Strong Pair Bonds Research Articles

Mutual mate guarding with limited sexual conflict in a sex-role-reversed shorebird.

Mate guarding is typically considered a male strategy to protect paternity. However, under some circumstances, females might also benefit from guarding their mate. Female mate guarding might be particularly important in socially polyandrous species in which females compete for access to care-giving males. Because males also benefit from being near their partner to avoid paternity loss, pair members may have a mutual interest in mate guarding in polyandrous species. We studied the time spent together and movements that lead to separation, as behavioral measures of mate guarding, in the classically polyandrous red phalarope (Phalaropus fulicarius). We equipped 64 breeding pairs with miniaturized telemetry loggers with GPS to assess variation in mate-guarding intensity in relation to breeding phenology and season, nest attendance, and the occurrence of extrapair paternity. We show that red phalarope pairs were almost continuously together in the days before clutch initiation with no sex bias in separation movements, indicating mutual contribution to mate guarding. Our results suggest that in red phalaropes, both pair members guard their mate, with limited sexual conflict arising through biases in the operational sex ratio and a trade-off with male nest attendance. We found no clear relationship between mate-guarding intensity and the occurrence of extrapair paternity. In this non-territorial socially polyandrous species, mutual mate guarding might be the process underlying the evolution of a brief but strong social pair bond, with no other purpose than producing a clutch for a care-giving male.

Dear territory or dear partner? Causes and consequences of breeding dispersal in a highly territorial bird of prey with a strong pair bond

Territorial species are unlikely to show extensive movements between breeding seasons. This is especially true for long-lived species, which often have strong pair bonding and can occupy the same territory for years. However, also in such species, individuals may face situations that can lead to a territory shift. Here, we use a comprehensive dataset documenting 40 years of breeding behavior in tawny owl (Strix aluco) – a long-lived species with high site tenacity and mate fidelity – to examine the factors affecting the decisions whether or not to move to another breeding territory and how far, as well as the fitness consequences thereof. We found that the likelihood and distance of movement in either sexes is strongly associated with a change of partner, indicating that mate loss may cause breeding dispersal. Moreover, mate change, not movement to a new territory, had negative effects on subsequent reproductive performance: individuals that changed partner were more likely to skip reproduction in the subsequent year and, in those cases they bred, they produced smaller clutches and raised fewer offspring. Our findings indicate that tawny owls change territory almost exclusively when searching for a new partner and that mate change has profound consequences on their subsequent breeding performance. Overall, our study provides evidence that in tawny owls territoriality and monogamy are associated and strongly linked to fitness, but mate fidelity may be more important than site fidelity, likely because sexes are involved in specific tasks and their cooperation ensures breeding success and, consequently, increases fitness.Significance statementBreeding dispersal, the movement of individuals between breeding sites, can entail high costs for animal fitness, especially for territorial species, which display strong site fidelity. We studied the factors associated with breeding dispersal and the consequences on breeding performances in tawny owl (Strix aluco), a highly territorial species. We found that tawny owls moved more frequently to another breeding territory when the mate died. Either sexes showed an equal probability to move, but the effect was stronger in females than in males after a mate change. Moreover, owls that changed partner showed delayed reproduction, smaller clutch and a higher probability to skip reproduction. Our findings show that in tawny owls territoriality and monogamy are associated and strongly linked to fitness, but mate fidelity may be more important than site fidelity, likely because sexes share the costs of holding the territory.

Clomiphene citrate treatment during perinatal development alters adult partner preference, mating behaviour and androgen receptor and vasopressin in the male mandarin vole Microtus mandarinus.

During development, many aspects of behaviour, including partner preferences and sexual behaviour, are "organized" by neural aromatization of androgen and oestrogen. This study aimed to analyse these processes in the mandarin vole (Microtus mandarinus); this is a novel mammalian model exhibiting strong monogamous pair bonds. Male pups were treated daily with a sesame oil only (MC) or the oestrogen receptor blocker-clomiphene citrate sesame oil mixture (MT) from prenatal day 14 to postnatal day 10. Female pups were treated daily with sesame oil only (FC). Partner preferences, sexual behaviour, and the expression of androgen receptor (AR) and arginine vasopressin (AVP) were examined when animals were 3months old. The MT and FC groups exhibited male-directed partner preferences and feminized behaviour. AR-immunoreactive neurons (AR-IRs) in the medial preoptic area (mPOA), bed nucleus of stria terminalis (BNST), and medial amygdaloid nucleus (MeA) were reduced in MT males compared to MC males, and there was no significant difference in the number of AR-IRs between MT males and FC females. AVP-immunoreactive neurons (AVP-IRs) in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) were reduced in MT males compared to MC males, and there were no significant differences in the number of AVP-IRs between MT males and FC females. The results indicate a significant role of hormone signalling in the development of male mate preference in the novel monogamous mammal model.

