Yolk removal generates hatching asynchrony in snake eggs

Abstract

Hatching synchrony is wide-spread amongst egg-laying species and is thought to enhance offspring survival, notably by diluting predation risks. Turtle and snake eggs were shown to achieve synchronous hatching by altering development rates (where less advanced eggs may accelerate development) or by hatching prematurely (where underdeveloped embryos hatch concurrently with full-term embryos). In Natricine snakes, smaller eggs tend to slow down metabolism throughout incubation in order to hatch synchronously with larger eggs. To explore the underlying mechanism of this phenomenon we experimentally manipulated six clutches, where half of the eggs were reduced in mass by removing 7.2% of yolk, and half were used as the control. The former experienced higher heart rates throughout the incubation period, hatched earlier and produced smaller hatchlings than the latter. This study supports the idea that developmental rates are related to egg mass in snake eggs and demonstrates that the relationship can be influenced by removing yolk after egg-laying. The shift in heart rates however occurred in the opposite direction to expected, with higher heart rates in yolk-removed eggs resulting in earlier hatching rather than lower heart rates resulting in synchronous hatching, warranting further research on the topic.

Figure 1

Egg mass trajectories were recorded every 10 days from laying to hatching in 30 water snake eggs from which 7.2% of yolk had been removed shortly after oviposition (red triangles) and 29 control eggs (blue circles). A repeated measure two-factor Anova with treatment and clutch of origin as factors and the successive egg masses as the repeated measure yielded: interaction term: F_{5.34, 53.38} = 1.43, P = 0.22; effect of treatment over time F_{1.78, 53.38} = 1.99, P = 0.15; effect of treatment F_{1, 30} = 24.05, P < 0.0001. As the assumption for sphericity was violated (W = 0.051, χ² ₉ = 84.33; P < 0.0001), results were corrected using Greenhouse-Geisser adjustments. Means ± SE are plotted.

Figure 2

Early effects of yolk removal on the relationship between egg mass and embryo heart rates (incubation day 10) in yolk-removed (red triangles) and control water snake eggs. Heart rate and egg mass were strongly correlated in both treatments (see text for statistical details). While heart rate increased in response to yolk removal, the relationship between heart rate and egg mass was essentially unchanged. Means ± SE are plotted.

Figure 3

Heart rate trajectories were recorded from laying to hatching in yolk-removed (red triangles) and control water snake eggs (blue circles). A repeated measure two-factor Anova with clutch of origin and treatment as factors and 7 consecutive heart rate recordings as the repeated measure yielded: global interaction term F_{12.92, 137.81} = 1.32, P = 0.21; effect of heart rates over time interaction; F_{4.31, 1137.81} = 0.66, P = 0.63; effect of treatment F_{1, 32} = 10.48, P < 0.0028. As the assumption for sphericity was violated (W = 0.026, χ² ₂₀ = 39.67; P < 0.0001), results were corrected using Greenhouse-Geisser adjustments. Means ± SE are plotted.