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BATS, BIRDS, ECHOLOCATION & SIGHT



On mechanisms of competitive exclusion in birds, bats and pterosaurs, 
Henri Rönkkö wrote...

>I believe birds do better than bats in daylight because
> birds rely on vision whereas bats rely on echolocation.
> Echolocation is neither as acute nor as effective as seeing,

Later on in the thread, Ron wrote...

> Let's not forget that not all bats echolocate, and some birds do.

Furthermore, let's not forget that many bats are far from deficient in 
terms of eyesight. Some bats, notably megadermatids, have large eyes, 
acute vision and hunt predominantly by sight, even gleaning *without* 
echolocating (as Ron notes). They catch large arthropods, small 
mammals and sleeping birds in this way. I need not say that megabats 
have large primate-like eyes and visual centres in the brain. 
Furthermore, as noted before, some microbats hunt in the day on 
islands devoid of predatory birds. 

As usual therefore, there are few black and white rules, but lots of 
shades of grey. Bats may do better at night because of 
thermoregulatory constraints, and they may actively avoid competition 
from birds, but this doesn't necessarily restrict them to nocturnal 
behaviour - there does not seem to be one simple rule that dictates a 
dichotomy in bird and bat behaviour. Perhaps the constraints of 
ancestry provide another answer: branch-leaping proto-bats (sensu 
Fenton) were probably nocturnal, and most descendants of these 
ancestors simply stayed this way. There are excellent text-books by 
Fenton, Altringham and Hill and Smith that discuss these areas in 
much greater depth.

To bring this round to pterosaurs: the (largely) naked patagia of bats 
apparently restrict their foraging period. Pterosaurs had naked patagia, 
but I find it very hard to think that they were nocturnal (diurnality is 
implied by their many visual display structures). Were pterosaurs better 
at temperature regulation than bats, perhaps because of their 
pneumaticity? Or is it bats which are strange in being mostly nocturnal, 
and in adopting a uniquely mammalian approach to the problems of 
being a flighted tetrapod? 

Ron - is the turtle book out yet?

"Craig's in the bedroom, asleep like a kitten - this man could sleep, for 
the whole of Great Britain" - - Caroline, 2000

DARREN NAISH 
PALAEOBIOLOGY RESEARCH GROUP
School of Earth & Environmental Sciences
UNIVERSITY OF PORTSMOUTH
Burnaby Building
Burnaby Road                           email: darren.naish@port.ac.uk
Portsmouth UK                          tel (mobile): 0776 1372651     
P01 3QL                                tel (office): 023 92842244