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Function and Development of Status Signals |
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Signals—traits that benefit the bearer
because they provide information to others and affect their behavior—are of widespread
interest for students of sexual selection and of social behavior. My lab has
spent over 15 years studying putative signals in the plumage of male house
sparrows, most notably the black throat patch (‘bib’). We are interested in
both the function (the potential fitness costs and benefits of signaling) and
the development of this signal. Function: We have used both field studies and
experiments in aviaries to better understand the function of male plumage
traits. Our findings: 1. Field
studies reveal no effect of bib size on timing of breeding, quality of mate,
or paternity. 2.
Mate choice trials in aviaries
uncovered evidence of preferences for particular males, but no link with any
plumage trait. 3.
Field studies found a positive
correlation between bib size and age, and a positive correlation between age
and probability of retaining occupancy of a nest site, but little evidence
that bib size influenced occupancy (Morrison
et al. 2008). 4.
An aviary experiment found that males
manipulated to have larger bibs were more likely to become owners of a
nest-box (Morrison 2003). 5.
Field observations of winter-time
social interactions revealed that males with larger bibs were dominant to
both males and females. Large-bibbed males received less aggression from male
opponents, but surprisingly received more aggression from female opponents (Hein
et al. 2003). 6.
The relationship between bib size and
dominance is messy and is only sporadically repeatable in aviary experiments
(Sanh Kin Diep unpublished) 7. Motivation
interactions with bib size to affect aggressive behavior, and females respond
differently than males. Development: A signal should develop through
mechanisms that reliably incorporate the information of functional relevance
into the magnitude of the signal. The bib of the house sparrow is a puzzling
signal; it is produced anew once a year, cannot change size until the next
year, but changes appearance as white tips on the black feathers, formed
during molt, wear off over time. We have undertaken an array of experiments
to test alternative mechanisms for signal development: 1. Sparrows
placed on an artificial diet just before molt produced similar bib dimensions
regardless of the total protein content or the level of melanin precursor
amino acids. Feathers from birds on diets deficient in phenylalanine and
tyrosine produced paler feathers, indicating that melanin synthesis was
affected but diet was not determining bib size (Poston
et al. 2005). 2.
10-fold manipulations of copper
content in diet had no affect on any plumage trait. 3.
Manipulation of calcium content in
diet affected bib size, but not in the expected direction; birds with more
calcium had smaller bibs (Stewart
and Westneat 2010). This result has been repeated using
a different presentation of calcium. 4. Subjects
housed in situations where they received more aggression produced bibs of
similar size to subjects who experienced little or no aggression. |
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Useful links |
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