White-striped Males Research Articles

Actuarial senescence in a dimorphic bird: different rates of ageing in morphs with discrete reproductive strategies.

It is often hypothesized that intra-sexual competition accelerates actuarial senescence, or the increase in mortality rates with age. However, an alternative hypothesis is that parental investment is more important to determining senescence rates. We used a unique model system, the white-throated sparrow (Zonotrichia albicollis), to study variation in actuarial senescence. In this species, genetically determined morphs display discrete mating strategies and disassortative pairing, providing an excellent opportunity to test the predictions of the above hypotheses. Compared to tan-striped males, white-striped males are more polygynous and aggressive, and less parental. Tan-striped females receive less parental support, and invest more into parental care than white-striped females, which are also more aggressive. Thus, higher senescence rates in males and white-striped birds would support the intra-sexual competition hypothesis, whereas higher senescence rates in females and tan-striped birds would support the parental investment hypothesis. White-striped males showed the lowest rate of actuarial senescence. Tan-striped females had the highest senescence rate, and tan-striped males and white-striped females showed intermediate, relatively equal rates. Thus, results were inconsistent with sexual selection and competitive strategies increasing senescence rates, and instead indicate that senescence may be accelerated by female-biased parental care, and lessened by sharing of parental duties.

New insights into the hormonal and behavioural correlates of polymorphism in white-throated sparrows, Zonotrichia albicollis

The white-throated sparrow is a promising model for behavioural neuroendocrinology and genetics because behaviour and endocrine function may be linked to a chromosomal rearrangement that determines plumage colour. The notion that the two colour morphs, tan-striped (TS) and white-striped (WS), differ predictably in aggression and parenting has been widely accepted, despite conflicting evidence. It is also hypothesized that morph-typic behaviour is hormone mediated, yet no field study has measured sex steroids and behaviour in the same birds. Here, we re-evaluate the TS and WS phenotypes, describe the conditions under which they differ and investigate relationships between sex steroids and behaviour. We report that (1) during territorial intrusions, WS males were more aggressive than TS birds, but this difference was restricted to singing; WS males sang more than TS males but showed identical levels of physical aggression. WS females sang more than TS females and were also more physically aggressive. (2) TS males provisioned young more frequently than did WS males, but only during first broods. The parental strategy of WS males was flexible, and during replacement broods, WS and TS males provisioned at equal rates. (3) Consistent with previous studies, we detected no morph difference in female provisioning. (4) Plasma testosterone and dihydrotestosterone were higher in WS males than in TS males during periods of peak territorial defence and during first broods; within breeding stages, male androgen levels were positively correlated with singing and negatively correlated with provisioning. Plasma oestradiol levels were higher in WS females than in TS females and higher during peak territorial defence; oestradiol levels tended to be positively correlated with singing. Overall, our results refine the TS and WS phenotypes, show that behavioural differences between them are restricted to periods with relatively high mating opportunity, and demonstrate an association between sex steroids and morph-typic behaviour. These results will inform future studies of this promising model.

Morph Matters: Aggression Bias in a Polymorphic Sparrow

In species with discrete morphs exhibiting alternative behavioral strategies, individuals may vary their aggressive behavior in competitive encounters according to the phenotype of their opponent. Such aggression bias has been documented in multiple polymorphic species evolving under negative frequency-dependent selection, but it has not been well-studied under other selection regimes. We investigated this phenomenon in white-throated sparrows (Zonotrichia albicollis), a passerine with plumage polychromatism maintained by disassortative mating. The two distinct color morphs differ with respect to reproductive strategy in that white-striped birds invest more in territorial aggression than tan-striped birds. Whether territorial aggression in this species is biased according to the morph of an intruder is less understood. We found that during peak territorial and mating activity, both color morphs and sexes can exhibit aggression bias, but whether they do so depends on the strategy (morph) of the intruder. During simulated territorial intrusions, resident white-striped males and tan-striped females, which represent the opposite ends of a continuum from high to low territorial aggression, altered their territorial responses according to intruder morph. Tan-striped males and white-striped females, which represent the middle of the continuum, did not show a bias. We propose that because of the disassortative mating system and morph differences in reproductive strategy, the fitness risks of intrusions vary according to the morphs of the resident and the intruder, and that aggression bias is an attuned response to varying threats to fitness.

