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Cameron, E. Z. (2004). Facultative adjustment of mammalian sex ratios in support of the Trivers-Willard hypothesis: evidence for a mechanism. Proc Biol Sci, 271(1549), 1723–1728.
Abstract: Evolutionary theory predicts that mothers of different condition should adjust the birth sex ratio of their offspring in relation to future reproductive benefits. Published studies addressing variation in mammalian sex ratios have produced surprisingly contradictory results. Explaining the source of such variation has been a challenge for sex-ratio theory, not least because no mechanism for sex-ratio adjustment is known. I conducted a meta-analysis of previous mammalian sex-ratio studies to determine if there are any overall patterns in sex-ratio variation. The contradictory nature of previous results was confirmed. However, studies that investigated indices of condition around conception show almost unanimous support for the prediction that mothers in good condition bias their litters towards sons. Recent research on the role of glucose in reproductive functioning have shown that excess glucose favours the development of male blastocysts, providing a potential mechanism for sex-ratio variation in relation to maternal condition around conception. Furthermore, many of the conflicting results from studies on sex-ratio adjustment would be explained if glucose levels in utero during early cell division contributed to the determination of offspring sex ratios.
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Bugnyar, T., Stöwe, M., & Heinrich, B. (2004). Ravens, Corvus corax, follow gaze direction of humans around obstacles. Proc. Roy. Soc. Lond. B Biol. Sci., 271(1546), 1331–1336.
Abstract: The ability to follow gaze (i.e. head and eye direction) has recently been shown for social mammals, particularly primates. In most studies, individuals could use gaze direction as a behavioural cue without understanding that the view of others may be different from their own. Here, we show that hand–raised ravens not only visually co–orient with the look–ups of a human experimenter but also reposition themselves to follow the experimenter's gaze around a visual barrier. Birds were capable of visual co–orientation already as fledglings but consistently tracked gaze direction behind obstacles not before six months of age. These results raise the possibility that sub–adult and adult ravens can project a line of sight for the other person into the distance. To what extent ravens may attribute mental significance to the visual behaviour of others is discussed.
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Griffiths, S. W., Brockmark, S., Höjesjö, J., & Johnsson, J. I. (2004). Coping with divided attention: the advantage of familiarity. Proc. Roy. Soc. Lond. B Biol. Sci., 271(1540), 695–699.
Abstract: The ability of an animal to perform a task successfully is limited by the amount of attention being simultaneously focused on other activities. One way in which individuals might reduce the cost of divided attention is by preferentially focusing on the most beneficial tasks. In territorial animals where aggression is lower among familiar individuals, the decision to associate preferentially with familiar conspecifics may therefore confer advantages by allowing attention to be switched from aggression to predator vigilance and feeding. Wild juvenile brown trout were used to test the prediction that familiar fishes respond more quickly than unfamiliar fishes to a simulated predator attack. Our results confirm this prediction by demonstrating that familiar trout respond 14% faster than unfamiliar individuals to a predator attack. The results also show that familiar fishes consume a greater number of food items, foraging at more than twice the rate of unfamiliar conspecifics. To the best of our knowledge, these results provide the first evidence that familiarity–biased association confers advantages through the immediate fitness benefits afforded by faster predator–evasion responses and the long–term benefits provided by increased feeding opportunities.
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Hunt, G. R., & Gray, R. D. (2004). The crafting of hook tools by wild New Caledonian crows. Proc. Roy. Soc. Lond. B Biol. Sci., 271, S88–S90.
Abstract: The 'crafting' of tools involves (i) selection of appropriate raw material, (ii) preparatory trimming and (iii) fine, three-dimensional sculpting. Its evolution is technologically important because it allows the open-ended development of tools. New Caledonian crows manufacture an impressive range of stick and leaf tools. We previously reported that their toolkit included hooked implements made from leafy twigs, although their manufacture had never been closely observed. We describe the manufacture of 10 hooked-twig tools by an adult crow and its dependent juvenile. To make all 10 tools, the crows carried out a relatively invariant three-step sequence of complex manipulations that involved (i) the selection of raw material, (ii) trimming and (iii) a lengthy sculpting of the hook. Hooked-twig manufacture contrasts with the lack of sculpting in the making of wooden tools by other non-humans such as chimpanzees and woodpecker finches. This fine, three-stage crafting process removes another alleged difference between humans and other animals.
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Gardner, A., West, S. A. (2004). Cooperation and Punishment, Especially in Humans. Americ. Natur., 164(6), 753–764.
Abstract: Explaining altruistic cooperation is one of the greatest
challenges faced by sociologists, economists, and evolutionary biologists.
The problem is determining why an individual would carry
out a costly behavior that benefits another. Possible solutions to this
problem include kinship, repeated interactions, and policing. Another
solution that has recently received much attention is the threat
of punishment. However, punishing behavior is often costly for the
punisher, and so it is not immediately clear how costly punishment
could evolve. We use a direct (neighbor-modulated) fitness approach
to analyze when punishment is favored. This methodology reveals
that, contrary to previous suggestions, relatedness between interacting
individuals is not crucial to explaining cooperation through punishment.
In fact, increasing relatedness directly disfavors punishing
behavior. Instead, the crucial factor is a positive correlation between
the punishment strategy of an individual and the cooperation it
receives. This could arise in several ways, such as when facultative
adjustment of behavior leads individuals to cooperate more when
interacting with individuals who are more likely to punish. More
generally, our results provide a clear example of how the fundamental
factor driving the evolution of social traits is a correlation between
social partners and how this can arise for reasons other than genealogical
kinship.
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Yacoub Khallad. (2004). Conceptualization in the pigeon: What do we know? International Journal of Psychology, 39, 73–94.
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PNR Powers[1], & AJ Harrison[1]. (2004). Influences of a rider on the rotation of the horse-rider system during jumping. Equine and Comparative Exercise Physiology, 1, 33–40.
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Paz-y-Miño C. G., Bond, A. B., Kamil, A. C., & Balda, R. P. (2004). Pinyon jays use transitive inference to predict social dominance. Nature, 430(7001), 778–781.
Abstract: Living in large, stable social groups is often considered to favour the evolution of enhanced cognitive abilities, such as recognizing group members, tracking their social status and inferring relationships among them. An individual's place in the social order can be learned through direct interactions with others, but conflicts can be time-consuming and even injurious. Because the number of possible pairwise interactions increases rapidly with group size, members of large social groups will benefit if they can make judgments about relationships on the basis of indirect evidence. Transitive reasoning should therefore be particularly important for social individuals, allowing assessment of relationships from observations of interactions among others. Although a variety of studies have suggested that transitive inference may be used in social settings, the phenomenon has not been demonstrated under controlled conditions in animals. Here we show that highly social pinyon jays (Gymnorhinus cyanocephalus) draw sophisticated inferences about their own dominance status relative to that of strangers that they have observed interacting with known individuals. These results directly demonstrate that animals use transitive inference in social settings and imply that such cognitive capabilities are widespread among social species.
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Shettleworth, S. J. (2004). Cognitive science: rank inferred by reason. Nature, 430(7001), 732–733.
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Fenton, B., & Ratcliffe, J. (2004). Animal behaviour: eavesdropping on bats. Nature, 429(6992), 612–613.
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