Study sheds light on the function of the penis bone in male competition

December 13, 2016

A new UCL study examines how the baculum (penis bone) evolved in mammals and explores its possible function in primates and carnivores - groups where many species have a baculum, but some do not.

The baculum has been described as "the most diverse of all bones", varying dramatically in length, width and shape in the male mammals where it is present.

The research, published today in the Royal Society journal Proceedings B, shows that the ancestral mammal, like humans, did not have a baculum - but both ancestral primates and carnivores did. The work uncovers that the baculum first evolved in mammals between 145 and 95 million years ago.

The study found that prolonged intromission - defined as penetration for longer than 3 minutes - was correlated with baculum presence across the course of primate evolution. Prolonged intromission was also found to predict a longer baculum in primates and carnivores.

High levels of postcopulatory sexual competition between males also predicted longer bacula in primates.

First author, Matilda Brindle (UCL Anthropology), said: "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."

The findings of the study may also provide clues as to why humans do not have a baculum.

When any cultural aspects of sex are removed and a male's aim is solely to ejaculate, humans have a short intromission duration.

In species where mating occurs between multiple males and multiple females (known as polygamy), there is acute competition between males to fertilise a female. However, human mating systems are not like this. Instead humans tend to be monogamous or, more rarely, polygynous (where one male mates with multiple females). In these circumstances, only one male has access to a female and postcopulatory competition between males is absent or very low level.

Brindle added: "Interestingly, humans have neither prolonged intromission durations, nor high levels of postcopulatory sexual competition. Given the results of our study, this may help to unravel the mystery of why the baculum was lost in the human lineage."

Chimpanzees and bonobos, humans' closest relatives, have very small bacula (between about 6-8mm) and short intromission durations (around 7 seconds for chimpanzees and 15 seconds for bonobos). However, they are characterised by polygamous mating systems, so they experience high levels of postcopulatory competition between males. The researchers suggest that this may be why these species have retained a baculum - albeit a small one.

Co-author, Dr Kit Opie (UCL Anthropology), commented: "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."
-end-
Notes to Editors:

1. For more information, copies of the paper, or interview requests please contact Ruth Howells in UCL Media Relations on mob: +44 (0)7990 675 947, email: ruth.howells@ucl.ac.uk

2. The research paper 'Postcopulatory sexual selection influences baculum evolution in primates and carnivores' is due to be published online in the Royal Society journal Proceedings B, embargoed until Wednesday 14 December 2016 00.01 (GMT)

About UCL (University College London)

UCL was founded in 1826. We were the first English university established after Oxford and Cambridge, the first to open up university education to those previously excluded from it, and the first to provide systematic teaching of law, architecture and medicine. We are among the world's top universities, as reflected by performance in a range of international rankings and tables. UCL currently has over 38,000 students from 150 countries and over 12,000 staff. Our annual income is more than £1 billion http://www.ucl.ac.uk | Follow us on Twitter @uclnews | Watch our YouTube channel YouTube.com/UCLTV

University College London

Related Primates Articles from Brightsurf:

When new males take over, these female primates hurry up and mature
Most mammals--including humans and other primates--reach sexual maturity early or late depending on lots of different factors, such as how much food there is to eat.

Primates aren't quite frogs
Researchers in Japan demonstrated for the first time the 'spinal motor module hypothesis' in the primate arm, wherein the brain recruits interneuronal modules in the spinal cord rather than individual muscles to create movement and different modules can be combined to create specific movements.

Researchers reversibly disable brain pathway in primates
For the first time ever, neurophysiologists of KU Leuven, Harvard and the University of Kyoto have succeeded in reversibly disabling a connection between two areas in the brains of primates while they were performing cognitive tasks, or while their entire brain activity was being monitored.

The larynx has evolved more rapidly in primates
The larynx is larger, more variable in size, and has undergone faster rates of evolution in primates than in carnivores, according to a study published August 11, 2020 in the open-access journal PLOS Biology by Daniel Bowling of Stanford University, W.

Single-shot COVID-19 vaccine protects non-human primates
A leading COVID-19 vaccine candidate, developed at Beth Israel Deaconess Medical Center, creates the groundwork for a newly launched COVID-19 vaccine clinical trial.

Research bias may leave some primates at risk
Recent primate research has had a heavy focus on a few charismatic species and nationally protected parks and forests, leaving some lesser known primates and their habitats at risk, according researchers at The University of Texas at Austin and Santa Clara University.

Showy primates have smaller testicles
Well-adorned or well-endowed -- but not both. Evolutionary biologists at the University of Zurich have for the first time demonstrated that male primates either have large testicles or showy ornaments.

Short birth intervals associated with higher offspring mortality in primates
Shorter intervals between primate births are associated with higher mortality rates in offspring, finds a new study of macaque monkeys.

HIV vaccine protects non-human primates from infection
New research shows that an experimental HIV vaccine strategy works in non-human primates.

Oldest-known ancestor of modern primates may have come from North America, not Asia
A new fossil analysis suggests the earliest-known ancestor of modern primates may have come from North America, not Asia, as previously thought.

Read More: Primates News and Primates Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.