Morph-Specific Variation in Baseline Corticosterone and the Adrenocortical Response in Breeding White-Throated Sparrows (Zonotrichia albicollis)

The adrenocortical response to perturbation varies considerably within and across avian species during breeding. Among the factors hypothesized to influence this variation is the degree of parental care expressed by an individual. To increase our understanding of the relationship between parental care and the adrenocortical response, we examined corticosterone secretion in the polymorphic White-throated Sparrow (Zonotrichia albicollis) during three breeding stages: prelaying, incubation, and nestling. In addition, we reevaluated previously described morph differences in parental behavior. Tan-striped males provisioned nestlings at higher rates than white-striped males, which is consistent with earlier findings. However, in contrast to previous studies, provisioning rates were similar between female morphs and higher than those exhibited by either male morph. We found that, in males, the more parental morph (tan-striped) expressed a reduced adrenocortical response during the nestling stage, but the less parental morph (white-striped) did not. Females of both morphs exhibited similar and reduced adrenocortical responses during parental stages. In addition, we observed morph differences in baseline corticosterone during the nestling stage: less parental birds (white-striped males) had higher baseline corticosterone levels than more parental birds (females and tan-striped males). These findings support the hypothesis that modulation of baseline corticosterone and the adrenocortical response during breeding is a function of the degree of parental care expressed by an individual. By influencing how parents balance self-maintenance and parental care on a daily basis, or in the face of perturbation, baseline corticosterone and the adrenocortical response may play a role in the alternative reproductive strategies of White-throated Sparrow morphs.

Corticosterone manipulations alter morph-specific nestling provisioning behavior in male white-throated sparrows, Zonotrichia albicollis

In the polymorphic white-throated sparrow ( Zonotrichia albicollis), tan-striped males provision nestlings at higher rates than do white-striped males. In a previous study, we found that tan-striped males had lower baseline corticosterone levels than white-striped males during the nestling stage. To determine if this variation in corticosterone influences morph-specific differences in nestling provisioning behavior, we used intraperitoneal osmotic pumps to increase baseline corticosterone levels in tan-striped males (TS CORT) and administer RU486, a glucocorticoid receptor antagonist, in white-striped males (WS RU486). These manipulations essentially reversed morph-specific nestling provisioning behavior in males. TS CORT males fed nestlings at lower rates than TS controls (vehicle-only implant), and at similar rates to WS controls (vehicle-only implant), while WS RU486 males fed nestlings at higher rates than WS controls, and at similar rates to TS controls. These results demonstrate that (1) increases in baseline corticosterone (i.e., below concentrations associated with the adrenocortical response to stress) can directly or indirectly inhibit nestling provisioning behavior, and (2) corticosterone influences morph-specific variation in parental behavior in male white-throated sparrows. This study contributes to the growing evidence that modulating baseline CORT mediates parental care and self-maintenance activities in birds, and thus may serve as a mechanism for balancing current reproductive success with survival.

Plasma Testosterone Correlates with Morph Type across Breeding Substages in Male White‐Throated Sparrows

White-throated sparrows (Zonotrichia albicollis) exhibit a genetic polymorphism that affects plumage and behavior in both sexes. White-striped morphs are more territorially aggressive, whereas tan-striped morphs provision nestlings at a higher rate. We investigated testosterone physiology in this species in an effort to understand hormonal mechanisms for the observed differences in aggression and parental care between the morphs. We found a small but significant difference in plasma testosterone between free-living white-striped and tan-striped males over the course of the breeding season. This difference correlates with previously observed differences in aggressive behavior and suggests that testosterone may mediate these differences. Testosterone remained higher in white-striped males relative to tan-striped males when males were provisioning nestlings and fledglings. Thus, testosterone may also contribute to the relatively reduced levels of parental care exhibited by white-striped males. In contrast to males, plasma testosterone did not differ between free-living white-striped and tan-striped females, which suggests that testosterone does not mediate differences in aggression between female morphs. Injection with gonadotropin-releasing hormone led to greater testosterone secretion in both captive and free-living males and captive females but did not differ by morph. Therefore, we conclude that differences in plasma testosterone between the morphs are due to differences in testosterone regulation upstream of the pituitary.

Endocrine correlates of alternative phenotypes in the white-throated sparrow ( Zonotrichia albicollis)

Many vertebrate species exhibit alternative phenotypes (or morphs), in which one sex displays phenotypic variation equal to or greater than the variation between the sexes. Males in such species typically display differences in reproductive strategies and morphology. Steroid hormones such as testosterone are known modulators of reproductive behavior and morphology and therefore are obvious candidates for the mediation of phenotypic differences between morphs. We conducted a year-round study in the white-throated sparrow ( Zonotrichia albicollis) that exhibits alternative phenotypes in plumage coloration and behavior in both sexes: during the breeding season, white-striped males and females are more aggressive and have higher song rates than tan-striped individuals. At the beginning of the breeding season, free-living white-striped males had higher plasma testosterone concentrations than tan-striped males. However, this finding might have been due to different social experiences because captive male morphs sampled at similar times of year did not differ in testosterone concentrations. Captive white-striped males had larger testis and cloacal protuberance sizes than tan-striped males, which might be related to the divergent mating strategies of the morphs. Male morphs showed similar increases in luteinizing hormone following injections of gonadotropin-releasing hormone, but white-striped males showed larger increases in testosterone, indicating differences between morphs in gonadal testosterone production. Females had low concentrations of testosterone, and morphs did not differ. Plasma dehydroepiandrosterone (DHEA) concentrations were elevated in both sexes and morphs during the breeding and non-breeding seasons. These data do not support the hypothesis that testosterone activates behavioral differences between alternative phenotypes in the white-throated sparrow. Alternative testable hypotheses include hormonal effects during early development and direct genetic effects.

