Many mammals have a bone called the baculum. Elephants and humans don't. Why?
Is their a survival advantage specific to elephants & humans?
When we look at the sequence of bones from homo erectus back through to neanderthal man and beyond, at any point is there a baculum?
The absence of a baculum in humans is probably related to the peculiarities of human mating strategies that differ from other primates, such as the lack of estrus in females. I'm not aware of a baculum being found amongst any hominin fossil remains.
Dawkins suggested a handicap theory in 'The Selfish Gene' (p. 307 in the 1989 edition). The idea is that only healthy men can have a stiff penis without a baculum, and that females select men for health that way. 'Natural erection pressure' so to say:-)
How did females start to discern erections achieved by more pressure and less bone? And why don't more other species also lose their baculum due to this kind of erection selection?
A pretty limp argument by Dawkins. "Health" of an individual varies over time. And the health conditions associated with erectile dysfunction were probably less common throughout most of human prehistory, and generally affect individuals at older ages when reproduction has already taken place. And there are other way in which females could judge "health" in potential mates.
Baculum -- is an interesting aspect of evolutionary enquiry. Certainly to get into some tangible answer will be difficult, especially in view of dearht of skeletal evidence across the species. the question how did homo spiense lost the Baculum, is very intrigueing and it will occupy some of us active to get the data and facts and that can give us some reasonable hypothesis or if better might lead some clues. Here structure and fuction relationship one of the Darwanian tenet for investigating evolution should work or has a basis from which one can take off. As I ponder over, possibly a sum of collective changes that had happend in the evolution of Hominins, e.g. erect posture, development of hands and fingers, brian development that helped man to find better tools, cave dwelling which allowed the early man to have protection and leisure are all in one way or the other related to the disappearance of baculum. Possibly one can built an argument based on the above various aspects.
Probably due to our unique sociosexuality as humans. There haven't been any hominin baculum bones. Alan Dixson extensively discusses this amidst his 2013 "Primate Sexuality: Comparative Studies of the Prosimians, Monkeys, Apes, and Humans". It is worth a read.
Dawkin's argument is weak in terms of what we know about evolutionary theory and it seems unlikely. There are other mechanisms leading women away from ill men who had conditions common in prehistory. Disease avoidance cognition seems to be one tightly-conserved mechanism that comes to mind here.
I suspect it was lost early in early Australopithecines or perhaps the common ancestor. Dixson suggests the common ancestor likely had one. I suspect having to walk upright, alongside shifting sociosexualities towards something less competitive, more multi-male, multi-female and likely with the components needed to formulate later pair bonding likely discourage the utility of a baculum. In such a model, the baculum is part of a suite of traits that evolve in species with high male-male competition. Some discussion of this can be found in Peter Gray's work, particularly his book "Fatherhood" published with Harvard University Press.
We don't know whether the early hominoids had a (small) baculum: possibly, the different apes re-evolved it. Bacula are seen in most Carnivora & Pinnipedia, Chiroptera, Rodentia, Insectivora, Dermoptera & Primates, but AFAIK not in tarsiers, spider monkeys & people. These latter spp often have vertical bodies, for hopping, swinging & walking resp. Apes (more orthograde than most monkeys) have rel.small bacula. Why? FWIW, occasionally bacula can fracture (a disaster for copulation), and perhaps the risk is greater when the baculum is not protected by a pronograde body? There are indications (fossil & comparative) that Mio-Pliocene hominoids (e.g. Morotopithecus c 18 Ma, Oreopithecus c 8 Ma, Australopithecus 4-1 Ma) were more orthograde than most apes today (humans & gibbons are (still?) +-orthograde, see attachment): in this case, the australopiths might have lacked bacula, and the apes might have redeveloped (small) bacula?
So why are (were) there so many tribes that had some mechanism to put the penis into a similar erect position? - depending upon the region and culture, either by tucking the foreskin under a taught waist string or even wearing an exaggerated gourd ornament. Even if you don't have a permanent bone-supported erection, you can still flaunt it through other strategies. Of course all the girls can tell the difference, but something must drive these traditional behaviors over millennia.
Are their embryological data on bacula in humans & other animals? There are many different possible explanations, but the only foolproof explanation IMO is what we see in other species. AFAIK, primates without bacula are humans, spider monkeys & tarsiers. Female capuchin monkeys have a clitoral bone, called the baubellum, which remarkably is stronger developed before than after puberty. The great apes have rel.small bacula, but whether their ancestors lacked bacula (like humans) or had instead larger bacula, we don't know (embryological data could possibly give more information here). What do humans, spider monkeys & tarsiers have in common (vs other primates)? A more erect posture: humans walking, spider monkeys hanging, tarsiers jumping? Are there higher risks of fracturing your baculum when your body is often upright? Does the semi-erect posture of apes help explain why they have rel.small bacula?
I have been thinking: why do most primates & bats have a penis pendulus? Most quadrupedal mammals have a penis appositus (forward pointing, protected by the belly?), but this could be hindering amid the branches, so a hanging penis would be safer in these 3D animals? Moroever, in bats, a penis appositus could possibly hinder aerodynamics??
