Testing Bergmann's rule in marine invertebrates: Using global brachiopod data during the Permian glacial-interglacial transition

Bergmann's rule states that organisms tend to be larger in cold higher latitudinal areas and smaller in warmer lower latitudes, and that this latitudinal gradient of body-size becomes steeper during colder time intervals. Originally formulated for warm-blooded animals, this theory has also been applied to ectotherms. However, few studies have been conducted to examine latitudinal patterns of marine invertebrate body-size, and the idea concerning the latitudinal gradient varying along with overall climate has received little attention. Based on body-size data of 26,112 brachiopods from the Permian glacial-interglacial transition, this paper finds that the latitudinal size distribution of brachiopods followed Bergmann's rule, but the changing pattern of latitudinal gradients along with climate was opposite to that expected in Bergmann's rule.

Except for several specific regions where trends were affected by paleoequatorial currents or there are limited data, brachiopod body-size generally increased as latitude increased. From the colder early Cisuralian (Asselian–Sakmarian) to the warmer Guadalupian, the latitudinal gradient of brachiopod body-size distinctly increased. The starvation-resistance hypothesis may explain this latitudinal pattern of brachiopod body-size and the variation in latitudinal gradients. In higher latitudinal areas, brachiopods tend to grow to larger size to cope with seasonal fluctuation in resource availability, resulting in higher proportion of brachiopods with large body-size in higher latitudinal areas. During the cold ice age, small individuals, which were more abundant in low latitudinal areas, were more vulnerable to, and heavily affected by, the highly seasonal climates. This resulted in heavier losses of small-sized brachiopods in lower latitudinal areas than in high latitudes, and therefore flatter latitudinal gradient of body-size than warmer periods. During the warmer late Cisuralian (Artinskian-Kungurian) and Guadalupian, more small-sized individuals started to flourish in low latitudinal areas, and cool high latitudinal areas with pronounced seasonality still favored large individuals over small ones. The increased proportion of small-sized individuals in lower latitudes then resulted in the steeper latitudinal gradients of brachiopod body-size in warmer periods. Metabolic rate and oxygen solubility have been used to explain the larger body-size of marine invertebrate in higher latitudes in previous studies, but these are considered to have played a less important role in the latitudinal patterns of body-size in this study, since they do not support the result that the latitudinal gradient became steeper when it got warmer.