Weather and sex ratios of head-started Agassiz’s desert tortoise Gopherus agassizii juveniles hatched in natural habitat enclosures

Abstract

Head-starting of Agassiz's desert tortoise, a means to aid recovery of this threatened species, may adversely affect offspring sex ratios via temperature-dependent sex determination combined with possible unnatural thermal conditions in head-start facilities. We determined sex ratios in juvenile tortoises hatched from first clutches of 4 annual cohorts at the head-start facility at the US Marine Corps Base, Twentynine Palms, California, USA, using non-fatal, endoscopic inspection of gonads. Cohort sexes ranged from 97% females (f:m ratio of 6.25:1) in 2008 to 84% males (f:m ratio of 0.19:1) in 2009, apparently primarily in response to local weather conditions during the temperature-sensitive phase of incubation. Warmer weather during development of a second clutch laid in 2009 led to fewer males (55%, f:m ratio of 0.82). Efforts to cool (artificially shade) some nesting burrows were unsuccessful in increasing the proportion of male hatchlings in 2009. Cohort sex ratios were associated with average daily air temperatures during incubation, such that more females were produced during warmer periods, in good agreement with published temperaturecontrolled laboratory experiments. These results suggest that weather played a major role in determining sex ratios, with apparently smaller or negligible influences resulting from initial location, structure and operation of the head-start facility; experimental shading of nests; and individual mothers' variation in the timing of egg laying and placement of nests within the natal burrows. These results, obtained from a remote, mostly natural field site, indicate the potentially great sensitivity of sex determination in nests of wild, free-living desert tortoises to changes in climate.

Head-started Agassiz’s desert tortoises Gopherus agassizii achieved high survival, growth, and body condition in natural field enclosures

“…These 72 small pens served both as private enclosures for gravid females to nest and lay their eggs during springs of 2006 through 2013 and as pens for the females' hatchlings during their first winter. This allowed us to identify the mother of every hatchling (Nagy et al 2016). Subsequently, we connected some 7.7 × 7.7 m pens by removing small partition sections to form larger communal enclosures for entire juvenile year classes (or cohorts).…”

“…When necessary, native fire ant colonies were controlled with ant-species-specific poison bait in tortoise-proof stations. We used weather records from the nearby National Weather Service station, located approximately 10 km away and having similar elevation and topography (Nagy et al 2016), to calculate long-term average annual rainfall. Because desert rains can be localized, we installed 3 rain gauges within TRACRS to obtain more accurate precipitation data.…”

“…We avoided close monitoring of females to avoid influencing when and where they nested, and we did not search burrows for nests to avoid disrupting or altering egg placement and nest conditions. Both of these factors may influence incubation temperatures, potentially altering hatchling sex ratios (see Nagy et al 2016 and references therein) and nest success. Females ovipo sited after 1−4 wk at TRACRS, were offered water to drink, and were released to their home burrow.…”

Temperature influences resource use by chestnut‐bellied scaled quail

“…, Nagy et al. ). The relative importance of operative vs. ground surface temperature in resource selection is unknown for most species.…”

“…In ground-dwelling organisms (e.g., reptiles), ground surface temperature contributes to body temperature in through conduction (Bakken 1989). Soil surface temperatures are often hotter than operative temperatures during midday (Williams et al 2012), with soil surface temperatures in semiarid environments typically exceeding 50°C during summer (Fulbright et al 1995, Nagy et al 2016). The relative importance of operative vs. ground surface temperature in resource selection is unknown for most species.…”