Mesoscopic model confirms strong base pair metal mediated bonding for T–Hg2+–T and weaker for C–Ag+–C

Metal mediated DNA are mismatches where a metal ion bridges the base pairs and are of interest for biosensors. Here, we study THg2+T, which is known to be stabilized by base pair metal mediated bonds, and CAg+C whose stabilization mechanism is less well understood. We use a mesoscopic model and published melting temperatures of sequences containing CC or TT mismatches in the presence of the metal ions. For THg2+T we obtain a strong base pair bond potential and moderate one for CAg+C, only one stacking potential for each configuration is stronger than usual.

Prosocial and antisocial choices in a monogamous cichlid with biparental care

Human society is cooperative and characterized by spontaneous prosociality. Comparative studies on endotherm vertebrates suggest that social interdependence causes the evolution of proactive prosociality. To test the generality of this hypothesis, we modify a prosocial choice task for application to the convict cichlid, Amatitlania nigrofasciata, a monogamous fish with biparental care and a strong pair bond. We also affirm that male subjects learn to favor prosocial choices when their mates are the recipients in a neighboring tank. When the neighboring tank is empty, males choose randomly. Furthermore, in the absence of their mates, males behave prosocially toward a stranger female. However, if the mate of the subjects is also visible in the third tank, or if a male is a potential recipient, then subjects make antisocial choices. To conclude, fish may show both spontaneous prosocial and antisocial behaviors according to their social relationships with conspecifics and the overall social context.

Effects of Mating and Social Exposure on Cell Proliferation in the Adult Male Prairie Vole (Microtus ochrogaster).

Microtus ochrogaster is a rodent with a monogamous reproductive strategy characterized by strong pair bond formation after 6 h of mating. Here, we determine whether mating-induced pair bonding increases cell proliferation in the subventricular zone (SVZ), rostral migratory stream (RMS), and dentate gyrus (DG) of the hippocampus in male voles. Males were assigned to one of the four groups: (1) control: males were placed alone in a clean cage; (2) social exposure to a female (SE m/f): males that could see, hear, and smell a sexually receptive female but where physical contact was not possible, because the animals were separated by an acrylic screen with small holes; (3) social exposure to a male (SE m/m): same as group 2 but males were exposed to another male without physical contact; and (4) social cohabitation with mating (SCM): males that mated freely with a receptive female for 6 h. This procedure leads to pair bond formation. Groups 2 and 3 were controls for social interaction. Male prairie voles were injected with 5-bromo-2′-deoxyuridine (BrdU) during the behavioral tests and were sacrificed 48 h later. Brains were processed to identify the new cells (BrdU-positive) and neuron precursor cells (neuroblasts). Our principal findings are that in the dorsal region of the SVZ, SCM and SE m/f and m/m increase the percentage of neuron precursor cells. In the anterior region of the RMS, SE m/f decreases the percentage of neuron precursor cells, and in the medial region SE m/f and m/m decrease the number of new cells and neuron precursor cells. In the infrapyramidal blade of the subgranular zone of the DG, SE m/m and SCM increase the number of new neuron precursor cells and SE m/m increases the percentage of these neurons. Our data suggests that social interaction, as well as sexual stimulation, leads to pair bonding in male voles modulating cell proliferation and differentiation to neuronal precursor cells at the SVZ, RMS, and DG.

Age-related differences in reproductive success support the selection hypothesis in a Mute Swan population

In many bird species, we observe age-related differences in reproductive success, which usually increases early in life and later decreases due to senescence. At the individual level, an early-life improvement in breeding performance may be associated with experience acquired during early reproductive events (experience hypothesis). At the population level, higher average reproductive success of older age cohorts can result from a disappearance of low-quality individuals from the population (selection hypothesis). Here, we tested these hypotheses in a wild population of the Mute Swan Cygnus olor from central Poland. In 1996–2016, we collected information on breeding success of 150 marked individuals (70 females and 80 males; 590 breeding attempts). At the population level, there was an initial increase in reproductive success (1–5 years), followed by a plateau (5–8 years), and then by a decrease in older age classes. Both within- and between-individual age variation contributed to the linear increase in reproductive success at the population level, but the latter effect was much more apparent. Short-term breeders (≤ 2 breeding events) had significantly lower reproductive success than long-term breeders (≥ 3 breeding events) during their first two breeding attempts, providing support for selection hypothesis and disappearance of low-quality phenotypes. After exclusion of short-term breeders, a positive age-related increase in reproductive success lost significance, suggesting that under-specific condition (rapid population growth and strong human disturbance) experience may have a limited effect on reproductive success in long-lived species with strong pair bonds, such as the Mute Swan.

