Were Pleistocene hippopotamuses exposed to climate-driven body size changes? PAUL P. A. MAZZA AND ADELE BERTINI Mazza, P. P. A. & Bertini, A. 2013 (January): Were Pleistocene hippopotamuses exposed to climate-driven body size changes? Boreas, Vol. 42, pp. 194–209. 10.1111/j.1502-3885.2012.00285.x. ISSN 0300-9483. This study proposes a working hypothesis that Mediterranean hippopotamuses, and perhaps European ones as well, reduced their size, sometimes even drastically, from the ‘Mid-Pleistocene Revolution’ (MPR, c.1.2–0.5 Ma) to the Late Pleistocene. In contrast to the Early Pleistocene, during this time period glacial/interglacial cycling was dominated by a 100-ka periodicity, with more extended glacial phases (up to 85 ka) alternating with shorter interglacial phases (up to 15 ka). These changes seem to have somehow affected the size of hippopotamuses. While relatively larger hippopotamuses have been found in warmer and somewhat more humid intervals, data seem to indicate that they might not have grown as large under less favourable conditions, namely during colder and comparatively drier times. This is a possible response to climate-driven fluctuations in food availability, but Pleistocene habitat fragmentation may also have had an influence. Environmental break-up during the late Quaternary led to the isolation of megafauna populations, which underwent modifications similar to those observed in insular mammals. Although the amount of remains available is still limited, it is nonetheless a fact that hippopotamuses changed their body size through time, normally becoming smaller. If the conclusions are confirmed as data continue to accumulate, hippopotamuses might cast doubt on the generality of Bergmann’s rule. Paul P. A. Mazza (paul.mazza@unifi.it) and Adele Bertini (adele.bertini@unifi.it), Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via G. La Pira 4, 50121 Firenze, Italy; received 22nd March 2012, accepted 14th July 2012. The adaptation of past organisms is a major topic of palaeoecological studies. The long-term ecological processes that promote community change over geo- logical time can be most effectively investigated by pin- pointing and tracking palaeobiological properties that are not connected with specific historical situations and constraints. Body size, for example, is a palaeobiolo- gical indicator that can convey substantial information about the functioning of key ecosystem components, and that can thus provide significant clues to past environmental conditions. A number of studies have revealed a coincidence of size change in various mammals (e.g. mastodonts, carnivores, rodents, eden- tates, bison and deer) with climate change (Wilson 1980, 1996; King & Saunders 1984; McDonald 1984; Martin 1993; Martin & Barnosky 1993; Barnosky et al. 2003; Saunders et al. 2010). Blois & Hadly (2009) linked body sizes to climate-change-related responses of vegetation and primary productivity. Most body- size changes at the last glacial/interglacial transition have been shown to be ecophenotypic rather than geno- typic (Barnosky 2005). Bergmann’s (1847) thermoregu- latory explanation of the latitudinal increase in animal body size proved valid for poikilotherm vertebrates, as well as for a number of invertebrate taxa (e.g. Ashton 2002; Makarieva et al. 2005), but surprisingly few studies tested the empirical validity of these body size changes (Ashton et al. 2000 and references therein). Meiri & Dayan (2003) found that over 72% of the birds and 65% of the mammals they examined fitted the pre- dictions of Bergmann’s rule and that smaller mammals (<500 g) conformed significantly less than larger ones. These and other such results meant that many authors cast doubt on Bergmann’s ‘thermal’ interpretation, concluding instead that body size is controlled by many factors rather than just by temperature (Thompson 1942; Scholander 1955; Rosenzweig 1968; James 1970; Simberloff & Boecklen 1981; Lande & Arnold 1983; Yom-Tov & Nix 1986; Hansson & Jaarola 1989; Steudel et al. 1994; Yom-Tov & Geffen 2006; Teplitsky et al. 2008; Huston & Wolverton 2011). During the last decade, several studies have analysed the empirical validity of ecogeographic rules (Blackburn & Gaston 1998; Blackburn et al. 1999; Angilletta & Dunham 2003; Cruz et al. 2005; Millien et al. 2006; Rodríguez et al. 2006, 2007; Gaston et al. 2007; Meiri & Gavin 2007; Meiri et al. 2007; Meiri 2011). Many factors pro- posed as alternatives to Bergmann’s rule were probably correlated with, or at least influenced, by the environ- mental and climatic changes that occurred during the Quaternary. The Pleistocene started with a significant expansion of the Northern Hemisphere ice cap and the activation of the typical glacial/interglacial cycling. The latter was first governed by a 40-ka (obliquity) domi- nant cyclicity, and then, from the middle Pleistocene onwards, by a ~100-ka (eccentricity) dominant cycli- city. The switch caused intensified glacial phenomena, increasing seasonality, and aridity, which had disrupt- ing consequences on both flora and fauna. Faunas underwent the Pleistocene’s most spectacular turnover: DOI 10.1111/j.1502-3885.2012.00285.x © 2012 The Authors Boreas © 2012 The Boreas Collegium