Renee A Duckworth
We report the first case of mycoplasmosis in the western range of the House Finch (Carpodacus mexicanus). This disease originated in the eastern United States and has been previously documented only in eastern introduced House Finch populations where it reached epizootic proportions causing extensive and widespread mortality. Documentation of this disease in western Montana suggests that previously disjunct eastern and western populations of House Finches are now mixing in the northern part of their range. More importantly, as native House Finches are highly susceptible to this novel pathogen, western populations may now be at risk of high mortality, similar to that experienced by non-native eastern populations. Close monitoring of this disease in the western part of the House Finch range will provide important insight into the dynamics of the emerging disease and evolution of resistance to the pathogen.
Selection should favor flexibility in reproductive tactics when the combination of sexual traits and reproductive behaviors that achieve the highest fitness differs between males within a population. Understanding the functional significance of variation in male reproductive tactics can provide insight into their evolution. Male house finches (Carpodacus mexicanus) in a Montana population display continuous variation in parental tactics: males with more elaborated (redder) plumage color provide little or no parental care compared to less elaborated (dull) males. Here, we first determined whether elevation of prolactin (a pituitary hormone) was related to variation in male parental tactics and, second, we used the relationship between prolactin levels and parental behavior to investigate why redder males avoid a high investment in parental care. We found that prolactin elevation was closely associated with paternal care. In addition, males with redder plumage color had low prolactin levels, whereas dull males, which provision twice as frequently, had high levels of prolactin. We also found that male condition was unrelated to plumage color but negatively related to prolactin levels. These results suggest that the low provisioning of redder males was not due to physiological constraints, but instead reflected a tactic to avoid the costs associated with parental care. The condition benefits accrued by redder males may explain their higher post-breeding survival compared to dull males. Moreover, dull males were previously shown to have higher pairing success than redder males, suggesting that the relationship between male plumage color and parental care may reflect individually optimized parental tactics.
Maternal modification of offspring sex in birds has strong fitness consequences, however the mechanisms by which female birds can bias sex of their progeny in close concordance with the environment of breeding are not known. In recently established populations of house finches (Carpodacus mexicanus), breeding females lay a sex-biased sequence of eggs when ambient temperature causes early onset of incubation. We studied the mechanisms behind close association of incubation and sex-determination strategies in this species and discovered that pre-ovulation oocytes that produce males and females differed strongly in the temporal patterns of proliferation and growth. In turn, sex-specific exposure of oocytes to maternal secretion of prolactin and androgens produced distinct accumulation of maternal steroids in oocyte yolks in relation to oocyte proliferation order. These findings suggest that sex difference in oocyte growth and egg-laying sequence is an adaptive outcome of hormonal constraints imposed by the overlap of early incubation and oogenesis in this population, and that the close integration of maternal incubation, oocytes' sex-determination and growth might be under control of the same hormonal mechanism. We further document that population establishment and the evolution of these maternal strategies is facilitated by their strong effects on female and offspring fitness in a recently established part of the species range.
Male investment into sexual ornamentation is a reproductive decision that depends on the context of breeding and life history state. In turn, selection for state- and context-specific expression of sexual ornamentation should favour the evolution of developmental pathways that enable the flexible allocation of resources into sexual ornamentation. We studied lifelong variation in the expression and condition-dependence of a sexual ornament in relation to age and the context of breeding in male house finches (Carpodacus mexicanus) - a species that develops a new sexual ornament once a year after breeding. Throughout males' lifetime, the elaboration of ornamentation and the allocation of resources to the development of sexual ornamentation depended strongly on pairing status in the preceding breeding season - males that were single invested more resources into sexual ornamentation and changed ornamentation more than males that were paired. During the initial (post-juvenile) moult, the expression of ornamentation was closely dependent on individual condition, however the condition-dependence of ornamentation sharply decreased throughout a male's lifetime and in older males expression of sexual ornamentation was largely independent of condition during moult. Selection for early breeding favoured greater ornamentation in males that were single in the preceding seasons and the strength of this selection increased with age. On the contrary, the strength of selection on sexual ornamentation decreased with age in males that were paired in the preceding breeding season. Our results reveal strong context-dependency in investment into sexual ornamentation as well as a high flexibility in the development of sexual ornamentation throughout a male's life.