Derek, you also asked: why do elephants lack bacula? FWIW, my speculation: elephants have a very curious way of copulating: their penis has to make a detour to reach its destination (see illustrations in textbooks). I wonder whether this has to do with their complex evolution: they probably had semi-aquatic ancestors (related to Sirenia) & according to some, they might even had arboreal ancestors (related to hyraxes, cf pectoral instead in inguinal mammae). Just some possibilites.
Hi Asjlkdf, I think that's too simple. In fact, the mammals with relatively the longest penises appear to have the longest bacula: pinnipeds. According to Wiki, mammals with bacula are: most primates, rodents (but not rabbits, hares etc.), insectivores, carnivores (except hyenas) & pinnipeds, bats. It's absent in ungulates & whales, elephants & sirenians, platypus & echidna, marsupials, lagomorphs etc. Wiki also says the baculum has evolved independently 9 times & lost in 10 separate lineages.
AFAICS the primates without bacula apparently are the ones with most frequently vertical bodies: humans, atelids (spider monkeys) & tarsiers (for walking, brachiating & hopping resp.). I'd think a baculum would be more vulnerable with a vertical body, and a broken baculum is an enormous disadvantage. Just FWIW.
Just to think of there are several question that we can ask and wonder -- one such has been raised was -- about baculum versus pendulus that we observe across species. Similar such questions can be asked for other traits as well. However to be able to get some reasonable insights for the presence of these traits is not going to be easy. Is not that Darwin (and of course Wallance) struggled hard to give or explain operation of natural selection. And Darwin himself is very well know that what is trying is hardly can explain natural selection leading to species diversification. It is more intuitive and appears reasonable explanation, but once cannot generalize in any sense one wants to do so, as there are too many examples where the explanation given by Darwin is not applicable and appear to countering the observations that Darwin choose to explain among others for the support of natural selection. However, whatever the complexity of biological process nature which hardly helps us to have a generalization to see the reasonable principles behind the working nature of Nature.
With respect to the question baculum vs pendulus, possibly the reasonable explanation that we look will be limited to our understanding of Nature and evolutionary mechanism. It will be partial explanation -- like the way Darwin explanation remains as also partial. But such curiosities should lead us for a more understanding of the biological mechanism -- In one partial sense one can see that the pendulus penis possibly more opt and best suited for the sexual activities related to Man as against baculum in another animals. But how the bony part got lost in the evolutionary time etc. will possibly require a through probe possibly the molecular genetics is supposed to help for a better understanding -- we are far from reaching such an understanding right now.
Hi Vasulu, a penis with a baculum (penis bone) can be pendulus (hanging), they't not exclusive, iOW, it's not "baculum vs pendulus".
But we have a lot of comparative data, e.g.
- Wiki says bacula have evolved independently 9 times, and got lost in 10 separate lineages.
- AFAIK the mammals with rel.the longest penises have the longest bacula: pinnipeds.
- Mammals with bacula are: most primates, rodents (but not rabbits & hares), insectivores, carnivores (except hyenas) & pinnipeds, bats.
- Mammals without bacula: ungulates & whales, elephants & sirenians, platypus & echidna, marsupials, lagomorphs etc.
- Apes generally have rel.shorter bacula than non-hominoid primates, e.g. chimp 7 mm, gorilla 12 mm, human 0 mm, vs mandril 24 mm, the small potto 21 mm!
- Curiously, female capuchin monkeys have a small clitoris bone (baubellum) which is larger before than after puberty!
- F.Ankel-Simons 2000 "Primate anatomy" Acad.Press p.401: the primates without bacula are atelids (spider monkeys etc.), tarsiers & humans.
FWIW, these 3 primates seem to be the ones with the body most frequently vertical: humans for walking (& wading see attachment), atelids for brachiating (hanging vertically) & tarsiers fore hopping: is a (projecting) baculum more vulnerable with a vertical body? A broken baculum is obviously an enormous disadvantage for procreation.
Kenny, do you have refs for saying that Bushmen have bacula?
Have you ever thought that perhaps the bacculum could be the epipubic bones of monotremes and marsupials? It is a work from Jellison, 1945, who suggested that and I'm still thinking about it.
Hi Gabby, I have no idea, but since bacula often appear & disappear evolutionarily (see my post of 23 July 2017) and, more generally, since ossification is a frequent evolutionary phenomenon, it's not very likely at first sight IMO. Perhaps embryological studies on different placentals, marsupilas & monotremes could learn us something here? Note marsupials are closer relatives of us (placentals) than of monotremes.
Yes, I'm very interested in find out more about it, and probably only with embryo or evo-devo analysis. The weird thing is that these bones have a lot of morphological variation, also between males and females just like bacula and clitoral bones (which is very understudied), and also some influence in reproduction. I also think it is unlikely, but it would be something interesting to answer even with a negative response.
Yes.
Clitoral bones (baubella) are seen in capuchin & some other monkeys. I read somewhere that, curiously, the baubellum in capuchin monkeys is more developed before puberty than in adult females.
FYI, I just found this recent study: "The baubellum is more developmentally and evolutionarily labile than the baculum" Lough‐Stevens, Schultz & Dean 2018 Ecol Evol 8:1073-83 doi 10.1002/ece3.3634.
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