A prison effect in a wild population: a scarcity of females induces homosexual behaviors in males

Abstract

The high frequency of same-sex sexual behaviors (SSB) in free-ranging animals is an evolutionary puzzle because fitness benefits are often unclear in an evolutionary context. Moreover, the physiological and genetic underpinnings of SSB remain unclear. We exploited an extraordinary natural experiment to examine the impact of environmental factors (local sex ratio [SR]) and testosterone (T) levels on SSB in a dense population of Hermann's tortoises monitored for 7 years. Under the combination of high density and extremely skewed SR (~50 females, >1000 males), males courted and mounted other males more frequently than females. They even exhibited extravagant sexual behaviors, attempting to copulate with dead conspecifics, empty shells, and stones. T levels remained within the species' normal range of variation. SSB was not observed in other populations where SR is not, or less skewed, and where density is lower. This study reports the first natural example of a "prison effect," whereby a high population density combined with female deprivation triggered SSB as a mere outlet of sexual stimulation. More generally, it supports the hypothesis that SSB can be a nonadaptive consequence of unusual proximate factors rather than reflecting physiological disorders.

Covariates streamline age‐specific early life survival estimates of two chelonian species

“…Every captured individual was sexed when sexually dimorphic, aged, and permanently marked using a notche‐code on the marginal‐scutes (Cagle, ). In the course of recaptures, age was re‐estimated blindly (for error rate and more details on ageing and general CR methodology see Bonnet et al ., and Arsovski et al ., ).…”

“…In adult Hermann's tortoises sex‐specific survival is not common apart from few populations with a bias in sex‐ratio (Hailey & Willemsen, ). Interestingly, the Golem Grad tortoise population has the highest reported bias in sex‐ratio of this species, reaching its maximum on the Plateau where only 5% of all adult individuals are female (Bonnet et al ., ). Our best‐fit model implies significantly lower survival of younger females from this locality from birth (Fig.…”

“…During 9 years (2008–2016), we studied a very dense (~60 adults per hectare, Bonnet et al ., ) population on Golem Grad Island (Prespa Lake, Macedonia, ~18 ha, 40°52′09″N, 20°59′17″E). Each year the area was intensively patrolled (2–12 observers).…”

Sex-specific ecophysiological responses to environmental fluctuations of free-ranging Hermann's tortoises: implication for conservation

“…Adult population density is estimated to be 4.2 tortoises/hectare J-M. B. and S. C. (unpublished data). This value is categorized as moderately high for French continental areas (Celse et al ., 2014) but is low compared with less impacted areas (Bonnet et al ., 2016b). This study site is relatively flat, with a mean elevation of 270 m (maximum 295 m).…”

Is sexual brutality maladaptive under high population density?

“…Males are smaller than females; they achieve copulation through intensive harassment, and sometimes damage the female's cloacal region with their sharp horny tail (Hailey, 1990). In this species OSR is generally biased towards males (on average 1F/3.2M, N = 21 populations; reviewed by Bonnet et al, 2016). In Greek populations, harassment of females intensified with an increase in density of males while OSR was strongly male biased: 1F/6.7M.…”