Tan and White Color Morphs of White-Throated Sparrows Differ in Their Non-Song Vocal Responses to Territorial Intrusion

White-throated Sparrows (Zonotrichia albicollis) occur in two distinct color morphs indicated by the color of the bird's median crown stripe. Tan-striped (TS) and white-striped (WS) sparrows differ in their aggressive behavior, with WS males generally behaving more aggressively than TS males. We measured the number and type of songs and call vocalizations used by TS and WS males in response to simulated territorial intrusion. TS males used significantly more low aggression calls compared to WS males. These results suggest that non-song call vocalizations, in addition to measures of song, may be valuable for future studies of territorial behavior and communication in birds.

Negative assortative mating in the white-throated sparrow, Zonotrichia albicollis: the role of mate choice and intra-sexual competition

Abstract. White-throated sparrows are dimorphic, mating disassortatively by morph. In laboratory choice trials, females of both morphs preferred tan-striped males when allowed to interact with males (two-way choice), but displayed no preferences when one-way mirrors did not allow stimulus males to interact with them (one-way choice). Males of both morphs displayed significant preferences for white-striped females in one-way choice trials, but not in two-way trials. During within-sex competition trials, white-striped birds were dominant to tan-striped birds of the same sex. These results suggest that negative assortative mating is maintained in part because white-striped females outcompete tan-striped females for access to the preferred tan-striped males. Tan-striped females then pair with the less preferred white-striped males. A three-step process is proposed to account for the high level of negative assortative mating seen in the white-throated sparrow.

Aggressive Performance as a Behavioral Correlate of Plumage Polymorphism in the White-Throated Sparrow (Zonotrichia Albicollis)

Abstract Morph and sex-specific differences in aggressive performance were examined in the white-throated sparrow. Among the four morph/sex classes, white-striped males had the strongest response to white-striped models and were the most likely to approach and attend song playbacks independently. The slightly weaker responses of tan-striped males and white-striped females were similar to each other. Tan-striped females were the least aggressive. Aggression by white-striped females enabled tan-striped male x white-striped female pairs to perform as well, or better, than white-striped male x tan-striped female pairs. Tan-striped males paired earlier than white-striped males. An increased prevalence of territorial tan-striped males corresponded to the arrival of females on the breeding grounds. The data are consistent with the hypothesis that negative assortative mating benefits tan-striped males through the supplemental aggression of white-striped females. However, the apparent preference of white-striped females for tan-striped males suggests that there are benefits to negative asortative mating other than those that might arise from differences in aggressive performance.

Differences in parental contribution among pair types in the polymorphic White-throated Sparrow

We compared relative contributions to nestlings between male and female parents among different pair types in the White-throated Sparrow (Zonotrichia albicollis). The amount of parental contribution between the two main pair types, white-striped (WS) male × tan-striped (TS) female and TS male × WS female, did not differ. However, we found that (a) pairs consisting of TS males and WS females showed biparental care in which the partners made approximately equal investment in feeding nestlings; (b) males of WS male × TS female pair types contributed far less than the female, and far less than a TS male; (c) in WS pairs, males and females contributed about the same as did WS birds in mixed pairings; and (d) secondary TS females nested in territories of already paired WS males, and these females raised young unassisted by a male. We suggest explanations for these differences in contribution to nestlings between the sexes and between morphs, and we discuss optimal reproductive strategies for each sex and morph.

POLYMORPHISM IN THE WHITE-THROATED SPARROW, ZONOTRICHIA ALBICOLLIS (GMELIN)

A sample of 286 museum and 199 live specimens of white-throated sparrows were examined for morphic differences. Two morphic types in both sexes are described according to the color of the median crown stripe. These are the white-striped morph and the tan-striped morph. Regardless of sex, white-striped morphs have more black on the lateral crown areas, less streaking on a wider and grayer chest band, less intense black on the malar markings of the white throat patch and brighter yellow on the superciliary stripe. The proportions of white-striped males and tan-striped females increased from Newfoundland to the Northwest region. White-striped morphs mate selectively with tan-striped morphs. Evidence was presented to show that these morphic types are genetically determined, and once attained, do not change with age. Mechanisms governing polymorphism are discussed in an attempt to explain the existence of the morphic types in the white-throated sparrow.