Background: Many small vertebrates on islands grow larger, mature later, lay smaller clutches/litters, and are less sexually dimorphic and aggressive than their mainland relatives. This set of observations is referred to as the 'Island Syndrome'. The syndrome is linked to high population density on islands. We predicted that when population density is low and/or fluctuating insular vertebrates may evolve correlated trait shifts running opposite to the Island Syndrome, which we collectively refer to as the 'reversed island syndrome' (RIS) hypothesis. On the proximate level, we hypothesized that RIS is caused by increased activity levels in melanocortin receptors. Melanocortins are postranslational products of the proopiomelanocortin gene, which controls pleiotropically pigmentation, aggressiveness, sexual activity, and food intake in vertebrates.Results: We tested the RIS hypothesis performing a number of behavioral, genetic, and ontogenetic tests on a blue colored insular variant of the Italian Wall lizardPodarcis sicula, living on a small island off the Southern Italian coast. The population density of this blue-colored variant was generally low and highly fluctuating from one year to the next.In keeping with our predictions, insular lizards were more aggressive and sexually dimorphic than their mainland relatives. Insular males had wide, peramorphic heads. The growth rate of insular females was slower than growth rates of mainland individuals of both sexes, and of insular males. Consequently, size and shape dimorphism are higher on the Island. As predicted, melanocortin receptors were much more active in individuals of the insular population. Insular lizards have a higher food intake rate than mainland individuals, which is consistent with the increased activity of melanocortin receptors. This may be adaptive in an unpredictable environment such as Licosa Island. Insular lizards of both sexes spent less time basking than their mainland relatives. We suspect this is a by-product (spandrel) of the positive selection for increased activity of melanocortins receptors.Conclusions: We contend that when population density is either low or fluctuating annually as a result of environmental unpredictability, it may be advantageous to individuals to behave more aggressively, to raise their rate of food intake, and allocate more energy into reproduction.

Cope's rule is the trend toward increasing body size in a lineage over geological time. The rule has been explained either as passive diffusion away from a small initial body size or as an active trend upheld by the ecological and evolutionary advantages that large body size confers. An explicit and phylogenetically informed analysis of body size evolution in Cenozoic mammals shows that body size increases significantly in most inclusive clades. This increase occurs through temporal substitution of incumbent species by larger-sized close relatives within the clades. These late-appearing species have smaller spatial and temporal ranges and are rarer than the incumbents they replace, traits that are typical of ecological specialists. Cope's rule, accordingly, appears to derive mainly from increasing ecological specialization and clade-level niche expansion rather than from active selection for larger size. However, overlain on a net trend toward average size increase, significant pulses in origination of large-sized species are concentrated in periods of global cooling. These pulses plausibly record direct selection for larger body size according to Bergmann's rule, which thus appears to be independent of but concomitant with Cope's.

Species response to environmental change may vary from adaptation to the new conditions, to dispersal towards territories with better ecological settings (known as habitat tracking), and to extinction. A phylogenetically explicit analysis of habitat tracking in Caenozoic large mammals shows that species moving over longer distances during their existence survived longer. By partitioning the fossil record into equal time intervals, we showed that the longest distance was preferentially covered just before extinction. This supports the idea that habitat tracking is a key reaction to environmental change, and confirms that tracking causally prolongs species survival. Species covering longer distances also have morphologically less variable cheek teeth. Given the tight relationship between cheek teeth form and habitat selection in large mammals, this supports the well-known, yet little tested, idea that habitat tracking bolsters morphological stasis.