Better to Divorce than Be Widowed: The Role of Mortality and Environmental Heterogeneity in the Evolution of Divorce.

AbstractDespite widespread interest in the evolution and implications of monogamy across taxa, less attention-especially theoretical-has been paid toward understanding the evolution of divorce (ending a socially monogamous pairing to find a new partner). Here, we develop a model of the evolution of divorce by females in a heterogeneous environment, where females assess territory quality as a result of their breeding success. Divorce results in females leaving poor territories disproportionally more often than good territories, while death of a partner occurs independent of territory quality, giving an advantage to divorce. Increasing environmental heterogeneity, a decreasing benefit of pair experience, and moderate survival rates favor the evolution of higher divorce rates, even in the absence of variance in individual quality and knowledge of available territories. Imperfect information about territory quality constrains the evolution of divorce, typically favoring divorce strategies that remain faithful to one's partner whenever successful reproduction occurs. Our model shows how feedbacks between divorce, widowhood, and the availability of territories are intricately linked in determining the evolutionary advantage of divorce. We detail testable predictions about populations that should be expected to divorce at high rates.

Mate fidelity improves survival and breeding propensity of a long-lived bird.

Evolutionary and behavioural ecologists have long been interested in factors shaping the variation in mating behaviour observed in nature. Although much of the research on this topic has focused on the consequences of mate choice and mate change on annual reproductive success, studies of a potential positive link between mate fidelity and adult demographic rates have been comparatively rare. This is particularly true for long-lived birds with multi-year, socially monogamous pair bonds. We used a 26-year capture-mark-recapture dataset of 3,330 black brent Branta bernicla nigricans to test whether breeding with a familiar mate improved future breeding propensity and survival. We predicted that experienced breeders nesting with a new partner would have rates of survival similar to familiar pairs because long-lived species avoid jeopardizing survival since their lifetime fitness is sensitive to this vital rate. In contrast, we expected that any costs of breeding with a new partner would be paid through skipping the subsequent breeding attempt. We found that unfamiliar pairs had lower subsequent breeding propensity than faithful partners. However, contrary to our expectations, individuals breeding with a new mate also suffered reduced survival. These results add to a small number of studies indicating that a positive relationship between mate retention and adult demographic rates may exist in a diverse array of avian species. Given these results, researchers should consider costs of mate change that extend beyond within-season reproductive success to fully understand the potential adaptive basis for perennial social monogamy. We caution that if mate retention enhances survival prospects, improvements in annual reproductive success with pair-bond length could be a secondary factor favouring perennial social monogamy, particularly in species with slower life-history strategies. Furthermore, some cases where annual reproductive success does not improve with pair-bond duration, yet multi-year pair bonds are common, could be explained by benefits afforded by mate fidelity to adult vital rates.

Site fidelity facilitates pair formation in aggregations of coral reef cardinalfish.

Colonial animals often form stable pair bonds, returning to the same site to breed with the same partner every year. Familiarity with both partner and breeding site has the potential to enhance an individual's reproductive success. However, it is often unknown whether the mating system arises because of site fidelity, mate fidelity or both. Here, observational and experimental studies are used to identify causal links between site fidelity and pair formation in a group-living coral reef cardinalfish, Sphaeramia nematoptera. A long-term field tagging study was undertaken to quantify site and mate attachment. This was followed by both mate removal and mate transplant experiments to test whether the prolonged association with home sites was primarily because of mate or site fidelity. Adult S. nematoptera exhibited a prolonged association with home sites and partners, with some pairs lasting more than 4 months at the same site. A Bayesian mixed effect model showed that individuals in pairs were more likely to remain site attached, regardless of sex and maturity. Following mate removal, 78% of S. nematoptera found a new partner within 2 weeks on the same site, supporting the hypothesis that individuals primarily exhibit site fidelity. This was confirmed by the partner translocation experiment, with only 1 of 24 fish following their translocated partner to a new site. In these cardinalfish, strong site attachment facilitates long-lasting pair bonds, as well as new pair formation when necessary, suggesting that site rather than mate fidelity is the major driver of the reproductive system.

Male fidelity expressed through rapid testosterone suppression of ultrasonic vocalizations to novel females in the monogamous California mouse.

