Is sexual brutality maladaptive under high population density?

Abstract

Coercive copulation is frequent in promiscuous mating systems, generating both benefits (e.g. higher copulation rate) and costs (e.g. injuries and fewer foraging opportunities). The negative consequences of sexual harassment are expected to increase with increasing population density with male-biased operational sex ratio (OSR). This study offers an example in which the frequency and severity of the injuries inflicted on female tortoises during forced copulations increased with population density and biased OSR. Male Hermann's tortoises (Testudo hermanni) harass females during courtship, ramming and biting them until they eventually force mating. They also use their horny and mobile tail to stimulate females, sometimes damaging their cloaca. We compared the frequency and severity of cloacal injuries between two populations in the Republic of Macedonia. Under high density and balanced OSR at one site (Konjsko), 25% of females exhibited cloacal injuries. Under very high density and extremely biased OSR at another site (Golem Grad Island), 75% of females were wounded, often severely. Furthermore, exclusively on Golem Grad, many small and immature females were courted and wounded. This behavioural shift toward juveniles generated costs without benefits. Perhaps brutal reproductive behaviours in male Hermann's tortoises evolved under low or moderate population densities? Under very high densities, coercive mating seems maladaptive, negatively affecting females and possibly further skewing OSR.

Male coercion and female injury in a sexually cannibalistic mantis

“…Male traits that facilitate coercive matings have the potential to harm females [4]. Indeed, male-induced injury has been documented in a wide range of taxa, including mammals [4,47], birds [49], reptiles [35,50,51], insects [29,30,34,37,38] and molluscs [31,52]. Harmful coercion also occurs in sexual cannibals, but is rare.…”

“…[ [29][30][31][32][33][34][35][36][37][38][39] or a result of selection favouring the harmful trait in an unrelated context [1,40].…”

Evolving social dynamics prime thermal tolerance during a poleward range shift

“…Social stress associated with high densities probably reflects both increased predation pressure and competition for food, basking and oviposition sites (Pechenik, 2006). For adult females, high densities also reflect more intense mating harassment overall (Golubović et al, 2018). Van Gossum et al (2001 found that mating harassment rates, and particularly the tendency of males to target common female morphotypes, increased at high population densities, suggesting that common morphs, and particularly gynochromes (Rivera & Sánchez-Guillén, 2007), will be most prone to experiencing the cold tolerance benefit of density-induced social stress.…”

Inside the shell: body composition of free-ranging tortoises (Testudo hermanni)

“…From 2008 until 2019, we studied the ecology and morphology of tortoises in two dense populations (> 20 adults / ha) of T. hermanni in two nearby sites (~4.5 km away from each other) of the Prespa Lake region in North Macedonia: Golem Grad Island (GG, ~18 ha; N 40°52'; E 20°59'), and Konjsko village (KV, ~20 ha, N 40°54'; E 20°99') (Djordjević et al, 2011;Arsovski et al, 2018a;Golubović et al, 2013Golubović et al, , 2017Golubović et al, , 2018.…”