The banded colour patterns of sea snakes discourage attack by predatory fishes, enabling Batesian mimicry by harmless species

The adaptive significance of large size at birth in marine snakes

Evolutionary shifts from one habitat type to another can clarify selective forces that affect life-history attributes. Four lineages of snakes (acrochordids and three clades within the Elapidae) have invaded marine habitats, and all have larger offspring than do terrestrial snakes. Predation by fishes on small neonates offers a plausible selective mechanism for that shift, because ascending to breathe at the ocean surface exposes a marine snake to midwater predation whereas juvenile snakes in terrestrial habitats can remain hidden. Consistent with this hypothesis, snake-shaped models moving through a coral-reef habitat in New Caledonia attracted high rates of attack by predatory fishes, and small models (the size of neonatal terrestrial snakes) were attacked more frequently than were large models (the size of neonatal sea snakes). Vulnerability to predatory fishes may have imposed strong selection for increased offspring size in marine snakes.

Carrying eggs uphill: are costs of reproduction stronger on steeper slopes?

Locomotor impairment during pregnancy is a well-documented cost of reproduction, but most empirical studies have not incorporated ecological complexity, such as locomotion on sloping inclines rather than horizontal surfaces. Biomechanical factors suggest that carrying a heavy burden-including shifts in the body's centre of mass-may impair locomotor ability even more when an animal is running uphill. If so, then measuring costs of reproduction on horizontal racetracks may underestimate these costs in nature for arboreal species. To evaluate this prediction, we measured the pregnancy-induced reduction in speed for jacky dragons (Amphibolurus muricatus) at inclines ranging from 0 to 45°. Both pregnancy and steeper slopes reduced lizard performance, but pregnancy did not exacerbate the locomotor decrement on steeper racetracks. An ability to maintain mobility on steep slopes during pregnancy may be a target of selection in arboreal taxa. To understand the evolutionary context of locomotion-based costs of reproduction, we also need studies on the relationship between organismal performance and ecologically relevant measures such as predation risk.

The banded colour patterns of sea snakes discourage attack by predatory fishes, enabling Batesian mimicry by harmless species

The evolution of bright 'warning' colours in nontoxic animals often is attributed to mimicry of toxic species, but empirical tests of that hypothesis must overcome the logistical challenge of quantifying differential rates of predation in nature. Populations of a harmless sea snake species (Emydocephalus annulatus) in New Caledonia exhibit colour polymorphism, with around 20% of individuals banded rather than melanic. Stability in that proportion over 20 years has been attributed to Batesian mimicry of deadly snake species by banded morphs of the harmless taxon. This hypothesis requires that banded colours reduce a snake's vulnerability to predation. We tested that idea by pulling flexible snake-shaped models through the water and recording responses by predatory fish. Black and banded lures attracted similar numbers of following fish, but attacks were directed almost exclusively to black lures. Our methods overcome several ambiguities associated with experimental studies on mimicry in terrestrial snakes and support the hypothesis that banded colour patterns reduce a non-venomous marine snake's vulnerability to predation.