Miguel A. Rodríguez-Gironés

SEXUAL DIMORPHISM AND PARENTAL ROLES IN THE THORN-TAILED RAYADITO (FURNARIIDAE)

Abstract Sexual dimorphism, mating system, and parental care are known for only a few species of the large passerine family Furnariidae. We conducted a study of sexual dimorphism in morphology, coloration, and parental roles during incubation and chick-rearing in the Thorn-tailed Rayadito (Aphrastura spinicauda), a characteristic resident ovenbird of the southern temperate rainforests of Chile and Argentina. Through molecular sexing, morphological measurements, and spectrophotometric analysis of body plumage and rectrices of reproductive adults captured on Chiloé Island (southern Chile), we determined that males were between 2% and 10% larger than females in mass, tarsus length, and wing length, while no difference was found for the length of the bill or the two longest central rectrices and their characteristic spines, or in plumage coloration. Heavy males were paired with heavy females and light males with light females. Males and females participated equally in all reproductive activities during the incubation and nestling phases, except removal of nestling feces, in which females were twice as active as males. In a study of habitat use on Navarino Island (extreme southern Chile) we found that the extended graduated tail, with rectrices that end in spines, which gives the species its name, was not used as a support while foraging and could be related to another function such as sexual or social signaling. The absence of sexual dimorphism in plumage and parental roles in rayaditos may be related to the use of the long, graduated tail as a signal of quality by both sexes, although this hypothesis requires confirmation through future mate choice studies.

The evolution of female sexuality

Females in monogamous species tend to be more sexually active than females in species with other mating systems. In this paper we consider the possibility that female sexuality has evolved because more sexually active females have received more male assistance. We develop a model in which there is no direct cue available to males indicating whether the female is fertile. Instead males might respond to female behaviour as an indirect cue. The latter could favour increased female sexuality if males tend to stay longer with more sexually active females. Our results show that female sexual behaviour can have a significant impact on social behaviour and that sexually active females and sexual behaviour outside fertile periods can evolve under some circumstances. We end with a discussion of theories of the evolution of female sexuality. We believe that theories based on male assistance fit empirical findings better than theories based on variation in male genetic quality.

Evolutionary stability of vigilance coordination among social foragers.

Coordination can greatly improve the efficiency of anti–predatory vigilance scans by increasing predator detection for a constant proportion of time spent vigilant. However, it has been rarely found in nature and most studies have detected or assumed independent scanning by group members. In this study, we analysed the functional consequences of the coordinated alternation of vigilance scanning by group foragers. We introduce coordination by assuming that interscan intervals (ISIs) follow a modified gamma distribution. Depending on the parameters of the distribution, successive scans can be evenly spaced (coordinated scanning) or may present a high overlap (uncoordinated scanning). Comparing evolutionarily stable strategies for animals that do not coordinate their scanning with animals that do coordinate their anti–predator behaviour shows that coordination has a marked effect on survival probability. Moreover, the coordinating strategy is quite robust against mutants that scan independently with exponential distributions of ISIs. However, coordination breaks down when animals can continuously adjust their level of coordination by deciding the proportion of time they spend monitoring the behaviour of other group members. In this case, coordination is only evolutionarily stable if it can be very easily achieved.

Relative Importance of Perceptual and Mnemonic Variance in Human Temporal Bisection

We investigated the relative contributions of perception and reference memory to behavioural variability in a temporal discrimination with human subjects. We used two temporal bisection tasks. In both tasks each trial consisted of a sequential presentation of three intervals, two standards, and a probe, and subjects were asked to judge the similarity of each probe against the two standards. In a “single bisection”, the standards’ duration was constant across trials. In a “roving bisection”, the two standards were trial unique. We compared our results with the predictions from a model related to Scalar Expectancy Theory, with the added assumption that the decision process minimizes the expected number of errors given the information available. The model shows that if errors in reference memory were dominant the psychometric function should be identical for single and roving tasks, and if perceptual errors were dominant the psychometric function should be steeper for the single than for the roving bisection. As we found that psychometric functions were steeper for the single than for the roving tasks, we concluded that perceptual errors are dominant.

Behavioral adjustment to modifications in the temporal parameters of the environment

We study the dynamics of behavioral transitions when European starlings (Sturnus vulgaris) experience stepwise changes in the value of a meaningful time interval. Subjects were primed to respond at a certain time T1. After extensive training, the primed time changed to a new value T2. In Experiment 1 subjects were reinforced on 40% of the trials and they experienced a single transition which lasted until asymptotic behavior was reached. Starlings showed a progressive adjustment to T2, with no obvious discontinuities. In Experiment 2, probability of reinforcement was initially 20%, and the schedule switched to extinction after a varied number of trials were reinforced at the post-transition time. The number of post-transition reinforcements was used as independent variable. Behavior was examined in extinction to judge the state of temporal performance after a controlled amount of experience. Under these conditions, adjustment to T2 took place in two stages, and there was an intermediate phase when behavior changed little. These results are consistent with the hypotheses that animals continuously update the subjective probabilities that reinforcement comes at any given time and that responding occurs when the current estimate is above a certain threshold. We show that in spite of the continuous updating of time estimates, responding can show either continuous or discontinuous adjustments depending on the vicinity of the pre- and post-transition times and the probability of reinforcement.