All the chimp (and one bonobo) presentations to see at the #AAPA2016 meeting


More or less in the order in which they appear in the program, here are all the chimpanzee (and the only one bonobo) behavioral ecology talks & posters to watch out for next week in Atlanta.

Research from Kibale (Kanyawara & Ngogo) and Gombe (Uganda and Tanzania, respectively) dominate the program, as usual, but there are also new data from Fongoli (Senegal) and the Goualougo Triangle (Congo)!

There are also some fascinating comparative studies that explicitly compare data on chimpanzee and human physiology and life history variation. Must-see!

(click on title for abstract and other info)

Dominance styles of eight alpha male chimpanzees at Gombe National Park, Tanzania

Relationship of dominance ranks to age and coalition formation among male chimpanzees at Ngogo

Dominance Rank, Male-Female Aggression, and Male Mating Success Among Chimpanzees at Ngogo

Predation by female chimpanzees at Gombe and Kibale: toward an understanding of sex differences in meat acquisition among early hominins

The diet of savanna-woodland chimpanzees (Pan troglodytes schweinfurthii) at Issa, western Tanzania

Chimpanzee laterality redux

Alternative reproductive tactics among male chimpanzees in Gombe National Park, Tanzania

Chimpanzees and the phylogenetic origins of multi-level sociality in humans

Parasite infections reveal costs and benefits of social behavior in a community of wild chimpanzees

Adolescent male chimpanzees do not form a linear dominance hierarchy with their peers

Social bonds in adolescent and young adult male chimpanzees at Ngogo, Kibale National Park, Uganda

Rethinking chimpanzee tool use: Niche construction and developmental bias in maintaining technological traditions among African apes

Comparing the spatial dimensions of gorilla and chimpanzee sleeping sites: Nearest-neighbor nest distances of sympatric apes along a conservation gradient

Paternal age at conception predicts offspring telomere length in chimpanzees to a greater degree than in humans

Dynamics of human-chimpanzee encounters at Fongoli, Senegal, 2006-2014

Nutritional composition of plant foods consumed by the chimpanzees of Gombe National Park, Tanzania

Low Mortality Rates among Ngogo Chimpanzees: Ecological Influences and Evolutionary Implications

Somatic senescence in female chimpanzees (Pan troglodytes) occurs earlier and more rapidly than in women

Hormonal Correlates of Sexual Swellings During Pregnancy in Wild Chimpanzees (Pan troglodytes schweinfurthii) of Kanyawara and Gombe

New data on female maturation milestones indicate longer development in wild chimpanzees

Testosterone, musculature, and development in Kanyawara chimpanzees and Tsimane forager-horticulturalists

Fecal stable isotopes (δ13C:δ15N, %N) used to track infant nutritional development and reveal average weaning age in wild chimpanzees

Infant handling and urinary oxytocin in sub-adult bonobos (Pan paniscus): support for the learning to mother hypothesis


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Macaques at the AAPA meeting (2016)

juv feeding on flower

Fan of macaques? Here are all the talks & posters you need to see at the AAPA meeting next week. Clink on titles to go to the abstract (with info on time and location of the presentation).

And follow the conference on Twitter (#AAPA2016)

Rhesus macaques

Social status, seasonality, and stress: Variation in glucocorticoid concentrations of high-ranking male rhesus macaques (Macaca mulatta)

Life in a new world: The Silver River rhesus macaques’ feeding ecology and encounters with humans

Infant weight growth and weaning age in free-ranging rhesus macaques (Macaca mulatta)

Evidence that male face shape in rhesus macaques (Macaca mulatta) is subject to inter-sexual selection

Maternal body composition predicts prepubertal fat mass accrual in female offspring


Other macaques 

Social Network Analysis of Stone Handling and Object Manipulation Among Long-Tailed Macaques (Macaca fascicularis) in Bali, Indonesia: A Preliminary Analysis

Network Position and Human Presence in Barbary Macaques of Gibraltar

Social network analysis of Barbary macaque (Macaca sylvanus) dominance structure

Object preferences and the function of object play behavior in a provisioned troop of long-tailed macaques (Macaca fascicularis) at Wat Khao Takieb, Thailand

The Monkeybar Project: Population Density of Long-Tailed Macaques (Macaca fascicularis) in Two Different Forest Types in Kudat District, Sabah, Malaysia

Comparison of the vaginal, fecal, and rectal microbiome in captive Pigtailed Macaques (Macaca nemestrina)

Using ancient DNA from museum specimens for phylogenetic correction to interpret island dwarfing in Macaca fascicularis


This month in primatology (Feb 2016)

At the end of February, here’s a non-exhaustive list of papers on primate behavioral ecology from across a variety of journals.

