The K-Pg event as a key to bat evolution

Bats are unique mammals. This note discusses some questions regarding bat evolution including why they are nocturnal and why they can echolocate. It is hypothesized that echolocation was necessary for bats to survive the period of limited visibility that followed the Cretaceous–Paleogene (K–Pg) extinction event. Introduction. Bats are amazing and unique animals: they are mammals which can fly and echolocate. There are over 1200 bat species [1, 2], out of some 5500 mammal species [3] making them the second largest order of mammals (after rodents). Some 70% of bat species feed on insects, namely are insectivores. From an evolutionary viewpoint it is natural to wonder about the origin of bats and ask: when and under what conditions did bats evolve? Further questions are more bat specific. What is the relation between the evolution of flight and that of echolocation? Why are bats nocturnal? Why are bats so relatively diverse among mammals? In short, we are asking: what is the reason for bats? By this we mean the conditions which made its defining characteristics necessary for survival. Already in the Origin of Species Darwin [4] recognized the problem presented by the sudden appearance of ‘completely developed’ bats. Echolocation in bats, discovered in 1958 [5], aggravates the problem, by requiring an explanation for the evolution of two related traits rather than one, see for example [6, 7]. Much is known about bats and their evolution, yet bat evolution remains a mystery. For instance “early evolution of the group remains poorly understood”, at least as of 2008 [8]. A 2001 summary of a major aspect of the puzzle [6] reads “Since there are two key behavioural traits, there are at least three scenarios for their evolution: echolocation may have evolved first ..., flight may have evolved first ..., or the two may have evolved in tandem ... This minimal view assumes that at least one of the behaviours evolved only once. Multiple origins of both traits ... could lead to much more complex interrelationships.” The ellipsis denote reference lists. 1 ar X iv :1 70 5. 03 40 3v 1 [ qbi o. PE ] 9 M ay 2 01 7 In this note we shall humbly suggest a resolution to this puzzle and mention possible tests. Analysis. Let us analyze the issue of the evolution of flight and echolocation in bats and the relation between them. Together they enable flight (and hunt) in the dark. The considerable diversity of bats is a testament to the advantage offered by this ability (in fact, the speciation is known to have been going on throughout the cenozoic era, as we mention later). If we consider each innovation separately we find that flight in its own is very useful, as clearly evidenced by the large number of flying species without echolocation including birds and insects. On the other hand, I view the rarity of terrestrial echolocating species as evidence for the low usefulness of echolocation without flight (on the ground). Therefore I choose to continue by assuming that flight was developed first. At this point I wish to pose a question: why was echolocation necessary for bats? Before discussing the question it is perhaps worthwhile to explain what is meant by necessary. Echolocation is clearly useful, yet it is also rather involved or specialized. Not only does it require various bat resources such as energy (for sound) and signal processing load on the brain, but also its evolution requires a large number of steps. There is evidence that all extant bats evolved from an echolocating ancestor, see e.g. [9]. Hence the considerable price of echolocation must have been outweighed by considerable advantages and selective pressures, namely by necessity. The problem is that echolocation appears to be unnecessary for the following reasons. First, isn’t it conceivable that the proto-bat would be active during the day (be diurnal rather than nocturnal) thereby making echolocation completely redundant? In fact, [6] has already advanced this fascinating and suggestive hypothesis. Secondly, it appears to be possible to get along at night without echolocation. In fact, nights are not completely dark, especially in the presence of moonlight, and owls are a living proof that heightened sight and hearing can very well suffice. The same conclusion can be strengthened from a different angle: echolocation allows bats to inhabit completely dark caves. Yet, it is doubtful that caves are attractive enough to drive echolocation evolution. In order to find a way out of this puzzle we resort to consider whether at any time in the past conditions were so markedly different from today, so that echolocation became truly necessary. A formative event for echolocating bats. Known facts about bat evolution offer hints towards identification of such a time. The currently oldest bat fossil was dated to some 52 million years ago (Mya) during the early Eocene period [8]. Recent genetic studies enable an improved determination of phylogenetic trees including time calibration. [10] assembled such a phy-