Multiple Paths to a Larger Mouth: Morphological Divergence in Two Island Populations of Australian Tiger Snakes (Notechis scutatus)

We studied ontogenetic variation in skull morphology in tiger snakes sampled from mainland South Australia (Coffin Bay), mainland Western Australia (Herdsman Lake), Franklin Island, and Hopkins Island. The snakes from the two islands were selected because of their unusually large absolute body and head size, likely attained as a result of dietary constraints and strong selective pressure during thousands of years of isolation on islands. We performed geometric morphometric analysis of the skulls from these snakes to identify and precisely quantify morphological differences. Island snakes are born larger (trunk length) than mainland snakes, and consequently also tend to have larger absolute head size at birth. The heads of island snakes also show a faster relative growth rate (positive allometry) compared to those of mainland snakes, likely to cope with the large prey items found on these islands. However, the snakes from the two islands differ from each other in terms of morphological change associated with ontogeny, especially with regard to a palatal bone (ectopterygoid), which, compared to mainland snakes, is relatively longer in the snakes from Hopkins Island but shorter in the snakes from Franklin Island. Some morphological traits potentially associated with promoting a larger gape were also observed in one of the mainland populations (Herdsman Lake), consistent with standing variability for selection. Our results reveal that discernible changes in morphology and allometry in skull bones have evolved in island populations of tiger snakes in less than 10 000 years, and suggest that the large heads of island tiger snakes might develop via slightly different paths through morphospace on different islands.