title: Macroevolutionary patterns of sexual size dimorphism in copepods.
abstract: Major theories compete to explain the macroevolutionary trends observed in sexual size dimorphism (SSD) in animals. Quantitative genetic theory suggests that the sex under historically stronger directional selection will exhibit greater interspecific variance in size, with covariation between allometric slopes (male to female size) and the strength of SSD across clades. Rensch's rule (RR) also suggests a correlation, but one in which males are always the more size variant sex. Examining free-living pelagic and parasitic Copepoda, we test these competing predictions. Females are commonly the larger sex in copepod species. Comparing clades that vary by four orders of magnitude in their degree of dimorphism, we show that isometry is widespread. As such we find no support for either RR or for covariation between allometry and SSD. Our results suggest that selection on both sexes has been equally important. We next test the prediction that variation in the degree of SSD is related to the adult sex ratio. As males become relatively less abundant, it has been hypothesized that this will lead to a reduction in both inter-male competition and male size. However, the lack of such a correlation across diverse free-living pelagic families of copepods provides no support for this hypothesis. By comparison, in sea lice of the family Caligidae, there is some qualitative support of the hypothesis, males may suffer elevated mortality when they leave the host and rove for sedentary females, and their female-biased SSD is greater than in many free-living families. However, other parasitic copepods which do not appear to have obvious differences in sex-based mate searching risks also show similar or even more extreme SSD, therefore suggesting other factors can drive the observed extremes.
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, UK Centre for Ocean Life, National Institute for Aquatic Resources, Technical University of Denmark, Kavalergården 6, Charlottenlund 2920, Denmark a.g.hirst@qmul.ac.uk.
authors.forename: Andrew G
authors.initials: AG
authors.lastname: Hirst
authors.affiliation: Centre for Ocean Life, National Institute for Aquatic Resources, Technical University of Denmark, Kavalergården 6, Charlottenlund 2920, Denmark.
authors.forename: Thomas
authors.initials: T
authors.lastname: Kiørboe
mesh_terms: D000818:Animals; D005075:Biological Evolution; D049628:Body Size; D033342:Copepoda / Q000033:anatomy & histology / Q000254:growth & development / Q000502:physiology*; D005260:Female; D008297:Male; D012727:Sex Characteristics; D015048:Zooplankton / Q000254:growth & development / Q000502:physiology
keywords: Copepoda; allometry; sex ratio; sexual size dimorphism
references.citation: Evolution. 1995 Apr;49(2):325-336
references.pmid: 28565015
references.citation: Oecologia. 2006 May;148(1):40-50
references.pmid: 16425044
references.citation: PLoS One. 2011 Apr 14;6(4):e18870
references.pmid: 21533149
references.citation: Nature. 1991 May 2;351(6321):58-60
references.pmid: 2027382
references.citation: Evolution. 1988 Mar;42(2):363-374
references.pmid: 28567841
references.citation: Trends Parasitol. 2006 Oct;22(10):475-83
references.pmid: 16920027
references.citation: Trends Ecol Evol. 1998 Apr 1;13(4):159-63
references.pmid: 21238243
references.citation: Am Nat. 2007 Feb;169(2):245-57
references.pmid: 17211807
references.citation: Evolution. 1996 Dec;50(6):2520-2523
references.pmid: 28565692
references.citation: Am Nat. 2013 Nov;182(5):630-9
references.pmid: 24107370
references.citation: Growth Dev Aging. 1992 Winter;56(4):269-81
references.pmid: 1487365
references.citation: Oecologia. 2010 Nov;164(3):627-35
references.pmid: 20811913
references.citation: Science. 1977 Jul 15;197(4300):215-23
references.pmid: 327542
references.citation: PLoS One. 2007 Sep 19;2(9):e897
references.pmid: 17878932