Divorce in the socially monogamous zebra finch: Hormonal mechanisms and reproductive consequences

Benefits of Pair-Bond Duration on Reproduction in a Lifelong Monogamous Cooperative Passerine

AbstractLong-term social and genetic monogamy is rare in animals except birds, but even in birds it is infrequent and poorly understood. We investigated possible advantages of monogamy in a colonial, facultative cooperatively breeding bird from an arid, unpredictable environment, the sociable weaver (Philetairus socius). We documented divorce and extrapair paternity of 703 pairs over 10 years and separated effects of pair duration from breeding experience by analyzing longitudinal and cross-sectional datasets. Parts of the colonies were protected from nest predation, thereby limiting its stochastic and thus confounding effect on fitness measures. We found that 6.4% of sociable weaver pairs divorced and 2.2% of young were extrapair. Longer pair-bonds were associated with more clutches and fledglings per season and with reproducing earlier and later in the season, when snake predation is lower, but not with increased egg or fledgling mass or with nestling survival. Finally, the number of helpers at the nest increased with pair-bond duration. Results were similar for protected and unprotected nests. We suggest that long-term monogamy is associated with a better capacity for exploiting a temporally unpredictable environment and helps to form larger groups. These results can contribute to our understanding of why long-term monogamy is frequently associated with unpredictable environments and cooperation.

Longitudinal covariation of testosterone and sperm quality across reproductive stages in the zebra finch

Birds that breed opportunistically maintain partial activation of reproductive systems to rapidly exploit environmental conditions when they become suitable for breeding. Maintaining reproductive systems outside of a breeding context is costly. For males, these costs are thought to include continual exposure to testosterone. Males of seasonally breeding birds minimise these costs by downregulating testosterone production outside of a breeding context. Opportunistically breeding birds trade off the need to rapidly initiate reproduction with the costs of elevated testosterone production. One way opportunistically breeding males could minimise these costs is through fine scale changes in testosterone production across discrete reproductive stages which have a greater or lesser requirement for active sperm production. Although spermatogenesis broadly depends on testosterone production, whether changes in testosterone levels across the reproductive stages affect sperm quality and production is unknown. Here, we measured testosterone, sperm quality, and body condition in male zebra finches at discrete stages within reproductive bouts (egg laying, incubation, nestling provisioning, and fledging) and across two consecutive reproductive events in captive male zebra finches (Taeniopygia castanotis). We also examined associations between male testosterone, sperm quality/production, body condition, and nestling body condition. We found that testosterone levels varied across discrete reproductive stages with the lowest levels during incubation and the highest following chick fledging. Testosterone levels were positively associated with sperm velocity and the proportion of motile sperm but were not associated with male body condition. We found no associations between paternal body condition, testosterone levels, or sperm traits with nestling body condition (a proxy for the reproductive quality of a male and his partner). This study is the first to show that opportunistically breeding males vary testosterone synthesis and sperm traits at discrete stages within a reproductive event.

Differential effects of continuous and intermittent daytime food deprivation periods on metabolism and reproductive performance in diurnal zebra finches

We investigated whether food availability effects on metabolism and reproduction are the result of the sum effect of daily feeding (food availability) and starvation (food deprivation) periods. Adult zebra finches were paired and subjected to a time-restricted feeding (TRF) regimen consisting of continuous and intermittent daytime food deprivation periods. Birds were given food during the 12-h day for a total of 4-h in the evening (1 *4-h, hour 8-12), or in 2 splits of 2 h each (2 * 2-h) or 4 splits 1 h each (4 * 1-h), with controls on food ad libitum, until they had the first egg clutch. TRF caused significant changes in hepatic expression of metabolism-associated sirt1, egr1, pparα and foxo1 genes despite no difference in the food intake, body mass and blood glucose levels. Importantly, TRF resulted in a significant reduction in plasma testosterone and estradiol levels, delayed nest-building and egg laying, and reduced clutch size. Concurrently, under TRF regimes, we found a significantly lower expression of th and mtr genes linked with motivation and affiliation (but not of dio2, dio3, gnrh1 and gnih genes linked with gonadal maturation) in the hypothalamus, and of star and hook 1 genes in the testes and star, cyp19 and erα genes in the ovary. These results demonstrate the importance of daily food deprivation times on the metabolism and reproduction, and suggest a possible provisioning of energy available from daily feeding for the maintenance of body condition at the expense of reproduction performance in diurnal animals.

No evidence of immediate fitness benefits of within-season divorce in monogamous birds

Individuals of socially monogamous species can correct for suboptimal partnerships via two secondary mating strategies: divorce and extra-pair mating, with the former potentially providing both genetic and social benefits. Divorcing between breeding seasons has been shown to be generally adaptive behaviour across monogamous birds. Interestingly, some pairs also divorce during the breeding season, when constraints on finding a new partner are stronger. Despite being important for a comprehensive understanding of the evolution of social monogamy, whether within-season divorce is adaptive and how it relates to extra-pair mating remains unknown. Here, we meta-analysed 90 effect sizes on within-season divorce and breeding success, extracted from 31 studies on 24 species. We found no evidence that within-season divorce is adaptive for breeding success. However, the large heterogeneity of effect sizes and strong phylogenetic signal suggest social and environmental factors—which have rarely been considered in empirical studies—may play an important role in explaining variation among populations and species. Furthermore, we found no evidence that within-season divorce and extra-pair mating are complementary strategies. We discuss our findings within the current evidence of the adaptiveness of secondary mating strategies and their interplay that ultimately shapes the evolution of social monogamy.

Serial monogamy benefits both sexes in the biparental convict cichlid

Monogamy can be either long-term or serial, with new pairs formed with each breeding bout. Costs and benefits are associated with each strategy. Because biparental convict cichlids (Amatitlania nigrofasciata) typically switch mates, exhibiting serial monogamy, we tested for the costs associated with forcing individuals to remain with the same mate. Convict cichlids were observed over two successive breeding bouts, either with the same or a new, equally experienced, mate. Parental behavior did not differ between breeding bouts, nor did brood size. Surprisingly, fish that remained with their original partner for a second bout took significantly longer to produce a brood compared to fish that paired with new partners. New partners were also more likely to successfully produce a second brood than re-mated partners. This is in contrast to the majority of bird studies that show many benefits to staying with the same partner for multiple broods. In convict cichlids, there seems to be no benefit associated with remaining with the same partner and switching mates reduces duration between broods for both males and females, potentially increasing overall reproductive success.

Acute exogenous corticosterone treatments have few effects on courtship and pair bonding in zebra finches

Stress impacts social relationships. In turn, social relationships buffer the stress response in some species. Studies that have investigated the role of corticosterone (CORT) on courtship, mate choice, mating, and pairing have found mixed results. We therefore tested the role of CORT in these steps of the pairing process in the monogamous zebra finch. Male and female zebra finches received either one of 2 doses of corticosterone (CORT, 10 μg and 20 μg, referred to as low and high dose) or a vehicle control (peanut oil). Subjects were then given the opportunity to pair in mixed sex aviaries. Courtship and pair bonding behaviors were observed over 3 days. Overall, zebra finches of both sexes were equally likely to pair or not pair regardless of treatment, although a high dose of CORT increased the latency to form a pair bond. There were no effects of CORT on courtship behavior in either sex, though the low dose increased undirected (non-courtship) singing in males relative to the high dose. Animals treated with CORT, regardless of dose, engaged in fewer copulations than did control animals. When we examined pairing behaviors, we found a decrease in co-nesting in low dose animals. Our results suggest that acute CORT has few effects on pair bonding, suggesting species-specific effects of CORT on behavior.