ALBERT

Description: It has long been believed that the paired circular markings ("eyespots") of Lepidoptera larvae, combined with their defensive postures (where the larvae swell their anterior body segments) protect them from potential predators. These traits could inhibit attacks by enhancing the similarity of the prey item to the predator’s own natural enemies (notably snakes), but alternatively, they may simply exploit the predator’s wariness of novel and/or conspicuous objects. To differentiate between these contrasting explanations, we evaluated the responses of naive chicks ( Gallus gallus domesticus ) to artificial caterpillar-like prey. In the first experiment, chicks were presented with model caterpillars either without eyespots or with eyespots positioned in 1 of 2 locations on their bodies: anteriorly or centrally. In the second experiment, chicks were presented with model caterpillars either lacking the thickening associated with the defensive posture, or with models in which a thickened section was placed centrally or anteriorly. In both cases, the chicks were significantly more wary of prey with anteriorly positioned defensive traits than either prey without these traits or prey with centrally positioned traits. Because prey with central eyespots and central thickening were equally novel and conspicuousness to those with these traits positioned in a more head-like anterior position, we infer that predator wariness was primarily influenced by the similarity of the prey to their potential natural enemies. These results support the idea that both caterpillar eyespots, and the defensive posture examined here, deter predators not simply because they are conspicuous, but because they enhance caterpillars’ resemblance to potentially dangerous vertebrates.

Description: Conspecific attraction is a form of social information use whereby individuals are attracted to the presence of conspecifics because they may indicate high-quality sites or resources. Conspecific attraction results in aggregation of individuals with similar needs and may therefore intensify competition, in particular, at high densities. Thus, the occurrence and strength of conspecific attraction may be dependent on density, but the effects of predicted intensity of future competition for resources on individual decisions have rarely been quantified. We studied realized early fecundity and oviposition site selection in the butterfly Pieris napi in relation to a density gradient of conspecific eggs on available host plants in an explicit laboratory experiment. Relying on conspecific assessment of host quality is expected to select for conspecific attraction, whereas competition avoidance is expected to select for avoidance of high conspecific densities. Presence of conspecific cues did not substantially affect realized fecundity as females exposed to an environment containing conspecific cues laid approximately equal number of eggs as females exposed to an environment lacking such cues. Instead, when females were able to choose among host plants with or without previously laid conspecific eggs, they preferred plants that already carried eggs in relation to egg-free host plants, independently of the initial egg density. Indeed, the maintenance of conspecific attraction, rather than avoidance, in P. napi implies that the possible benefits of conspecific attraction in oviposition site selection may outweigh the costs of competition in the wild.

Description: Animals with parental care defend their offspring with an intensity reflecting parental investment. Parental investment theory predicts that parents should take risks relative to their residual reproductive value. Therefore, parental defense should change consistently with age reaching a peak at middle age, and it should vary consistently with age at start and end of reproduction. We recorded the intensity of parental defense of offspring in 410 female goshawks Accipiter gentilis throughout their lives, ranging from timid females that barely approached a human intruder at the nest to aggressive females that physically attacked the human. Females were consistent in their level of defense throughout life, and aggressive females were mated to aggressive males. Investment in reproduction as reflected by laying date, clutch size, and brood size showed a bell-shaped relationship with age. Females that started to breed at a young age were less aggressive than females that started late. Likewise, females that finished reproduction at a young age behaved less aggressively than females that finished at an old age. The intensity of defense of offspring peaked at an intermediate age followed by a decrease into old age and senescence. Females that started to breed early during the season were more aggressive than late breeders. These findings are consistent with the hypothesis that the intensity of parental defense of their offspring reflects parental investment and patterns of aging.

Description: Raptors are known for asymmetric parental roles, with the female brooding and feeding offspring, and the male providing food, usually assisted by the female from the latter part of the rearing period. Yet, the evolution of this role asymmetry is poorly understood. From filming prey delivery and handling in 61 nests of 9 raptor species, we show that the female role is related to prey size and prey type. Within species, nestlings were less likely to ingest larger prey unassisted, independent of increasing ability of unassisted feeding with increasing age. Feeding time increased with prey size and was longer for avian than for mammalian prey of the same size when nestlings fed unassisted. Across species, the female partitioned prey and fed offspring for a longer portion of the rearing period as prey size increased and as the diet contained more birds. Providing for the family selects for small body size; hence, extended female confinement as sedentary food processor for offspring would leave greater potential for differential selection on male and female body size. This may explain the female-biased size dimorphism among raptors, which becomes larger as diet changes from insects via reptiles and mammals to birds and as relative prey size increases. Symmetric parental roles and no female-biased size dimorphism would be expected in birds providing insects or other food that nestlings swallow without maternal assistance. Prey type and prey size would be important also for sexual conflicts and evolution of polygamy and mate desertion in birds providing for offspring.

Description: The reliability of sexual signaling may change across age classes due to shifts in resource allocation patterns. Two contrasting hypotheses exist regarding how the condition dependence of ornaments may shift with age, and both have received empirical support. On one hand, ornaments may more reliably reflect condition and quality in older individuals, because younger individuals of high quality invest in survival over signaling effort. On the other hand, the condition dependence of ornaments may decline with age, if older individuals in poor condition terminally invest in ornaments, or if resource constraints decline with age. Further, the expression and condition dependence of different ornaments may shift with age in unique ways, such that multifaceted sexual displays maintain reliable signaling across age classes. In yellow warblers ( Setophaga petechia ) of both sexes, we assessed how relationships between carotenoid- and phaeomelanin-based sexual pigmentation, prenesting body reserves, and condition at molt (reflected by growth bars and feather quality) vary across age classes. Melanin coverage correlated with condition at molt across age classes in males and showed high repeatability in both sexes. In contrast, carotenoid saturation increased longitudinally with age in males and correlated with condition at molt in different age classes in the 2 sexes. Specifically, carotenoid saturation correlated positively with condition at molt in younger, but not older males, whereas in females, the situation was reversed, with a positive correlation present only in older females. Results suggest that age-dependent signaling may promote maintenance of multifaceted sexual displays and that age-dependent signaling dynamics depend on sex.