Climate effects on nesting phenology in Nebraska turtles

A frequent response of organisms to climate change is altering the timing
of reproduction. In particular, advancement of reproductive timing has
been a common response to warming temperatures in temperate regions. Over
the past three decades in Nebraska, USA, the timing of nesting of the
Common Snapping Turtle (Chelydra serpentina) was negatively correlated
with mean December maximum temperatures of the preceding year and mean May
minimum and maximum temperatures in the nesting year, and positively
correlated with precipitation in July of the previous year. Increased
temperatures during the late winter and spring likely permit earlier
emergence from hibernation, increased metabolic rates and feeding
opportunities, and accelerated vitellogenesis, ovulation, and egg
shelling, all of which could drive earlier nesting. However, for the
Painted Turtle (Chrysemys picta), the timing of nesting was positively
correlated with mean minimum temperatures in September, October, December
of the previous year, February of the nesting year, and April
precipitation. These results suggest warmer fall and winter temperature
may impose an increased metabolic cost to painted turtles that impedes
fall vitellogenesis, and April rains may slow the completion of
vitellogenesis through decreased basking opportunities. For both species,
nest deposition was highly correlated with body size, where larger females
nested earlier in the season. Although average annual environmental
temperatures have increased over the last four decades at our study site,
spring temperatures have not yet increased, and hence, nesting phenology
has not advanced at our site for Chelydra. While Chrysemys exhibited a
weak trend toward later nesting, this response was likely due to increased
recruitment of smaller females into the population due to nest protection
and predator control (Procyon lotor) in the early 2000s. Should climate
change result in an increase in spring temperatures, nesting phenology
would presumably respond accordingly, conditional on body size variation
within these populations.