According to the differential investment hypothesis, females paired with attractive mates are expected to invest more in the current reproduction relative to females paired with unattractive males. I experimentally tested this hypothesis in the peafowl (Pavo cristatus) by providing females with males that differed in sexual attractiveness. In agreement with the differential allocation hypothesis, females paired with more ornamented males laid larger eggs, and deposited higher amounts of testosterone into the egg yolk, independently of the sex of the embryo. Recently, I conduced experiments on a captive population of the Houbara bustard.
Animal communication involves a multitude of signals ranging from
morphological to behavioural traits. In spite of the diversity of traits used in
animal signalling, most studies of sexual selection have focused on single male
traits. Moreover, the two forces of sexual selection (male–male competition and
female preference) may target different traits and favour the diversification of male
signalling. Empirical and theoretical studies have only recently begun to examine how both processes of sexual selection (male–male competition and female choice)
may interact to promote the evolution of multiple signals (see Arnold & Wade
1984a,b and references therein). Several studies on the ring-necked pheasant,
Phasianus colchicus, tried to identify male traits used in male–male competition or
for attracting females but failed in establishing a clear distinction between them as
the same traits (including spur and tail length, wattle display duration) seem to be
used both in intra- and intersexual selection. However, in the yellow-browed leaf warbler, Phylloscopus inornatus, it has been shown that females use more than one character to choose their mates, and these traits differ from those used during male contests. Female scarlet-tufted malachite sunbirds, Nectarinia johnstoni, and female red-collared widowbirds, Euplectes ardens, show a strong preference for one signal (tail elongation), whereas another trait (pectoral feather tufts or the red collar respectively) is used during male agonistic interactions .
Male sexual advertisements affecting either male–male
competition or female choice often involve several
morphological and behavioral traits.
Much emphasis has been put on the role played by female
choice in the evolution of multiple male traits. Verbal and
mathematical models have been used to explore the different
scenarios under which multiple male traits are selected
and maintained by female mating preference.
Whereas some models have suggested that multiple male
traits are unlikely to be “handicap revealing”– unless the cost of mate choice is low – others have shown that good
gene selection can drive the evolution of multiple male
traits. Empirical tests of these predictions
are still rare. The
tail length and the red
carotenoid collar badge in red-collared widowbirds
(Euplectes ardens) were involved in inter- and intrasexual
selection respectively. Interestingly, the two traits
were negatively correlated and this trade-off might explain the coexistence of multiple honest signals. In other species, multiple male traits serve only in the process of female choice. In this case multiple displays could signal different properties of male condition (multiple message hypothesis).
The peacock has long been considered as a prime example of the strength of sexual selection on the evolution of multiple sexual traits with males exhibiting a complex array of morphological traits, including feather ornaments and colors, and behavioral displays. Train length and tarsus length are determinant in the process of territory establishment in the lek (intra-sexual selection), and females use the number of ocelli in the train as well as display activity as cues during mate choice (inter-sexual selection).
Soon to come