Males, on the other hand, are "heterogametic," with one X chromosome and one Y chromosome. For example, in many turtle species, eggs from cooler nests hatch as all males, and eggs from warmer nests hatch as all females. If such a snail is removed from its attachment, it will become female.
The results, which found evidence of TSD in four out of five species, confirmed that some vertebrate species exhibit TSD. TSD may be advantageous and selected for in turtles, as embryo energy efficiency and hatchling size are optimized for each sex at single-sex incubation temperatures and are indicative of first-year survivorship.
Hormones and temperature show signs of acting in the same pathway, in that less hormone is required to produce a sexual shift as the incubation conditions near the pivotal temperature. Occurring during the middle third of incubation, the TSP lasts for days, species-dependent . An Introduction to Behavioral Endocrinology.
New Jersey: Pearson Education, One possible explanation that TSD is common in amniotes is phylogenetic inertia — TSD is the ancestral condition in this clade and is simply maintained in extant lineages because it is currently adaptively neutral or nearly so.
The lizards were female in appearance and were capable of bearing offspring, despite having the ZZ chromosomes usually associated with male lizards. Berlin: Springer-Verlag,
Individuals with low levels of aromatase during the thermosensitive period will develop male characteristics. Nature has provided many variations on her masterpiece. The highlands are colder with a higher magnitude of annual temperature fluctuation and a shorter activity season, delaying maturity, thus GSD is favored so sex ratios are not skewed.
Reptiles in which both incubation temperature and sex chromosomes interact to determine sex may represent "transitional" evolutionary states between two end points: complete GSD and complete TSD. Other species show disproportionately high female production at both high and low temperatures, with intermediate temperatures causing mostly male development.
The scope of the model for other reptiles, however, requires further research. As of , sixty-five of seventy-nine tested species of turtles were found to exhibit TSD. Very near or at the pivotal temperature of sex determination, mixed sex ratios and more rarely intersex individuals. Washington: Smithsonian Books, Genetics Selection Evolution.