Capsule: Early male arrival on the breeding grounds results in early pairing but not early nesting in Chiffchaffs Phylloscopus collybita, and Chiffchaffs can nest, fail, and re-nest before Willow Warblers Phylloscopus trochilus begin nesting. Aims: To quantify the consequences of timing of arrival for the subsequent timing of pairing, nesting, and re-nesting of short-distance (Chiffchaff) and long-distance (Willow Warbler) migrants. Methods: The arrival dates of 118 Chiffchaffs and 20 Willow Warblers were measured from March to June over 10 weeks in Foxley Wood nature reserve, Norfolk. Colour-ringing of 56 Chiffchaffs (55 males, 1 female) and 11 Willow Warblers (10 males, 1 female) was used to relate individual arrival dates to timing of male pairing, clutch initiation, and re-nesting. Results: Male Chiffchaffs started to arrive in early March and increased rapidly in number until early April, while the arrival of male Willow Warblers began in early April. Early-arriving male Chiffchaffs paired earlier than later-arriving individuals, but timing of clutch initiation was unrelated to male arrival dates. Early nesting by Chiffchaffs allowed replacement clutches following nest loss, the earliest of which occurred 12 days after the first Willow Warbler male had paired. Conclusions: Although early arrival in male Chiffchaffs does not translate into earlier nesting, timing of nesting in Chiffchaffs was sufficiently early to allow time for replacement clutches following nest loss. The later arrival of Willow Warblers is likely to mean fewer opportunities for replacement clutches following nest loss. This difference in breeding phenology could therefore contribute to differences in productivity between the species, especially if nest failure rates are high.

The standard evolutionary theory of ageing predicts a negative relationship (trade-off) between fecundity and longevity. However, this relationship can become positive: (i) under the influence of longevity-enhancing mutations; (ii) when individuals have unequal resources; and (iii) in eusocial insects, in which reproductive queens outlive less- or non-reproductive workers. Developmental diet is likely to be central to determining trade-offs as it affects key fitness traits such as adult body size, but its exact role remains uncertain. For example, in Drosophila melanogaster fruit flies, changes in adult diet can affect fecundity, longevity, and gene expression throughout life, but it is unknown how changes in developmental (larval) diet affect fecundity-longevity relationships or gene expression in adults. Using D. melanogaster, we therefore tested the hypothesis that variation in developmental diet alters the directionality of fecundity-longevity relationships in adults, and characterised associated gene expression changes. We reared D. melanogaster larvae on low (20%), medium (100%), and high (120%) SYA (Sugar Yeast Agar) diets, and transferred adult females developing from these larvae to a common (110% SYA) adult diet. We measured life-time fertility (realised fecundity) and longevity of individual adult females and, using mRNA-seq, profiled gene expression changes across two time-points. Adult females raised on the different larval diets exhibited fecundity-longevity relationships that were significantly different in directionality, i.e., varied from positive to negative, despite minimal differences in mean life-time fertility or longevity. Treatments also differed in age-related gene expression, including expression of genes known to be associated with ageing. Hence, this study shows that the sign of fecundity-longevity relationships in adult insects can be altered and even reversed by variation in larval diet quality. Furthermore, larval diet quality may be a key mechanistic factor underpinning positive fecundity-longevity relationships observed in species such as eusocial insects.

Background The standard evolutionary theory of ageing proposes that ageing occurs because of a trade-off between reproduction and longevity. Eusocial insect queens exhibit positive fecundity-longevity associations and so have been suggested to be counter-examples through not expressing costs of reproduction and through remodelling conserved genetic and endocrine networks regulating ageing and reproduction. If so, eusocial evolution from solitary ancestors with negative fecundity-longevity associations must have involved a stage at which costs of reproduction were suppressed and fecundity and longevity became positively associated. Using the bumblebee (Bombus terrestris), we experimentally tested whether queens in annual eusocial insects at an intermediate level of eusocial complexity experience costs of reproduction, and, using mRNA-seq, the extent to which they exhibit a remodelling of relevant genetic and endocrine networks. Specifically, we tested whether costs of reproduction are present but latent, or whether a remodelling of relevant genetic and endocrine networks has already occurred allowing queens to reproduce without costs. Results We experimentally increased queens’ costs of reproduction by removing their eggs, which caused queens to increase their egg-laying rate. Treatment queens had significantly reduced longevity relative to control queens whose egg-laying rate was not increased. Reduced longevity in treatment queens was not caused by increased worker-to-queen aggression or by increased overall activity in queens. In addition, treatment and control queens differed in age-related gene expression based on mRNA-seq in both their overall expression profiles and the expression of ageing-related genes. Remarkably, these differences appeared to occur principally with respect to relative age, not chronological age. Conclusions This study represents the first simultaneously phenotypic and transcriptomic experimental test for a longevity cost of reproduction in eusocial insect queens. The results support the occurrence of costs of reproduction in annual eusocial insects of intermediate social complexity and suggest that reproductive costs are present but latent in queens of such species, i.e. that these queens exhibit condition-dependent positive fecundity-longevity associations. They also raise the possibility that a partial remodelling of genetic and endocrine networks underpinning ageing may have occurred in intermediately eusocial species such that, in unmanipulated conditions, age-related gene expression depends more on chronological than relative age.