Assorted mammalian bacula
Despite the assumptions behind certain slang terms, human males don't really have a penis bone, or baculum, unlike some of our closest mammalian relatives such as chimpanzees and bonobos. The reason why was not really clear until now although evolutionary explanations have been suggested.
The penis bone first put in appearance in mammalian evolution between 145 and 95 million years ago, so was present in the last common ancestor of apps and carnivores, so at some point, and in the case of humans after we diverged from the chimpanzee/bonobo line, the penis bone was lost.
It has been suggested that, partly as a consequence of our upright gait which puts the male genitalia on display, female sex selection may have caused a larger penis to evolve. Riding on the back of this may have been an incidental selection for good health, in that poor health tends to cause loss of erectile function. Losing the penis bone not only allows the larger penis to shrink back to a more manageable size when not in use but also allows erectile function and fitness to be on display. Now researchers may have identified another reason, at least for why the bone was not retained.
According to Matilda Brindle and Christopher Opie of the University College Department of Anthropology, London, UK, there is strong correlation in primates between the presence of the baculum and the amount of time the penis remains in the vagina (intromission time) during intercourse, and this in turn depends on the mating strategy.
Our findings suggest that the baculum plays an important role in supporting male reproductive strategies in species where males face high levels of postcopulatory sexual competition. Prolonging intromission helps a male to guard a female from mating with any competitors, increasing his chances of passing on his genetic material.
Matilda Brindle, UCL Anthropologist. Co-author.
Quoted in Phy.Org
Quoted in Phy.Org
After the human lineage split from chimpanzees and bonobos and our mating system shifted towards monogamy, probably after 2mya, the evolutionary pressures retaining the baculum likely disappeared. This may have been the final nail in the coffin for the already diminished baculum, which was then lost in ancestral humans.
Dr Kit Opie, UCL Anthropology. Co-author.
Quoted in Phy.Org
Quoted in Phy.Org
Abstract
The extreme morphological variability of the baculum across mammals is thought to be the result of sexual selection (particularly, high levels of postcopulatory selection). However, the evolutionary trajectory of the mammalian baculum is little studied and evidence for the adaptive function of the baculum has so far been elusive. Here, we use Markov chain Monte Carlo methods implemented in a Bayesian phylogenetic framework to reconstruct baculum evolution across the mammalian class and investigate the rate of baculum length evolution within the primate order. We then test the effects of testes mass (postcopulatory sexual selection), polygamy, seasonal breeding and intromission duration on the baculum in primates and carnivores. The ancestral mammal did not have a baculum, but both ancestral primates and carnivores did. No relationship was found between testes mass and baculum length in either primates or carnivores. Intromission duration correlated with baculum presence over the course of primate evolution, and prolonged intromission predicts significantly longer bacula in extant primates and carnivores. Both polygamous and seasonal breeding systems predict significantly longer bacula in primates. These results suggest the baculum plays an important role in facilitating reproductive strategies in populations with high levels of postcopulatory sexual selection.
1. Introduction
The morphology of male intromittent organs is argued to be subject to more rapid divergent evolution than any other form in the animal kingdom [1]. The baculum, or penis bone, does not buck this trend and has been described as ‘the most diverse of all bones’ ([2], p. 1), varying dramatically in length, width and shape across the Mammalia.
The baculum is not uniformly present across mammals. It was thought only to exist in eight of the mammalian orders: Afrosoricida, Carnivora, Chiroptera, Dermoptera, Erinaceomorpha, Primates, Rodentia and Soricomorpha [3,4]. However, it has recently been discovered that a Lagomorph, the American pika (Ochonta princeps), also has a small baculum [5]. This discovery suggests that baculum presence may be more prevalent across mammals than historically assumed. Certain orders have a mixture of baculum presence and absence across species; these are the Carnivora, Chiroptera, Primates and Rodentia. In Primates, for example, humans, tarsiers and several Platyrrhines lack a baculum. The Lagomorphia may be similarly divided. Aside from documenting the presence and absence of the baculum across the mammalian orders, the evolutionary history of the baculum had not been studied until recently, leaving many questions unanswered.
Genital (and hence bacular) morphology is suggested to be subject to sexual selection [6]. The few empirical tests conducted to date may support this hypothesis. Stockley et al. [7] found that baculum width in polygamous house mice (Mus domesticus) was a significant predictor of male reproductive success. Simmons & Firman [8] were able to manipulate baculum width experimentally by altering the level of sexual selection pressure in populations of house mice. After 27 generations, populations with artificially enforced high levels of postcopulatory sexual selection pressure had significantly thicker bacula than populations in which monogamy was enforced and sexual selection pressure was therefore absent. These studies indicate that intra-sexual selection, in particular postcopulatory sexual selection pressure, may be driving bacular evolution. If this is the case, the bacula of populations under high levels of intra-sexual selection pressure, such as those with polygamous (multi-male, multi-female) or seasonal mating systems, should be subject to stronger evolutionary forces.
