Stephen Pruett-Jones

Independent versus nonindependent mate choice: do females copy each other?

There is increasing evidence from both observational and experimental studies that females may copy each others mating decisions. Female copying can be defined as a type of nonindependent choice in which the probability that a female chooses a given male increases if other females have chosen that male and decreases if they have not. The important characteristic of copying behavior that separates it from other similar processes is that the change in the probability of choice is strictly because of the actions of other females and not the consequences of those actions (e.g., a males behavior changing as a result of successful matings). A gametheory model suggests that the adaptive significance of female copying may depend primarily on the ratio of the costs to the benefits of active mate choice. Copying behavior, and more generally conspecific cueing, may be important in many behavioral processes beyond mate choice.

MEASURING THE EFFECTS OF PAIRING SUCCESS, EXTRA-PAIR COPULATIONS AND MATE QUALITY ON THE OPPORTUNITY FOR SEXUAL SELECTION

Sexual selection can act through variation in the number of social mates obtained, variation in mate quality, or variation in success at obtaining extra‐pair fertilizations. Because within‐pair fertilizations (WPF) and extra‐pair fertilizations (EPF) are alternate routes of reproduction, they are additive, rather than multiplicative, components of fitness. We present a method for partitioning total variance in reproductive success (a measure of the opportunity for selection) when fitness components are both additive and multiplicative and use it to partition the variance into components that correspond to each mechanism of sexual selection. Computer simulations show that extra‐pair fertilizations can either increase or decrease total variance, depending on the covariance between within‐pair and extra‐pair success. Simulations also suggest that for socially monogamous species, extra‐pair fertilizations have a greater effect than variation in mate quality or pairing status on the opportunity for selection. Application of our model to data gathered for a population of red‐winged blackbirds (Agelaius phoeniceus) indicates that most of the variance in male reproductive success was attributable to within‐pair sources of variance. Nevertheless, extra‐pair copulations increased the opportunity for selection because males varied both in the proportion of their social young that they sired and in the number of extra‐pair mates that they obtained. Furthermore, large and positive covariances existed between the number of extra‐pair mates a male obtained and both social pairing success and within‐pair paternity, indicating that, in this population, males preferred as social mates also were preferred as extra‐pair mates.

PROMISCUITY DRIVES SEXUAL SELECTION IN A SOCIALLY MONOGAMOUS BIRD

Abstract Many socially monogamous species paradoxically show signs of strong sexual selection, suggesting cryptic sources of sexual competition among males. Darwin argued that sexual selection could operate in monogamous systems if breeding sex ratios are biased or if some males attract highly fecund females. Alternatively, sexual selection might result from promiscuous copulations outside the pair bond, although several recent studies have cast doubt on this possibility, in particular by showing that variance in apparent male reproductive success (number of social young) differs little from variance in actual male reproductive success (number of young sired). Our results from a long-term study of the socially monogamous splendid fairy-wren (Malurus splendens) demonstrate that such comparisons are misleading and do not adequately assess the effects of extra-pair paternity (EPP). By partitioning the opportunity for selection and calculating Bateman gradients, we show that EPP has a strong effect on male annual and lifetime fitness, whereas other proposed mechanisms of sexual selection do not. Thus, EPP drives sexual selection in this, and possibly other, socially monogamous species.

Genetic similarity of social mates predicts the level of extrapair paternity in splendid fairy-wrens

Why females mate with multiple males, particularly in socially monogamous species, is one of the central unanswered questions in sexual selection and behavioural ecology. Recent theory suggests that socially monogamous females may improve the genetic quality of their offspring by mating with extrapair males with whom they are more genetically dissimilar (relative to their social mates), because decreased genetic similarity between mates may lead to decreased inbreeding depression of offspring and other benefits. We found that levels of extrapair paternity in splendid fairy-wren, Malurus splendens, broods were predicted by genetic similarity between social mates. Females whose entire broods were sired by extrapair males were more genetically similar to their social mates than they were to their extrapair mates, but females were no less similar to their extrapair mates than they were to the average male from the population. Within-pair young (WPY) from mixed-paternity broods showed higher levels of inbreeding and lower heterozygosity than did their extrapair nestmates or WPY from broods without extrapair paternity. Several processes could account for these patterns, including female choice of genetically dissimilar mates, differential sperm investment by males or differential survival of embryos as a function of genetic dissimilarity between mates, or postcopulatory processes such as sperm competition, cryptic female choice, or interactions between sperm and ova. Regardless of the mechanism, our results suggest that female splendid fairy-wrens may indirectly benefit from extrapair mating because it leads to lower levels of inbreeding and increased heterozygosity in their offspring.

