Mass-related differences in metabolic rate and fasting endurance explain divergence in seasonal activity of Mediterranean lizards

We analysed the effects of body mass on the monthly activity patterns of six Mediterranean lacertid lizard taxa, four relatively small species, the Italian wall lizard (Podarcis siculus), the common wall lizard (Podarcis muralis), the blue-throated keeled lizard (Algyroides nigropunctatus), and the Ionian wall lizard (Podarcis ionicus), and two larger species, the western green lizard (Lacerta bilineata) and the Balkan green lizard (Lacerta trilineata). The highest number of observations for all six species occurred in April and May and the lowest in July and August, the latter being the hottest and driest months of the year. The two larger species were mainly active from March to July, whereas the four smaller species had an additional period of high activity from September to November. As all six species reproduce during spring, the increase in activity of the smaller species in autumn was consequently unrelated to reproductive behaviour. There was no difference in seasonal activity of the two smaller Italian species at sites with or without the larger green lizards. It is therefore unlikely that interference competition/predation by green lizards caused the increased autumnal activity of the smaller lizards. We suggest that due to their lower mass-specific metabolic rates, larger species can obtain sufficient lipid stores over a shorter annual activity to ensure successful reproduction the subsequent spring. By contrast, smaller species have greater need to replenish their lipid reserves after summer fasting and therefore resume much higher activity levels in September to November to attain this goal.