Competition over mates takes many forms and has far-reaching consequences for many organisms. Recent work suggests that relative reproductive rates of males and females, sperm competition and quality variation among mates affect the strength of sexual selection. Song, other display, body size, visual ornaments and material resource offerings are often sexually selected. There is much empirical evidence of mate choice, and its evolution is clarified by mathematical models. Recent advances in theory also consider costs of choice, effects of deleterious mutations, fast and slow evolution of preferences and preferred traits, and simultaneous preferences for several traits. Contests over mates are important; so is sperm competition, scrambles, endurance rivalry, and coercion. The latter mechanisms have received less attention than mate choice. Sexual selection may explain puzzling aspects of plant pollination biology.

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In spite of strong evidence for viability-based sexual selection and sex ratio adjustments, the blue tit, Parus caeruleus, is regarded as nearly sexually monomorphic and no epigamic signals have been found. The plumage coloration has not, however, been studied in relation to bird vision, which extends to the UV-A waveband (320^400 nm). Using molecular sex determination and UV/VIS spectrometry, we report here that blue tits are sexually dichromatic in UV/blue spectral purity (chroma) of the brilliant crown patch. It is displayed in courtship by horizontal posturing and erected nape feathers. A previously undescribed sexual dimorphism in crown size (controlling for body size) further supports its role as an epigamic ornament. Against`grey-brown' leaf litter and bark during pair formation in early spring, but also against green vegetation, UV contributes strongly to conspicuousness and sexual dimorphism. This should be further enhanced by the UV/bluish early morning skylight (`woodland shade') in which blue tits display. Among 18 breeding pairs, there was strong assortative mating with respect to UV chroma, but not size, of the crown ornament. We conclude that blue tits are markedly sex dimorphic in their own visual world, and that UV/violet coloration probably plays a role in blue tit mate acquisition.

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1 1 1 hrwin's iind Fisher's throry ofsexual selection, frmalrs prcfrr ornnmrnted males; thr rvolution of larger orn~inirnts is 1iiiiitc.d ti) increasrd mortality, lor rxsmplr through prrdation. An adornment of given sizr should often raise mort;ilii> morr in low tlxin in high quality phenotypes. Possible consrqurncrs Ibr tlir c.volution and optimal sizr of ornament^ are herr examined with mathrmatical nlotlcls.Fisher suggrstrd that ;in ormiime~it m ; i } rvol\-c ifit initially improves male survival. Fern;ilr prefrrrnre then sprr.ids Ii)r ;idorncd malrs. Their conseqiirnt iniiting .idvantage furthcrs propagation of thr prrli.rrcd trait. ; h i iiltcrmtive, tlir 'handicqi mrrhmism'. suggests that only those males brst :ihlr to survive c.in do s o \\ i t h II large, h;indic:ipping ortimicnt. Chwsirig ;idorncd malrs. li.m;iles niiglic therchrc I)r;ir olIspring with high genrr;il survivorship. h i t sons also inhrrit thr h;indicapping ornmirnt. \Vlirn it rcduces survival more in low than i n high quality phenotyprs. thr handicap niec1i;inisrn 1 iii conjunciion with the Fislierian mating advantage) bccomes piwrrfiil with lower lirritdiilit) of litwss than previously suppised. I t still rrquirrs that fitncss has some hrritaldity, fi)r which thrrr is indircrt rvidrnce. hut lield meiisurrmrnts iirr larking.In 1)arwin'k mid Fisher's theory. hoth inatiiig advant:igr arid mortality disiidviintagr inrrriise with orn,imcnt side, and 1iaI;inrr :it its optiniiil drvrlopinrnt. I f ;I given iidtmimcnt rrducrs survivd most in l(iw qu;ilit! phriintyim, the optimum incrr~isrs with plicnotypic qualit!. Ornamrnt sizr thrn ma! providr ;I mc;isurc of fitness. nnd c;in I)r used in mate choice. A male can usually not gain litnrss 11) dcvrlopiiig liir-ger ;tdorninrnts thiiii other males or the same quality.Ornametits c;in pr(~b;ibl). itlso evolve through dirert competition brtwern malrs, without frmale prrfrrcncrs for adorned m;ilrs. Field rxprrimcnts iirr nrrdcd to rlaril) this and other q i r c t s of srxual srlrrtion.