Male–male aggression peaks at intermediate relatedness in a social spider mite

Abstract

Theory predicts that when individuals live in groups or colonies, male-male aggression peaks at intermediate levels of local average relatedness. Assuming that aggression is costly and directed toward nonrelatives and that competition for reproduction acts within the colony, benefits of aggressive behavior are maximized in colonies with a mix of related and unrelated competitors because aggression hurts nonkin often, thereby favoring reproduction of kin. This leads to a dome-shaped relation between male-male aggression and average relatedness. This prediction has been tested with bacteria in the laboratory, but not with organisms in the field. We study how male-male aggression varies with relatedness in the social spider mite Stigmaeopsis miscanthi. We sampled 25 populations across a wide geographic range between Taiwan and Japan, representing a gradient of high to low within-population relatedness. For each population the weaponry of males was measured as the length of the first pair of legs, and male-male aggression was tested by placing pairs of nonsibling males together and scoring the frequency of male death over a given period. As these two morphological and behavioral variables correlate strongly, they both reflect the intensity of male-male conflict. Our data on the social spider mite show that male-male aggression as well as weapon size strongly peak at intermediate, average relatedness, thereby confirming theoretical predictions.

Molecular identification of seven species of the genus Stigmaeopsis (Acari: Tetranychidae) and preliminary attempts to establish their phylogenetic relationship

“…2). Sakagami et al (2009) Two behavioral phenotypes of S. miscanthi, the low-aggression (LW) and high-aggression (HG) phenotypes based on male aggressiveness, were previously reported (Saito & Sahara 1999;Sato et al 2008Sato et al , 2013. The two phenotypes were basically shown to be two separate clades based on their nuclear 28S gene (Sakagami et al 2009) and COI gene of mtDNA (Ito & Fukuda 2009).…”

“…S. miscanthi has two behavioral phenotypes based on differences in male pugnacity against conspecific males. The two phenotypes have been called the low-aggression (LW) phenotype and the high-aggression (HG) phenotype (Saito & Sahara 1999;Sato et al 2008Sato et al , 2013, and these phenotypes formed distinct groups in phylogenetic trees of the 28S rRNA gene (Sakagami et al 2009) and COI gene (Ito & Fukuda 2009;Ito et al 2011). S. longus also forms two distinct groups in a phylogenetic tree based on the COI gene; one is distributed in the northern part of Japan (from Hokkaido Prefecture to Yamagata Prefecture) and the other is distributed in the southern part of Japan (from Yamagata Prefecture to the Kyushu district), suggesting that there were two routes of expansion of this species (Ito et al 2011).…”

Phylogeography of lethal male fighting in a social spider mite

“…Nevertheless, in several species, males do fight to the death (Crespi, 1988;Hamilton, 1979;Saitō, 1990;Zenner, O'Callaghan, & Griffin, 2014). Why and how such lethal male fight evolves may be explained by several adaptive mechanisms, for example, extremely high benefit that each mating event contributes to lifetime reproduction of the males (Enquist & Leimar, 1990), low encounter rate with male competitors (Innocent, West, Sanderson, Hyrkkanen, & Reece, 2011;Murray, 1987Murray, , 1989, and difference in genetic relatedness among males (Hamilton, 1979;Kapranas, Maher, & Griffin, 2016;Reinhold, 2003;Saito, 1995;Sato, Egas, Sabelis, & Mochizuki, 2013). Few studies have addressed a reconstruction of the evolutionary process, despite both approaches being important for understanding evolution of lethal fighting.…”

“…Males are aggressive not only against predators but also against other conspecific males inside the nests and even kill rival males to establish their own harem ( Figure 1b). The frequency of male killing (hereafter, male-male aggressiveness) varies among populations, and male-male aggressiveness is low in colder regions, high in warmer regions, and mild in subtropical regions in Japan (Saito, 1995;Saito & Sahara, 1999;Sato, Egas, et al, 2013). Populations with low and high aggressiveness are discriminated as LW and HG forms, because of their behavioral, ecological, and morphological differences (Saito & Sahara, 1999;Saito, Sakagami, & Sahara, 2002;Yano, Saito, Chittenden, & Sato, 2011), their molecular phylogeny (Ito & Fukuda, 2009;Sakagami, Saito, Kongchuensin, & Sahara, 2009;Sakamoto et al, 2017) and also incomplete but strong reproductive isolation between them (Sato, Breeuwer, Egas, & Sabelis, 2015;Sato, Saito, & Mori, 2000a, 2000b.…”

“…Populations with low and high aggressiveness are discriminated as LW and HG forms, because of their behavioral, ecological, and morphological differences (Saito & Sahara, 1999;Saito, Sakagami, & Sahara, 2002;Yano, Saito, Chittenden, & Sato, 2011), their molecular phylogeny (Ito & Fukuda, 2009;Sakagami, Saito, Kongchuensin, & Sahara, 2009;Sakamoto et al, 2017) and also incomplete but strong reproductive isolation between them (Sato, Breeuwer, Egas, & Sabelis, 2015;Sato, Saito, & Mori, 2000a, 2000b. Recently, populations with mild male-male aggressiveness and intermediate male weapon morph were found (Sato, Egas, et al, 2013), yet their ecological and phylogenetic relationships with the LW and HG forms are unclear . Using a different set of sampled populations, Sakamoto et al, (2017) identified a third clade in S. miscanthi, which consisted of a population collected from Fuzhou, China, by phylogenetic analysis based on COI and 18S-28S genes.…”