Different mating systems generate variation in levels of postcopulatory sexual selection pressure and therefore morphological variability; for example, species with high levels of sperm competition tend to have large testes relative to their body mass [9]. Residual testes mass is thus considered to be a reliable measure of the mating system of a population and therefore the degree of sexual selection pressure [9]. Orr & Brennan [10] found that relative testes mass was a significant predictor of baculum presence across Chiroptera, Eulipotyphla, Primates and Rodentia. However, the same study found no relationship between baculum presence or width and mating system, indicating that a third variable may be at play. Ramm [11] tested for a relationship between testes mass and baculum length in four orders by first establishing the level of phylogenetic dependency between species and then conducting appropriately corrected regressions. A positive relationship between testes mass and baculum length was noted in Rodentia and Carnivora, but the same test found no relationship in Primates or Chiroptera.
The adaptive function of the baculum, under high levels of intra-sexual selection, is yet to be established. A potential strategy by which a male could increase their reproductive success, by outcompeting rival males, is through prolonging intromission and consequently delaying a female mating with another male [12]. The prolonged intromission hypothesis argues that the baculum helps to facilitate this prolonged duration of intromission by supporting the penis [13].
In this context, the proximate mechanism of the baculum is to act as a supportive rod, strengthening the penis and protecting the urethra during prolonged intromission [12]. A recent study on three different species of bat found that the baculum formed a functional unit with the corpora cavernosa, which protected the glans tip and the shaft of the penis when erect [14]. The authors posit that the baculum also helps to limit constriction of the distal urethra and urethral opening in the erect penis during intromission, facilitating sperm flow. In many species of primate in which the baculum is elongated, the distal end of the bone projects slightly from the urethra while the penis is erect [15]. This could bring the baculum into contact with a female's cervix during intromission, facilitating the transfer of semen into the cervical canal [12,15].
Evidence for the prolonged intromission hypothesis has so far proven controversial. Early studies found that prolonged intromission was correlated with elongated bacula in primates and carnivores [13,16]; however, these studies did not take into account the statistical non-independence of data that arises due to a shared evolutionary history between species. A later study, corrected to account for phylogeny, tested for a correlation between prolonged intromission and baculum length in North American carnivores, but did not find support for the hypothesis [17]. However, data to test this hypothesis were only available for 18 species, of which only two were characterized as having short intromission duration. Dixson et al. [18] argue that this sample is not representative enough to decisively refute the prolonged intromission hypothesis, and carried out their own phylogenetically corrected analysis in a sample of 57 species of mammal. This time, a significant correlation was found between the two variables. Although both studies were corrected for phylogeny, if the degree of phylogenetic dependency is not established before an analysis is adjusted, correcting for phylogeny can produce misleading or incorrect results, as the level of relatedness between species varies across a phylogeny [19]. Bayesian Markov chain Monte Carlo (MCMC) analyses enable species' phylogenies to be incorporated into an analysis, rather than simply correcting for phylogeny, and can thus produce more reliable results [20,21].
In a new study, Schultz et al. [22] used the earlier phylogenetic methods of stochastic mapping to model the presence and absence of the baculum in 954 mammalian species, and argue that the baculum independently evolved a minimum of nine times in mammals. However, this sample is unlikely to reflect the course of evolution across the entire mammalian class, particularly as baculum absence was only noted in 103 species.
Here, we use phylogenetic comparative methods within a Bayesian MCMC framework [23] to examine the evolutionary history of the baculum and investigate the hypothesis that increased levels of intra-sexual selection affect baculum evolution. We first reconstructed the evolutionary trajectory of the baculum across the entire mammalian class and examined the rate of bacular evolution in the primate order. Then, we tested for a relationship between baculum length and testes mass in both primates and carnivores. We then further tested for correlated evolution between baculum presence and intromission duration in primates, before conducting phylogenetic t-tests to establish whether primates and carnivores with prolonged intromission durations have longer bacula than those with short intromission durations. Finally, we used the same tests to examine whether increased levels of postcopulatory sexual selection pressure caused by (i) polygamous mating systems and (ii) seasonal breeding patterns led to an increase in baculum length in primates. Primates and carnivores are likely to be particularly rewarding groups to study, because there is a mixture of baculum presence and absence within each order. This means that differences between those with and without bacula can be tracked at the species level, rather than across orders. Furthermore, as these orders are arguably more extensively studied than other mammalian groups, there are more data available.
If the baculum facilitates prolonged intromission and increased proximity to the cervix in order to reduce the level of sperm competition and increase reproductive success, then several predictions can be made and tested. It would be expected that there would be a relationship between baculum length and testes mass, which can be used as a proxy for the level of postcopulatory sexual selection pressure in a population. Intromission duration would be expected to correlate with baculum presence across the course of evolution. Species with prolonged intromission durations should have elongated bacula. Finally, groups in which postcopulatory sexual selection pressure is highest, such as those in which mating is polygamous or occurs seasonally, should have longer bacula than groups with lower levels of postcopulatory sexual selection pressure.
Matilda Brindle, Christopher Opie
Postcopulatory sexual selection influences baculum evolution in primates and carnivores
Proceedings of The Royal Society B DOI https://doi.org/10.1098/rspb.2016.1736
© 2016 The Authors.
Reprinted under the terms of the Creative Commons Attribution 4.0 International License (CC BY 4.0)
So it seems probable that human males lost their baculum after the split from the other apes and after the mating system changed to become more monogamous or at least polygynous so there was reduced male postcopulatory competition. This may have been as recent as 2 million years ago so it will be interesting to see if traces of the baculum are ever found in Australopithecines such as Australopithecus afarensis or Au. sediba. This in turn would give us a clue as to their mating strategies and so to their tribal/family structure.
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