What makes a pair bond in a Neotropical primate: female and male contributions.

Pair living and pair bonding are rare in mammals, and the mechanisms of their maintenance remain a puzzle. Titi monkeys, a ‘textbook example’ for ‘monogamous’ primates, have strong pair bonds and extensive male care. To investigate mechanisms of pair-bond maintenance, we studied seven wild groups of red titis (Plecturocebus cupreus) in Peruvian Amazonia over a period of 14 months. We analysed pair bonds by measuring proximity, grooming and approaches/leaves within pairs, and collected data on intergroup encounters. Females contributed to grooming more than males, especially during infant dependency, when most of the grooming within pairs was done by females. Females were also more active in controlling proximity between pair mates, making most of the approaches and leaves. Males, on the other hand, invested more in territorial defences. They participated in more intergroup encounters than females and were more active during these encounters. Our data is most consistent with the ‘male-services’ hypothesis for pair-bond maintenance, where a female contributes more to the proximity and affiliation maintenance while a male provides beneficial services.

Parental care and neuropeptide dynamics in a cichlid fish Neolamprologus caudopunctatus

The hypothalamic neuropeptides arginine vasopressin (AVP) and oxytocin (OT) modulate social behavior across a wide variety of species. However, the role of arginine vasotocin (AVT) and isotocin (IT, the teleost homologs of AVP and OT) in regulating biparental care especially in the context of monogamy is not well studied. Here, using high-performance liquid chromatography (HPLC), we investigated how bioactive whole brain AVT and IT neuropeptide levels vary in relation to the phase of the breeding cycle and sex, in a monogamous biparental cichlid fish, Neolamprologus caudopunctatus. Since non-caring individuals of this species readily cannibalize eggs, but caring parents never do, we further investigated whether there might be changes in AVT/IT whole brain levels that correspond to the transition from a non-breeding, egg cannibal to an egg caring parent. We found that AVT levels were higher in females than in males and that AVT levels were highest when the need to defend the young was greatest. Breeding pairs that had a strong pair-bond and a higher frequency of nest care had the highest levels of AVT, whereas individuals that spent little time close to their breeding partner, displayed aggression towards their partner and neglected their nest duties (signs of a weak pair bond), had lower whole brain AVT levels. Isotocin (IT) levels did not differ between sexes and we did not detect any variation across the breeding cycle, with pair-bonding scores or with parental behavior. Our results show that whole brain AVT levels are linked to the breeding cycle, nest maintenance and pair-bonding score in this species. Furthermore, our study highlights species and sex-specific nonapeptides patterns in bi-parental caring fish and contributes to the increasing knowledge on neuroendocrinological mechanisms underlying parental care.

Female vocalizations predict reproductive output in brown-headed cowbirds (Molothrus ater).

Pair bonds are often maintained through the reciprocal and coordinated exchange of communicative signals. The ability to recognize and appropriately respond to a partner’s signals will define a pair’s ability to reproduce. Individual variation in responsiveness, by shaping the formation and maintenance of strong pair bonds, will ultimately influence an individual’s reproductive output. Throughout the breeding period, female cowbirds (Molothrus ater) respond to male song displays using a vocalization known as the chatter. In this study, we investigated whether variation in chatters remained repeatable across years and predicted reproductive performance. A flock of cowbirds housed in a large aviary complex was observed during the spring of 2011 to 2012. We recorded courtship interactions, including singing behavior for males, and chatters and eggs laid by females. The rate with which females responded to song using chatters remained consistent across years, with some females predictably responding to more songs using chatters than others. During 2012, chattering predicted the number of eggs females laid and her paired status. Paired females were more likely to respond to songs with chatters, and there was a strong positive relationship between the number of eggs laid and the proportion of songs she responded to using chatters. Overall, these findings suggest that variation in female vocal behavior is associated with their reproductive success.