The steroid hormone testosterone (T) is a well-known mediator of male sexual behavior in vertebrates. However, less is known about T's rapid effects on sexual behavior, particularly those involving ultrasonic vocalizations (USVs), a mode of communication that can influence mate acquisition in rodents. Using the monogamous California mouse, Peromyscus californicus, we tested whether T rapidly alters male USV production by giving T or saline injections to non-paired (sexually naïve) males and paired (paternally experienced and pair-bonded) males immediately prior to a brief exposure to an unrelated, novel female. Among non-paired males, no differences in the total number of USVs were observed; however, T increased the proportion of simple sweeps produced. Among paired males, T decreased the number of USVs produced, and this change was driven by a reduction in simple sweeps. These results suggest a differential rapid effect of T pulses between non-paired and paired males upon exposure to a novel female. Additionally, we observed a positive correlation in the production of USVs made between males and novel females, and this relationship was altered by T. Given the importance of USVs in sexual communication, our study supports an essential concept of monogamy in that mate fidelity is reinforced by decreased responsiveness to prospective mates outside of the pair bond. The central mechanism in pair bonded males that decreases their responsiveness to novel females appears to be one that T can trigger. This is among the first studies to demonstrate that T can inhibit sexually related behaviors and do so rapidly.

Breeding populations of Marbled Godwits and Willets have high annual survival and strong site fidelity to managed wetlands.

The Prairie Pothole Region of central Canada supports a diverse community of breeding waterbirds, but many species have declining populations and the demographic mechanisms driving the declines remain unknown. We conducted a 7-year field study during 1995-2001 to investigate the demographic performance of Marbled Godwits (Limosa fedoa) and Willets (Tringa semipalmata) breeding in managed wetlands near Brooks, Alberta. Mark-recapture analyses based on Cormack-Jolly-Seber models revealed that the annual rates of apparent survival for Marbled Godwits ( ϕ ^  = 0.953 ± 0.012SE) and Willets ( ϕ ^  = 0.861 ± 0.015SE) are among the highest rates of survivorship reported for any breeding or nonbreeding population of large-bodied shorebirds. Our estimates of life expectancy for males were comparable to longevity records in godwits (17.3 years ±5.8SE vs. 25-29+ years) and willets (7.7 ± 1.5SE vs. 10+ years). The two species both showed strong breeding site fidelity but differed in rates of mate fidelity. Pairs that reunited and males that switched mates usually nested <300 m from their previous nests, whereas females that switched mates usually moved longer distances >1.1-1.5 km. Returning pairs usually reunited in godwits (85%) but not in willets (28%), possibly because of species differences in adult survival or patterns of migration. Baseline estimates of annual survival for banded-only birds will be useful for evaluating the potential effects of new tracking tags or the environmental changes that have occurred during the past 20 years. Conservation strategies for large-bodied shorebirds should be focused on reduction of exposure to anthropogenic mortality because low rates of natural mortality suggest that losses to collisions at breeding sites or harvest at nonbreeding areas are likely to cause additive mortality.

Breeding site fidelity is lower in polygamous shorebirds and male-biased in monogamous species.

Sex-bias in breeding dispersal is considered the norm in many taxa, and the magnitude and direction of such sex-bias is expected to correlate with the social mating system. We used local return rates in shorebirds as an index of breeding site fidelity, and hence as an estimate of the propensity for breeding dispersal, and tested whether variation in site fidelity and in sex-bias in site fidelity relates to the mating system. Among 111 populations of 49 species, annual return rates to a breeding site varied between 0% and 100%. After controlling for body size (linked to survival) and other confounding factors, monogamous species showed higher breeding site fidelity compared with polyandrous and polygynous species. Overall, there was a strong male bias in return rates, but the sex-bias in return rate was independent of the mating system and did not covary with the extent of sexual size dimorphism. Our results bolster earlier findings that the sex-biased dispersal is weakly linked to the mating system in birds. Instead, our results show that return rates are strongly correlated with the mating system in shorebirds regardless of sex. This suggests that breeding site fidelity may be linked to mate fidelity, which is only important in the monogamous, biparentally incubating species, or that the same drivers influence both the mating system and site fidelity. The strong connection between site fidelity and the mating system suggests that variation in site fidelity may have played a role in the coevolution of the mating system, parental care, and migration strategies.

