Heart rates increase after hatching in two species of Natricine snakes

Abstract

Experimental studies have shown heart rates to decrease from embryo to hatchling stage in turtles, remain steady in skinks, and increase in birds. However, no snake species has been studied in this regard. I recorded heart rate evolution trajectories from embryo to juvenile stage in 78 eggs from two species of European Natricine snakes. Unexpectedly, snakes behaved more like birds than turtles or lizards: heart rates increased after hatching in both N. maura and N. natrix, respectively by 43.92 ± 22.84% and 35.92 ± 24.52%. Heart rate shift was not related to an abrupt elevation of metabolism per se (snakes that increased their heart rates the most sharply grew the least after birth), but rather due to a number of smaller eggs that experienced lower than normal heart rates throughout the incubation and recovered a normal heart rate post-birth. This finding is discussed in the light of hatching synchrony benefits.

Figure 1. Mass evolution was measured in 28 eggs and hatchlings of the grass snake N. natrix (open circles) and 51 eggs and hatchlings Viperine snake N. maura (grey triangles).

A repeated measure ANOVA with species as factor and egg/hatchling mass as the repeated measure over time yielded F_{7, 469} = 7.38; P < 0.001. Egg masses were measured every week. Snake masses were measured at birth and 3 weeks old. Snakes were unfed. Means ± SE are plotted.

Figure 2. Heart rates were measured in 28 eggs and hatchlings N. natrix (open circles) and 51 eggs and hatchlings N. maura (grey triangles).

A repeated measure ANOVA with species as factor and heart beats as the repeated measure over time (every 10 days from laying and in hatchlings aged 24.35 ± 3.74 days) yielded: effect of species F_{1, 76} = 0.80, P = 0.38; effect of time F_{4, 304} = 101.66, P < 0.001; interaction term F_{4, 304} = 8.40, P < 0.001. Hatching occurred between the 41^st and 45^th days post laying (dash box). All measurements were made at a constant 28°C. Means ± SE are plotted.

Figure 3. Linear regression of mean embryo heart rates against mean egg mass in 28 N. natrix eggs (R = 0.54; F_{1, 26} = 10.66; P < 0.003).

Figure 4. Relative amplitude of the shift in heart rates (% of increase) against heart rates averaged across the incubation period in N. natrix (open circles; R = 0.57; F_{1, 26} = 12.42; P < 0.002) and N. maura (grey triangles; R = 0.58; F_{1, 48} = 24.75; P < 0.001).