Cloacal protuberances and extreme sperm production in Australian fairy-wrens

In many passerine species males have enlarged cloacal protuberances during the breeding season. One hypothesis for the evolution of cloacal protuberances posits that they are a response to sperm competition and thus predicts that both within and between species the size of the protuberance correlates with the number of sperm stored. Here we provide the first intraspecific test of this hypothesis. In Australian fairy-wrens (Aves: Maluridae) females regularly mate outside of their social group resulting in intense sperm competition among males. Male fairy-wrens develop enlarged cloacal protuberances, and in a study of three species, splendid fairy-wren, white-winged fairy-wren, and variegated fairy-wren, we found significant intraspecific correlations between the size of a male’s protuberance and the stored sperm reserves in two of the three species. Males of these species had extreme numbers of sperm in their cloacal protuberances, up to 8.3 billion for splendid fairy-wrens, which should be available for a single ejaculate and is the most ever reported in an avian species. Studies of both captive and wild males showed that individuals can produce as many as 2 billion sperm per day. These data support the sperm competition hypothesis for the evolution of cloacal protuberances and highlight extreme sperm production as one possible outcome of sperm competition.

Sperm Competition Selects for Sperm Quantity and Quality in the Australian Maluridae

When ejaculates from rival males compete for fertilization, there is strong selection for sperm traits that enhance fertilization success. Sperm quantity is one such trait, and numerous studies have demonstrated a positive association between sperm competition and both testes size and the number of sperm available for copulations. Sperm competition is also thought to favor increases in sperm quality and changes in testicular morphology that lead to increased sperm production. However, in contrast to sperm quantity, these hypotheses have received considerably less empirical support and remain somewhat controversial. In a comparative study using the Australian Maluridae (fairy-wrens, emu-wrens, grasswrens), we tested whether increasing levels of sperm competition were associated with increases in both sperm quantity and quality, as well as an increase in the relative amount of seminiferous tubule tissue contained within the testes. After controlling for phylogeny, we found positive associations between sperm competition and sperm numbers, both in sperm reserves and in ejaculate samples. Additionally, as sperm competition level increased, the proportion of testicular spermatogenic tissue also increased, suggesting that sperm competition selects for greater sperm production per unit of testicular tissue. Finally, we also found that sperm competition level was positively associated with multiple sperm quality traits, including the proportion of motile sperm in ejaculates and the proportion of both viable and morphologically normal sperm in sperm reserves. These results suggest multiple ejaculate traits, as well as aspects of testicular morphology, have evolved in response to sperm competition in the Australian Maluridae. Furthermore, our findings emphasize the importance of post-copulatory sexual selection as an evolutionary force shaping macroevolutionary differences in sperm phenotype.

Multiple shifts between violet and ultraviolet vision in a family of passerine birds with associated changes in plumage coloration

Colour vision in diurnal birds falls into two discrete classes, signified by the spectral sensitivity of the violet- (VS) or ultraviolet-sensitive (UVS) short wavelength-sensitive type 1 (SWS1) single cone. Shifts between sensitivity classes are rare; three or four are believed to have happened in the course of avian evolution, one forming UVS higher passerines. Such shifts probably affect the expression of shortwave-dominated plumage signals. We have used genomic DNA sequencing to determine VS or UVS affinity in fairy-wrens and allies, Maluridae, a large passerine family basal to the known UVS taxa. We have also spectrophotometrically analysed male plumage coloration as perceived by the VS and UVS vision systems. Contrary to any other investigated avian genus, Malurus (fairy-wrens) contains species with amino acid residues typical of either VS or UVS cone opsins. Three bowerbird species (Ptilonorhynchidae) sequenced for outgroup comparison carry VS opsin genes. Phylogenetic reconstructions render one UVS gain followed by one or more losses as the most plausible evolutionary scenario. The evolution of avian ultraviolet sensitivity is hence more complex, as a single shift no longer explains its distribution in Passeriformes. Character correlation analysis proposes that UVS vision is associated with shortwave-reflecting plumage, which is widespread in Maluridae.

