off mechanism for females whose total reproductive investment is governed by pre‐breeding state.

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

“…Finally, an overlooked issue is whether helpers’ presence affects how mothers distribute resources within clutches. Eggs’ fitness value can vary with laying order and latter-laid eggs commonly have lower survival chances (Nager et al 2000 ; Acevedo et al 2020 ). In addition, variability in egg size and contents across the laying sequence has been well demonstrated (Slagsvold et al 1984 ; Schwabl 1993 ; Kozlowski and Ricklefs 2010 ).…”

Individual heterogeneity in fitness in a long‐lived herbivore

“…Hatch date plays a prominent role in such variation because early hatching goslings grow more rapidly (Cooch et al, 1991;Lindholm et al, 1994;Sedinger & Flint, 1991), resulting in larger adult body sizes (Larsson & Forslund, 1991). Further, the size of eggs produced by female geese explains some variation in gosling size at fledging (Acevedo et al, 2020). While much of the relevant variation appears heritable (Larsson & Forslund, 1992), the general lack of response to apparently strong selection for larger body sizes (Riecke, 2020) and associated reproductive variables suggests that either such heritability does not have a substantial additive genetic basis, or there are unknown associated negative genetic covariances (Hoffmann & Merilä, 1999;Larsson, 1993;Price & Liou, 1989).…”

“…Over the past three decades, clutch size for brant has remained relatively constant, indicating that clutch size has not responded to selection (Figure 4). We propose that an increase in clutch size would require an increase in adult body size to reduce the effects of a trade-off between number and quality of offspring that exists currently (Acevedo et al, 2020). Differences in environmental conditions during growth largely determine heterogeneity in body size and individual quality which exists within (Riecke et al, 2018;Sedinger & Chelgren, 2007) and among (Lohman et al, 2019) cohorts.…”

“…Recently, experimental manipulations of clutch and brood size in black brant (Branta bernicla nigricans, hereafter brant), which have precocial young, demonstrated diminishing fitness returns as brood size increased (Sedinger et al, 2017) and costs to future reproduction of producing broods larger than the most common brood size of four (Leach et al, 2019). Additionally, egg size has a positive association with the size of goslings at fledging (Acevedo et al, 2020), which strongly influences first-year survival (Sedinger & Chelgren, 2007) and recruitment into the breeding population (Riecke et al, 2018;Sedinger et al, 2004). When nutrient reserves are limited, variation in egg size among females (Flint & Sedinger, 1992;Lemons et al, 2011) results from individual trade-offs between the quantity and quality of offspring produced (Williams, 2001).…”

Estimating survival and adoption rates of dependent juveniles

“…Chick survival may also be higher in years with more cumulative winter precipitation because the herbaceous understory responds positively to more moisture, and sage‐grouse rely on grasses and forbs for energetic demands as well as for cover (Gibson et al, 2017 ; Wann et al, 2020 ). We suggest that, as a result of the relative lack of attention to this vital rate, the role of pre‐fledging survival in population dynamics and individual fitness has been under‐appreciated in sage‐grouse (Dahlgren et al, 2016 ; Taylor et al, 2012 ) and other species with precocial young (Acevedo et al, 2020 ; Cooch et al, 2001 ).…”