Convict cichlid parents that stay with the same mate develop unique and consistent divisions of roles.

Previous studies, largely on avian species, have suggested that pairs that are permanently monogamous and have biparental care develop a coordination over time that enhances offspring survival. If this is the case, we predicted that a parent involved in biparental care would develop a pattern of biparental care specific to a particular mate and remain consistent in that pattern over time but would lose this pattern if it were to change mates. We tested this prediction with the convict cichlid fish (Amatitlania nigrofasciata) which has biparental care that is both complex and flexible. In this species, each parent can perform all parental roles but typically shows a division of labor in which males typically defend against offspring predators while the female typically provides direct care to the offspring. At various times, the parents briefly switch roles. Our experiments revealed that pairs that remained together for two consecutive broods were more consistent in their parental behaviors, including time they spent near the intruder and in the nest compared to pairs that were comprised of individuals that had previously mated with other partners. Also individuals that remained with the same partner were also more consistent as a parental unit, maintaining their sex-specific roles of males defending aggressively against an intruder and females spending more time directly caring for young. While our experiment clearly support our prediction that individuals do develop unique coordination with specific individuals, convict cichlids in nature appear to be largely serially monogamous in which they mate only once before changing partners. Thus, it is likely this coordination may be available in many species that have biparental care but become adaptive when repeated matings become common.

Mate fidelity in a polygamous shorebird, the snowy plover (Charadrius nivosus).

Social monogamy has evolved multiple times and is particularly common in birds. However, it is not well understood why some species live in long-lasting monogamous partnerships while others change mates between breeding attempts. Here, we investigate mate fidelity in a sequential polygamous shorebird, the snowy plover (Charadrius nivosus), a species in which both males and females may have several breeding attempts within a breeding season with the same or different mates. Using 6 years of data from a well-monitored population in Bahía de Ceuta, Mexico, we investigated predictors and fitness implications of mate fidelity both within and between years. We show that in order to maximize reproductive success within a season, individuals divorce after successful nesting and re-mate with the same partner after nest failure. Therefore, divorced plovers, counterintuitively, achieve higher reproductive success than individuals that retain their mate. We also show that different mating decisions between sexes predict different breeding dispersal patterns. Taken together, our findings imply that divorce is an adaptive strategy to improve reproductive success in a stochastic environment. Understanding mate fidelity is important for the evolution of monogamy and polygamy, and these mating behaviors have implications for reproductive success and population productivity.

The role of extragroup encounters in a Neotropical, cooperative breeding primate, the common marmoset: a field playback experiment.

In cooperatively breeding species, encounters with intruders may serve multiple functions ranging from reaffirming group territory ranges to facilitating assessments for additional breeding opportunities. While these distinctive events offer the opportunity to investigate the delicate balance of these social dimensions within animal societies, their unpredictable occurrence makes witnessing and controlling these events in the wild particularly challenging. Here we used a field playback approach to simulate conspecific territorial incursions in cooperatively breeding common marmosets (Callithrix jacchus) to distinguish between the three following non-mutually exclusive functions of intergroup encounters in this species of New World primate: territorial defense, mate defense, and assessment of breeding opportunities. For these experiments, we systematically broadcast species-typical long-distance contact calls - phees - commonly used in intergroup interactions from the core and periphery of the groups' territories using either male or female vocalizations. Consistent with a territorial defense hypothesis, a group's reaction was independent of the simulated intruder's sex and the response strength was greater when the playback stimulus was broadcast from the core areas of groups' territories relative to stimulus broadcast from periphery areas. However, sex differences in some facets of their responses suggest that this is not the only potential function for these encounters. Mated males and females started to move first in response to simulated intruders of the opposite sex, suggesting that these events offered opportunities to assess extra-pair breeding opportunities, while the occurrence of females' piloerection towards simulated female intruders is suggestive of mate-guarding. These data provide unique experimental evidence for the theory that excursions by conspecific intruders may serve multiple functions in a cooperatively breeding vertebrate and are reflective of the known complexities of common marmoset sociobiology.