Thesis (PhD (Botany and Zoology))—University of Stellenbosch, 2009. / Females of many bird species prefer to mate with males exhibiting elaborate
ornamentation, which serves as an indicator of male quality. Such ornaments, called
secondary sexual traits, could act as signals to females that males could confer direct
and/or indirect genetic benefits (when offspring inherit superior genes), on offspring. In
particular, it has been suggested that these signals relate to male ability to resist infections,
as only high quality individuals are able to invest both in high immune defence and
elaborate ornament expression.
The ostrich (Struthio camelus) is the largest living bird and is a member of the family
of flightless birds, the ratites. They are sexually dimorphic, males displaying black
plumage, and a pink-coloured neck and bill; whereas females display dull-brown plumage
(both sexes have white feathers). Little is known about the mating system of ostriches: they
are promiscuous and in the wild, males and females have multiple partners. The communal
nesting system of ostriches is unique in that only the major female and major male provide
parental care, in the form of incubation and guarding the offspring until independence.
Furthermore, a remarkable feature of cohorts is that offspring may differ greatly in size,
and these size differences are likely to have a genetic basis arising from differing parental
genotypic differences.
As a trade-off between immune response and life-history traits has been documented
in various bird species, I examined the relationships between male secondary sexual traits
(and specifically colouration) and maternal investment; levels of immunocompetence in
both parents and chicks; and chick growth. This study showed that females invest more at
the egg stage in response to traits involved in the male courtship display: the colour of the
neck, white and black body feathers, and the brightness of black feathers. As these traits,
which are exposed during the courtship display as well as during male-male interactions,
were related to male immune responses, I suggest that only high quality males will be able to display their condition optimally. Chicks with higher growth rates were found to have
intermediate responses to stimulation of their humoral immune system with diphtheria and
tetanus vaccines, suggesting that not only fitness benefits, but also costs are associated
with mounting an immune response; and that variation in humoral responses and growth
rates relates to how individuals trade off these costs and benefits. In addition, chick
humoral responses were found to be related to the humoral response of both parents, but
through different antibody responses (maternal responses to tetanus and paternal responses
to diphtheria), suggesting that this component of the immune system is heritable. As the
colouration of white feathers predicted chick growth rates, as well as a male’s ability to
raise an antibody response, I suggest that this visual cue could serve as a signal to females
of male humoral immunocompetence, therefore forming the basis of mate choice whereby
females could increase the fitness of their offspring through higher growth rates.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/1257 |
| Date | 03 1900 |
| Creators | Bonato, Maud |
| Contributors | Cherry, Michael I., Evans, Matthew R., Cloete, Schalk W. P., University of Stellenbosch. Faculty of Science. Dept. of Botany and Zoology. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | University of Stellenbosch |
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