Testing sexual and other selection pressures influencing male size in sexually size dimorphic spiders

Quinones Lebron, Shakira Guani (author), Kuntner, Matjaž (mentor), Kralj Fišer, Simona (comentor)

Sexual size dimorphism (SSD) refers to the phenotypic difference in body size between males and females and it has puzzled scientists since Darwin first described it in 1871. The processes underlying the evolution of SSD have been extensively studied in vertebrates, where male-male competition commonly favours larger males. In many arthropod groups, however, the females of many species are much larger than males. Several hypotheses have been proposed to explain the evolution of SSD in spiders, including natural and sexual selection pressures. The hypotheses related to natural selection pressures include the fecundity hypothesis, the differential mortality hypothesis, and the gravity hypothesis. Here, we investigate the interplay of several natural and sexual selection pressures influencing male size in SSD spiders, specifically in Nephilingis cruentata. We found no directional selection towards smaller male sizes in most of the tested hypotheses. Small male sizes do not confer an advantage in either climbing or bridging ability. In fact, larger males are faster climbers. Similarly, we found no correlation between size and time of maturation. In other words, earlier maturation does not pose a size constraint in males and we therefore reject the scramble competition hypothesis as a possible explanation for the maintenance of small male sizes. Surprisingly, male size heritability is low, and depended mostly on food supply and maternal effects. Furthermore, we found evidence for a dynamic trade-off between size and fitness, mediated by sexual cannibalism. Larger males are more likely to get cannibalized, truncating their future mating opportunities. However, cannibalized males transferred more sperm. The fitness of smaller males almost doubles when cannibalized, but they are less likely than larger males to be attacked by females. Both the rate of sexual cannibalism and the occurrence of male-male contests were relatively low. Generally, larger males have an advantage when fighting off rivals, however fights are rare. Also, the presence of a rival, regardless of his size, did not affect male mating success. In fact, most of the time, males ignored the presence of a competitor. Lastly, we found that the co-occurrence of Trichonephila species suggests reproductive interference. Larger males might be outcompeting both heterospecific and conspecific rivals, but this hypothesis remains to be tested. In conclusion, male size is a highly variable trait that is maintained by both sexual selection and developmental plasticity.

• Language: English
• Keywords: Sexual size dimorphism, developmental plasticity, sexual selection, natural selection, Nephilidae
• Work type: Doctoral dissertation
• Organization: BF - Biotechnical Faculty
• Year: 2020
• PID: 20.500.12556/RUL-115280
• COBISS.SI-ID: 14590723
• Publication date in RUL: 22.04.2020

Secondary language

• Language: Slovenian
• Title: Testiranje spolnih in drugih selekcijskih pritiskov, ki vplivajo na velikost samcev pajkov s spolnim velikostnim dimorfizmom

Abstract

Spolni velikostni dimorfizem (SSD) se nanaša na fenotipsko razliko v telesni velikosti med samci in samicami, znanstvenike pa je fasciniral že odkar ga je Darwin prvič opisal leta 1871. Procesi, na katerih temelji evolucija SSD, so podrobno raziskani pri vretenčarjih, kjer kompeticija med samci običajno daje prednost večjim samcem. V številnih skupinah členonožcev pa najdemo vrste, pri katerih so samice veliko večje od samcev. Evolucijo SSD pri pajkih razlaga več hipotez, ki vključujejo naravne in spolne selekcijske pritiske. Hipoteze, ki se nanašajo na naravno selekcijo, vključujejo hipotezo plodnosti, hipotezo o različni smrtnosti in gravitacijsko hipotezo. V tej nalogi raziskujemo medsebojno delovanje več naravnih in spolnih selekcijskih pritiskov, ki vplivajo na velikost samcev pri pajkih z SSD, natančneje Nephilingis cruentata. Za večino testiranih hipotez nismo našli usmerjene selekcije za izbor samcev manjše telesne velikosti. V nasprotju s pričakovanji so večji samci celo boljši plezalci. Prav tako v nasprotju s pričakovani nismo našli korelacije med telesno velikostjo in časom odraščanja. Drugače povedano, zgodnje odraščanje pri samcih za telesno velikost ne predstavlja omejitve, zato zavračamo hipotezo o kompeticiji za samice kot možno razlago za vzdrževanje majhne telesne velikosti pri samcih. Presenetljivo je dednost velikosti pri samcih nizka, odvisna pa je predvsem od preskrbe s hrano in materinega učinka. Našli smo tudi podporo za dinamičen kompromis med telesno velikostjo in biološkim fitnesom, ki je odvisen od spolnega kanibalizma. Verjetnost spolnega kanibalizma je višja za večje samce, kar sicer izniči njihove naknadne paritvene možnosti, vendar so kanibalizirani samci prenesli več sperme. Biološki fitnes kanibaliziranih majhnih samcev se skoraj podvoji, vendar je v primerjavi z večjimi samci manj verjetno, da jih bo samica napadla. Tako stopnja spolnega kanibalizma, kakor pojavnost kompeticije med samci je bila razmeroma nizka. V splošnem so imeli večji samci v neposrednih spopadih s tekmeci prednost, čeprav so takšni spopadi redki. Prav tako prisotnost tekmeca, ne glede na njegovo velikost, ne vpliva na samčev uspeh parjenja. Ugotovili smo tudi, da sočasni pojav več vrst rodu Trichonephila, verjetno pomeni reproduktivno interferenco. Večji samci verjetno izpodrivajo tako heterospecifične kot konspecifične tekmece, vendar je to hipotezo še treba testirati. Sklenemo lahko, da je velikost samcev variabilna lastnost, ki jo vzdržujejo tako spolna selekcija kot razvojna plastičnost.

• Keywords: Spolni velikostni dimorfizem, razvojna plastičnost, spolna selekcija, naravna selekcija, Nephilidae