Plumage coloration, ejaculate quality and reproductive phenotype in the red-backed fairy-wren

Understanding how pre- and postcopulatory sexually selected traits covary can provide insight into the evolution of male ornamentation and female mate choice. In this study, we examined ejaculate quality and investment in testicular tissue in relation to plumage colour and social status in the genetically promiscuous red-backed fairy-wren, Malurus melanocephalus. In this species, males exhibit one of three alternative reproductive phenotypes during the breeding season: males can breed in red and black plumage, breed in brown plumage, or act as brown-plumed, nonbreeding auxiliaries. We found that red/ black breeders invested more heavily in spermatogenic tissue, had larger sperm reserves, and tended to have greater numbers of sperm in ejaculate samples, when compared to brown breeders and auxiliaries. Within red/black breeders, plumage redness and saturation (i.e. long wavelength hue and increased red chroma) were negatively correlated with ejaculate sample sperm density. In addition, ejaculate motility appeared to be related to variation in plumage coloration such that, overall, males with less elaborate ornamentation showed greater ejaculate quality. These results suggest that pre- and postcopulatory traits negatively covary in red/black plumed red-backed fairy-wrens and indicate a possible trade-off between investment in plumage ornaments and investment in ejaculates.

SEXUAL SELECTION AND TAIL-LENGTH DIMORPHISM IN SCISSOR-TAILED FLYCATCHERS

Abstract Scissor-tailed Flycatchers (Tyrannus forficatus) exhibit elongated tails in both sexes, and sexual dimorphism in tail length. At Fort Sill, Oklahoma, during 1991 and 1992, Scissor-tailed Flycatchers exhibited sexual dimorphism (male–female) in tail length (1.48), with more moderate sexual dimorphism in wing length (1.09) and beak length (1.04). Based on an analysis of museum specimens, immature birds (<1 year of age) of both sexes in their first calendar year exhibited significantly shorter tails than adults (measured in the field). Furthermore, tail length was highly variable among both sexes relative to other morphological traits. Male tail length was correlated with early clutch initiation by the males mate and, in 1991, with larger clutch size. Similarly, female tail length was correlated with early clutch initiation, and, in one year, larger clutches. Longer-tailed females also tended to arrive earlier on the breeding grounds in 1992, the only year for which such data were available. Assortative mating by tail length was observed. Those findings support the hypothesis that tail length is a sexually selected trait in this socially monogamous species, and that female tail length may be correlated with measures of female quality (e.g. early arrival and breeding, large clutches). However, confounding effects of age on tail length make it difficult to distinguish among various hypotheses for evolution of elongated tails in this species.

PATTERNS OF FLOCK SIZE, DIET, AND VIGILANCE OF NATURALIZED MONK PARAKEETS IN HYDE PARK, CHICAGO

Abstract We examined patterns of diet, foraging group size, and vigilance effort of naturalized Monk Parakeets (Myiopsitta monachus) in Hyde Park, a neighborhood of Chicago, Illinois. Parakeets exhibited a highly seasonal and varied diet consisting of fruits, seeds, and buds, and they fed almost exclusively on birdseed provided at backyard feeding stations during the winter months. Birds foraged in groups of 1 to 31 birds, but most flocks were of 10 birds or less. Foraging group size was greatest in the fall and early winter, and smallest at the beginning of the spring when breeding began. Monk Parakeets adjusted their vigilance effort with changes in flock size. Individual vigilance effort declined with increasing flock size through a decrease in time spent scanning as well as the number of times scans were initiated. The number of parakeets exhibiting vigilance in a flock at any given time also decreased with flock size. It is likely that the highly adaptable and varied diet of Monk Parakeets contributes greatly to the persistence and growth of populations in a variety of North American habitats.