Social structure of Coal Tits Periparus ater in temperate deciduous forest

Analysis of the impacts of social structure on the behaviour and life history of birds is a rapidly developing area of ornithology. Such studies commonly focus on the Paridae (tits and chickadees) as a model group, but detailed assessment of the basic social structure is lacking for many parids, particularly in the non-breeding season. Such baseline information is essential for understanding the organisation of bird populations, including associations between individuals within social and spatial networks. We assessed the non-breeding (autumn–winter) social structure and that during spring (i.e. the breeding season) for Coal Tits Periparus ater in broadleaved forest in southern Britain. Coal Tits were found to be resident and sedentary in this habitat, with birds remaining close to their spring territories during the non-breeding season and associating in small groups averaging 3–4 individuals. Associations were variable, however, with no evidence of stable flock membership, group territoriality or strong pair bonds during autumn–winter. The non-breeding social structure during a year of high population density did not differ from that during a year of low population density, being most similar to a ‘basic flock’ organisation. This contrasted with that predicted for a food-hoarding parid, highlighting the potential differences in social behaviour between different populations of bird species. Such variation in social behaviour can inform our understanding of the evolution of avian social structures and associated behavioural traits, such as food hoarding, and aid the interpretation of intra- and interspecific differences in behaviour.

Sex differences in hippocampal mineralocorticoid and glucocorticoid receptor mRNA expression in response to acute mate pair separation in zebra finches (Taeniopygia guttata).

Mate separation has been shown to mediate changes in physiological and behavioral processes via activation of the hypothalamo-pituitary-adrenal (HPA) axis in both mammalian and avian species. To elucidate the neural mechanisms associated with changes in the HPA axis in response to social stress, we investigated the effects of mate pair separation on circulating corticosterone concentrations as well as gene expression levels of mineralocorticoid receptor (MR), glucocorticoid receptor (GR), and corticotropin releasing hormone (CRH) in the hypothalamus and hippocampus of both male and female zebra finches, a species that forms strong pair bonds. Zebra finches (Taeniopygia guttata) were housed three to a cage (a mated pair plus a stimulus female), and were assigned to one of three new housing treatment groups: (1) male or female removed from their respective mate and placed in a cage with a new opposite sex conspecific and stimulus female (2) male or female that remained with their mate, but a new stimulus female was introduced, or (3) the subjects were handled but not separated from their mate or the stimulus female. After 48 hr in the new housing condition, we observed significant increases in plasma corticosterone concentrations in response to both mate pair and stimulus female separation. No significant differences in MR, GR, or CRH mRNA expression in the hypothalamus were observed in response to any treatment for both males and females. Females exhibited a significant up regulation in hippocampal MR, but not GR mRNA, whereas males exhibited a significant down regulation of both hippocampal MR and GR mRNA in response to mate pair separation. Thus, the hippocampus appears to play a key role in regulating sex specific responses to social stressors.

Strong pair bonds and high site fidelity in a subarctic-breeding migratory shorebird

ABSTRACT Interannual mate and site fidelity is common in migratory shorebirds with monogamous mating systems. After long-distance migrations and separation during the winter, birds often relocate their former mate at their previous breeding territory. Although pairs frequently reunite, new pairs are also formed. Why birds change mates is still not completely understood. Mate change can involve active decisions, in which one or both mates actively chooses to divorce from a previous mate, but can also be related to arrival timing or mate availability at the breeding grounds. We explored possible causes of mate change in the Pacific subspecies of the migratory shorebird Dunlin (Calidris alpina pacifica) breeding at the subarctic Yukon-Kuskokwim Delta, Alaska, USA (61°36′N, 165°12′W). Interannual return rates of Dunlin to their breeding grounds were higher for males (74%) than for females (54%) and were 14% higher for birds with high previous breeding success. Mate change was rare if both birds returned to th...

