Early evolution of body size in bats

Abstract

Size is the single most important factor affecting physiology, locomotion, ecology, and behavior of mammals. Understanding evolution of size is especially important in groups like bats which exhibit many unique or energetically expensive behaviors (e.g., powered flight, echolocation, torpor and hibernation, long-distance migration). In addition, bats have the most diverse array of dietary habits of any mammalian Order. Most bat species are small: the central tendency in size in extant bats, as estimated by the median value, is around 14 g. However, bat size spans three orders of magnitude, with a few species exceeding one kilogram. Variation is not evenly distributed across groups, and there is no specific hypothesis accounting for size variation in bats. In search of evolutionary patterns, we first estimated mass in key Eocene fossils via allometric relationships. Least midshaft diameter of limb bones yielded accurate models of variation in size (body mass) in extant bats at the interspecific level (error <2%), thus providing a solid basis for size estimation in fossils. We then mapped size on current bat phylogenies including Eocene fossils. On these phylogenies, mass decreased along stem chiropteran nodes until a range of 14 -17 g was achieved in the crown clade including Palaeochiropteryx and extant bats (or microbats, depending on the topology). Remarkably, this estimated range includes the median of size for extant bats and was conserved since the Early Eocene along the backbone of all major bat clades with minor variations, strongly suggesting that an efficient combination of factors, including energy expenditure and cost of transport, was achieved at the base of the crown clade and was maintained through the evolutionary history of bats. Departures from this range were reconstructed as nested within bat families and were associated with major changes in diet, particularly carnivory and frugivory.