Evolutionary treasures hidden in the West Indies: Comparative osteology and visceral morphology reveals intricate miniaturization in the insular genera Mitophis Hedges, Adalsteinsson, & Branch, 2009 and Tetracheilostoma Jan, 1861 (Leptotyphlopidae: Epictinae: Tetracheilostomina)

The genera Mitophis and Tetracheilostoma comprise two extant lineages of small-sized threadsnakes that exclusively inhabit several islands of the West Indies. Even though leptotyphlopids are known for their extremely reduced size, miniaturization has only been hypothesized to reflect insular dwarfism for the genus Tetracheilostoma. Herein, we aim to describe the comparative osteology and visceral morphology of both genera, investigating and discussing their several internal morphological simplifications and novelties. Our results indicate that these taxa exhibit several autapomorphies mostly concentrated in the dorsoposterior skull elements and maxillae, as well as in their axial skeleton and viscera. These novelties and simplifications are most likely a result of extreme miniaturization driven by the evolutionary constraints or ecological opportunities possibly imposed by the "island rule." Both Mitophis and Tetracheilostoma distinguish from all other Epictinae in lacking a dentigerous process in the maxillae, by having the prootic fused to the otooccipital, and by the lack (except in comparison to a few Epictia) of a cervical vertebrae intercentrum I. Additionally, Mitophis can be distinguished from other Epictinae by the participation of the unpaired supraoccipital in the dorsal border of the foramen magnum, by the absence of the pleurapophyses in the caudal vertebrae, by a higher number of liver segments, and by the extreme degeneration of the pelvic rudiments. Tetracheilostoma differs from other Epictinae by lacking a distinct supraoccipital, which is fused to the parietal. Thus, our results reinforce that morphological characters are extremely valuable for leptotyphlopid systematics given their extremely conserved external morphology.

Plants obey (and disobey) the island rule

The island rule predicts that small animals evolve to become larger on islands, while large animals evolve to become smaller. It has been studied for over half a century, and its validity is fiercely debated. Here, we provide a perspective on the debate by conducting a test of the island rule in plants. Results from an extensive dataset on islands in the southwest Pacific illustrate that plant stature and leaf area obey the island rule, but seed size does not. Our results indicate that the island rule may be more pervasive than previously thought and that support for its predictions varies among functional traits.

Minimum area thresholds for rattlesnakes and colubrid snakes on islands in the Gulf of California, Mexico

We expand a framework for estimating minimum area thresholds to elaborate biogeographic patterns between two groups of snakes (rattlesnakes and colubrid snakes) on islands in the western Gulf of California, Mexico. The minimum area thresholds for supporting single species versus coexistence of two or more species relate to hypotheses of the relative importance of energetic efficiency and competitive interactions within groups, respectively. We used ordinal logistic regression probability functions to estimate minimum area thresholds after evaluating the influence of island area, isolation, and age on rattlesnake and colubrid occupancy patterns across 83 islands. Minimum area thresholds for islands supporting one species were nearly identical for rattlesnakes and colubrids (~1.7 km²), suggesting that selective tradeoffs for distinctive life history traits between rattlesnakes and colubrids did not result in any clear advantage of one life history strategy over the other on islands. However, the minimum area threshold for supporting two or more species of rattlesnakes (37.1 km²) was over five times greater than it was for supporting two or more species of colubrids (6.7 km²). The great differences between rattlesnakes and colubrids in minimum area required to support more than one species imply that for islands in the Gulf of California relative extinction risks are higher for coexistence of multiple species of rattlesnakes and that competition within and between species of rattlesnakes is likely much more intense than it is within and between species of colubrids.

Benefits of the destinations, not costs of the journeys, shape partial migration patterns

The reasons that lead some animals to seasonally migrate, and others to remain in the same area year-round, are poorly understood. Associations between traits, such as body size, and migration provide clues. For example, larger species and individuals are more likely to migrate. One explanation for this size bias in migration is that larger animals are capable of moving faster (movement hypothesis). However, body size is linked to many other biological processes. For instance, the energetic balances of larger animals are generally more sensitive to variation in food density because of body size effects on foraging and metabolism and this sensitivity could drive migratory decisions (forage hypothesis). Identifying the primary selective forces that drive migration ultimately requires quantifying fitness impacts over the full annual migratory cycle. Here, we develop a full annual migratory cycle model from metabolic and foraging theory to compare the importance of the forage and movement hypotheses. We parameterize the model for Galapagos tortoises, which were recently discovered to be size-dependent altitudinal migrants. The model predicts phenomena not included in model development including maximum body sizes, the body size at which individuals begin to migrate, and the seasonal timing of migration and these predictions generally agree with available data. Scenarios strongly support the forage hypothesis over the movement hypothesis. Furthermore, male Galapagos tortoises on Santa Cruz Island would be unable to grow to their enormous sizes without access to both highlands and lowlands. Whereas recent research has focused on links between traits and the migratory phases of the migratory cycle, we find that effects of body size on the non-migratory phases are far more important determinants of the propensity to migrate. Larger animals are more sensitive to changing forage conditions than smaller animals with implications for maintenance of migration and body size in the face of environmental change.