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

Abstract

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.

The adaptive significance of large size at birth in marine snakes

“…An observer (RS) swam 3 to 5 m behind the lure to record attacks (lure seized) and follows (fish orients to lure and follows it, sometimes approaching very closely, but does not seize it). Follows as well as attacks indicate predatory intent, because all follows were by large predatory fish of species that sometimes attacked our lures (as also reported in the study by Goiran et al [42]). The observer estimated body size of fishes by comparison with the length of the adjacent lure.…”

“…Each lure consists of 12 linked segments to create a sinuous movement that mimics the swimming action of a snake (see for video of the lure's action). Hooks were removed and replaced with lead weights to ensure negative buoyancy (see [ 42 ] for details). All lures were painted black to resemble the most common colour morph of the locally abundant sea snake Emydocephalus annulatus (mean SVL at birth 300 mm [ 42 , 43 ]).…”

“…Hooks were removed and replaced with lead weights to ensure negative buoyancy (see [ 42 ] for details). All lures were painted black to resemble the most common colour morph of the locally abundant sea snake Emydocephalus annulatus (mean SVL at birth 300 mm [ 42 , 43 ]). Three replicate lures of each size (200-mm or 300-mm long) were used during the trials to avoid stimulus pseudoreplication, with lure sizes changed between successive blocks of three trials per lure.…”

“…(see [42] for details). All lures were painted black to resemble the most common colour morph of the locally abundant sea snake Emydocephalus annulatus (mean SVL at birth 300 mm [42,43]).…”

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

“…To place locomotor costs into an ecological context, we need empirical studies on the relationship between an individual's locomotor performance and its vulnerability to predation [35]. Such studies pose considerable difficulties from both logistical and animal-welfare perspectives, but some systems facilitate direct manipulation of locomotor attributes of model prey to document rates and outcomes of attack by predators [36].…”