Where should we meet? Mapping social network interactions of sleepy lizards shows sex-dependent social network structure

Social network analyses allow researchers to describe patterns of social interactions and their consequences in animal societies. Since direct observations in natural settings are often difficult, researchers often use tracking technologies to build proximity-based social networks. However, because both social behaviour (e.g. conspecific attraction) and environmental heterogeneity (e.g. resources attracting individuals independently) affect rates of interaction, identifying the processes that shape social networks is challenging. We tracked sleepy lizards, Tiliqua rugosa, using global positioning system (GPS) telemetry to investigate whether they show conspecific attraction or avoidance beyond any shared space use driven by environmental heterogeneity. Since these lizards have strong pair bonds and nonoverlapping core home ranges, we predicted different interaction rates between inter- and intrasex dyads and compared social network indices among dyad types (male–male, female–female and intersex) using node-identity permutation tests. We also mapped interactions onto the home ranges (using distance from the centre as an index) and contrasted observed social networks with those expected from a spatially explicit null model. We found that dyad types differed in their interaction patterns. Intersex dyads had stronger connections (higher edge weight) than a null expectation, and stronger than for same-sex dyads. Same-sex dyads did not differ in edge weight from the null expectation, but were significantly more common (higher degree). Males had larger home ranges than females and consequently male–male dyads interacted further away from their home range centres. Moreover, the locations of these interactions also differed from the null expectations more strongly than other dyad types. Hence, we conclude that males predominantly interacted with each other at the peripheries of their home range, presumably reflecting territorial behaviour. By applying a novel analysis technique, we accounted for the nonsocial component of space use and revealed sex-specific interaction patterns and the contribution of conspecific attraction to the social structure in this species. More generally we report how mapping the locations of nonrandom interaction rates provides broad information on the behaviours they represent.

Imaging, Behavior and Endocrine Analysis of "Jealousy" in a Monogamous Primate.

Understanding the neurobiology of social bonding in non-human primates is a critical step in understanding the evolution of monogamy, as well as understanding the neural substrates for emotion and behavior. Coppery titi monkeys (Callicebus cupreus) form strong pair bonds, characterized by selective preference for their pair mate, mate-guarding, physiological and behavioral agitation upon separation, and social buffering. Mate-guarding, or the "maintenance" phase of pair bonding, is relatively under-studied in primates. In the current study, we used functional imaging to examine how male titi monkeys viewing their pair mate in close proximity to a stranger male would change regional cerebral glucose metabolism. We predicted that this situation would challenge the pair bond and induce "jealousy" in the males. Animals were injected with [18F]-fluorodeoxyglucose (FDG), returned to their cage for 30 min of conscious uptake, placed under anesthesia, and then scanned for 1 hour on a microPET P4 scanner. During the FDG uptake, males (n=8) had a view of either their female pair mate next to a stranger male ("jealousy" condition) or a stranger female next to a stranger male (control condition). Blood and cerebrospinal fluid samples were collected and assayed for testosterone, cortisol, oxytocin, and vasopressin. Positron emission tomography (PET) was co-registered with structural magnetic resonance imaging (MRI), and region of interest analysis was carried out. Bayesian multivariate multilevel analyses found that the right lateral septum (Pr(b>0)=93%), left posterior cingulate cortex (Pr(b>0)=99%), and left anterior cingulate (Pr(b>0)=96%) showed higher FDG uptake in the jealousy condition compared to the control condition, while the right medial amygdala (Pr(b>0)=85%) showed lower FDG uptake. Plasma testosterone and cortisol concentrations were higher during the jealousy condition. During the jealousy condition, duration of time spent looking across at the pair mate next to a stranger male was associated with higher plasma cortisol concentrations. The lateral septum has been shown to be involved in mate-guarding and mating-induced aggression in monogamous rodents, while the cingulate cortex has been linked to territoriality. These neural and physiological changes may underpin the emotion of jealousy, which can act in a monogamous species to preserve the long-term integrity of the pair.

Ultrasonic vocalization production and playback predicts intrapair and extrapair social behaviour in a monogamous mouse

Most studies examining rodent ultrasonic vocalizations (USVs) have investigated pericopulatory vocal behaviour in polygynous rodents, while vocalizations related to pair bond maintenance in monogamous rodents remain unexplored. In the monogamous California mouse, Peromyscus californicus, we used ultrasonic playbacks and post-playback social interactions to assess possible functions of USVs. We found that females responded with approach towards USVs of an unfamiliar male (bonded male from another pair) compared to a noise control, but displayed no difference in response to calls of their partner versus noise. Responsiveness to unfamiliar males does not appear to reflect an interest in extrapair copulations because during post-playback social interactions, females displayed more agonistic behaviours and fewer affiliative behaviours towards unfamiliar, sexually naive ‘stranger’ males than towards their partners. We speculate that approach to unfamiliar male USVs instead may be related to territorial defence. We further explored associations within the data set. Interestingly, female affiliation with her partner was predicted by USV output, particularly a higher number and proportion of complex call types, produced during the male partner USV elicitation phase. Female approach towards USVs was related to syllable duration of one call type in partner USVs (not in unfamiliar USVs) but no other features, and sufficient variation exists in syllable duration to allow females to theoretically distinguish between individuals based on this measure. Similarly, while pregnancy state did not influence female social behaviour, it decreased approach to playback of partner USVs but not to that of unfamiliar USVs. Overall, our results illuminate concepts about vocal communication in monogamous rodent species with strong pair bonds and suggest that functions of USVs in rodents can extend beyond mate choice.

