Size does matter

[Soxy]

Hahahaha

 

Size Does Matter in Bats' Evolution

 

By WILLIAM KATES, Associated Press Writer Tue Jan 24, 8:06 PM ET

 

SYRACUSE, N.Y. - For some male bats, sexual prowess comes with a price — smaller brains. A research team led by Syracuse University biologist Scott Pitnick found that in bat species where the females are promiscuous, the males boasting the largest testicles also had the smallest brains. Conversely, where the females were faithful, the males had smaller testes and larger brains.

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"It turns out size does matter," said Pitnick, whose findings were published in December in "Proceedings of the Royal Society: Biological Science," an online journal.

 

The study offers evidence that males — at least in some species — make an evolutionary trade-off between intelligence and sexual prowess, said David Hoskens, a biologist at the Centre for Ecology and Conservation at the University of Exeter in England and a leading authority on bats' mating behavior.

 

"Bats invest an enormous amount in testis, and the investment has to come from somewhere. There are no free lunches," said Hoskens, who did not participate in the study.

 

The relationship between the breeding system and relative brain size has received little investigation, said Pitnick, who teaches evolution and population biology and researches topics such as sexual selection and sexual conflict.

 

Bats are the second largest group of mammals (behind rodents) with about 1,000 known species. Because of their exceptional navigational and flying abilities, bats have been the subject of countless studies, providing Pitnick and his colleagues — Kate Jones of Columbia University and Gerald Wilkinson of the University of Maryland — with a bounty of data without having to slink off into caves.

 

Pitnick's team looked at 334 species of bats and found a convincing contrast in testes size. In species with monogamous females, males had testes starting at 0.11 percent of their body weight and ranging up to 1.4 percent. But in species where the females had a large number of mates, Pitnick found testes ranged from 0.6 percent to 8.5 percent of the males' mass (in the Rafinesque's big-eared bat).

 

"If female bats mate with more than one male, a sperm competition begins," Pitnick said. "The male who ejaculates the greatest number of sperm wins the game, and hence many bats have evolved outrageously big testes."

 

Promiscuity is known to make a difference in testicle size in some other mammals. For example, chimpanzees are promiscuous and have testicles that are many times larger than those of gorillas, in which a single dominant male has exclusive access to a harem of females.

 

Large brains, meanwhile, are metabolically costly to develop and maintain. Pitnick's research suggested that in those bat species with promiscuous females, the male's body used more of its energy to enhance the testes — giving it the greater adaptive advantage — and lacked the energy it needed to further develop the brain.

 

The study found that in more monogamous species, the average male brain size was about 2.6 percent of body weight, while in promiscuous species, the average size dipped to 1.9 percent.

[FlaSoxxJim]

I saw that and it's a pretty neat study. I do have one issue with it in that there is an alternative explanation that is not noted (at least not in this lit review – I have yet to read the actual paper). At least i don't think they have the whole story.

 

I don't think the issue is entirely one of energetics – that metabolic resources allocated to fecundity necessarily limits metabolic resources placed elsewhere. Consider the chimpanzee that this report notes exhibits males with relatively larger testes related very likely to evolutionary sperm competition arising from promiscuous reproductive behavior. But the chimpanzee brain, like that of all the great apes, is conspicuously large, so if there is an energetic cost associated with increased gonadal mass it seems not to be paid for by a reduction in brain size. In fact Chimpanzees, orangutans, gorillas and early hominids for that matter (Australopithecus have about the same brain/body ratio.

 

What is lacking in this explanation is the need to keep total body mass down in bats, as they are flying mammals. Since keeping a minimal body weight is critical an increase in testis mass pretty much demands that the difference be made up in a loss of mass elsewhere. Yes, these authors show some of it is made up in the brain size. But without giving actual numbers (rather than relative percentages), a reader cannot tell if there is a 1:1 mass trade off between brain and testes. Perhaps some mass reduction is seen elsewhere in the promiscuous bat species. If the entire gamut of meristics and morphometrics is examined, that may well be the case.

 

Even if it is not, I think a solely energetic explanation is lacking because it really could be the need to keep below a critical threshold weight so that flight is not compromised.

 

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As an aside, Soxy, I have a bit of a bone to pick with you. You left me hanging a couple of months ago when you made a post about language (or lack thereof) in social insects. I took the time to note that the von Frisch "waggle dance" honeybee language hypothesis is as robust as ever, made a couple of quick citations, kicked the olfaction school of thought around a bit, and rolled up my sleeves for a good dialogue with you that, sadly, never came.

 

Yes, this is a sad commentary on my social life, but I was looking forward to some stimulating bee talk.

[Soxy]

Sorry! I haven't been around much. But if you link it, I'll read and debate you as to why I don't think animals have language in the traditional sense of human language.