Partner fidelity and reciprocal investments in the mating system of a simultaneous hermaphrodite

Simultaneous hermaphrodites that exchange gametes within pairs allow us to test whether equity of resource exchange is positively associated with duration of cooperative partnerships. The chalk bass (Serranus tortugarum) provides an excellent model system for such research because they spawn daily, trading sex roles with the same mating partners in repeated spawning bouts (a tactic called parceling). Even so, they live in site-attached aggregations with opportunities for extra-pair matings. We asked whether individuals spawn preferentially with 1 long-term partner and whether partner fidelity was associated with matching in the number of eggs produced in a single day by individuals within pairs. In addition, we observed random focal individuals during spawning to determine whether egg parceling was coordinated within pairs: we asked whether parcel number (per day) was well matched within pairs and whether parcels were exchanged in nonrandom sequences. We did not witness any break-ups in our long-term pairs; during each monthly census, all identified pairs were found spawning and courting with their mating partner until one or both of them disappeared. Daily egg production and the number of egg parcels exchanged within primary partnerships were coordinated. The sequence of parcel exchange was fairly regular, although ca. 20% of focal individuals fertilized or exchanged a few egg parcels with another partner(s); parceling allows individuals the option to exchange eggs outside of the primary pair. Our study provides novel empirical evidence of strong pair bonds and resource matching even in relatively large social groups with extra-pair trading opportunities.

Mates but not sexes differ in migratory niche in a monogamous penguin species.

Strong pair bonds generally increase fitness in monogamous organisms, but may also underlie the risk of hampering it when re-pairing fails after the winter season. We investigated whether partners would either maintain contact or offset this risk by exploiting sex-specific favourable niches during winter in a migratory monogamous seabird, the southern rockhopper penguin Eudyptes chrysocome. Using light-based geolocation, we show that although the spatial distribution of both sexes largely overlapped, pair-wise mates were located on average 595 ± 260 km (and up to 2500 km) apart during winter. Stable isotope data also indicated a marked overlap between sex-specific isotopic niches (δ¹³C and δ¹⁵N values) but a segregation of the feeding habitats (δ¹³C values) within pairs. Importantly, the tracked females remained longer (12 days) at sea than males, but all re-mated with their previous partners after winter. Our study provides multiple evidence that migratory species may well demonstrate pair-wise segregation even in the absence of sex-specific winter niches (spatial and isotopic). We suggest that dispersive migration patterns with sex-biased timings may be a sufficient proximal cause for generating such a situation in migratory animals.

Reproductive behavior of the great hornbill (Buceros bicornis).

Great hornbills (Buceros bicornis) are a long-lived, monogamous species that forms strong pair-bonds, and mate compatibility is thought to be important for successful reproduction. Within AZA, great hornbills are listed as a red SSP. The population consists of a limited number of individuals that do not breed reliably, and improving reproduction is a top priority for the Coraciiformes TAG. To better understand mating behavior and evaluate mate compatibility, this study documented the behavior of pairs of great hornbills during and immediately after courtship. Using live observations, the study followed one female, an experienced and successful breeder, as she was paired with four successive males over 11 breeding seasons. Initially, males frequently vocalized, investigated the nest, and approached the female. As the female spent more time in the nest, these behaviors were replaced by regurgitation and food offering. The female was most often observed plastering and vocalizing. Behavioral differences between successful and unsuccessful pairs, possibly indicative of pair compatibility, included rates of approaching, billing, and biting. Numerous behaviors occurred more frequently during years that a chick hatched, including pseudoregurgitation, regurgitation, offering food items, and nest investigation. Males also spent more time in proximity to both the female and the nest during years that a chick hatched. Together, these results suggest that the amount of time pairs spend in proximity, the amount of time a male spends near the nest, and the frequency of certain behaviors may help evaluate compatibility and the likelihood of successful reproduction for pairs of great hornbills.

Sexual Stimulation and Sexual Selection.

Sexual Stimulation and Sexual Selection.