Dominance relations & social competition

The effect of intergroup competition on intragroup affiliation in primates

Dominance relationships in male Nepal gray langurs (Semnopithecus schistaceus)

The dark side of the red ape: male-mediated lethal female competition in Bornean orangutans

Breaking the succession rule: the costs and benefits of an alpha-status take-over by an immigrant rhesus macaque on Cayo Santiago

Obstacles and catalysts of cooperation in humans, bonobos, and chimpanzees: behavioural reaction norms can help explain variation in sex roles, inequality, war and peace


Promiscuous primates engage in same-sex genital interactions



Enduring voice recognition in bonobos

Bonobos (Pan paniscus) vocally protest against violations of social expectations

Social familiarity affects Diana monkey (Cercopithecus diana diana) alarm call responses in habitat-specific ways

Food-Associated Calling in Gorillas (Gorilla g. gorilla) in the Wild

Copying hierarchical leaders’ voices? Acoustic plasticity in female Japanese macaques


Life history & development

Weaned age variation in the Virunga mountain gorillas (Gorilla beringei beringei): influential factors

Male Infanticide in the Golden Snub-Nosed Monkey (Rhinopithecus roxellana), a Seasonally Breeding Primate

Infant Mortality Risk and Paternity Certainty Are Associated with Postnatal Maternal Behavior toward Adult Male Mountain Gorillas (Gorilla beringei beringei)

Early-life social adversity and developmental processes in nonhuman primates

Non-Maternal Infant Handling in Wild Formosan Macaques of Mount Longevity, Taiwan


Social networks, sociality, cooperation

Social structure and Escherichia coli sharing in a group-living wild primate, Verreaux’s sifaka

Network centrality and seasonality interact to predict lice load in a social primate

Bystanders, parcelling, and an absence of trust in the grooming interactions of wild male chimpanzees

The ties that bind: Maternal kin bias in a multilevel primate society despite natal dispersal by both sexes


Culture & innovation

Chimpanzee accumulative stone throwing

The reluctant innovator: orangutans and the phylogeny of creativity

The coevolution of innovation and technical intelligence in primates

Animal and human innovation: novel problems and novel solutions

A multidisciplinary view on cultural primatology: behavioral innovations and traditions in Japanese macaques


Ecology, variation & conservation

Nutritional consequences of folivory in a small-bodied lemur (Lepilemur leucopus): Effects of season and reproduction on nutrient balancing

Adapting to Florida’s riverine woodlands: the population status and feeding ecology of the Silver River rhesus macaques and their interface with humans

South to south learning in great ape conservation

Is Tickling Torture? Assessing Welfare towards Slow Lorises (Nycticebus spp.) within Web 2.0 Videos

Chimpanzee research and conservation in Bossou and the Nimba Mountains: a long-term international collaborative effort in West Africa


This month in primatology (January 2016)

Gratuitous photo of rhesus macaque juveniles doing a bit of rough-and-tumble play

We are only a month into 2016, and primatologists have been very busy! I’ve compiled a list of the most important/interesting/exciting/cool articles that have been released since the start of the year (in press/early view/advance access etc.). Papers that were released electronically in 2015 and have now been assigned to an actual issue in a journal are not counted. Obviously, this is a subjective list, reflecting my main interests in primatology (behavioural ecology & sociality) so no doubt many excellent studies that deal with topics I do not follow too closely are left out. I’ve organised the studies under several non-mutually exclusive categories – depending on the primary theme of each article.

Now that’s cleared up, here’s my primate-themed January reading list (in almost no particular order):

Sexual selection & reproductive ecology

Sexual Selection and the differences between the sexes in Mandrills (Mandrillus sphinx)

Male endocrine response to seasonally varying environmental and social factors in a neotropical primate, Cebus capucinus

Vocal complexity influences female responses to gelada male calls

Female chimpanzees adjust copulation calls according to reproductive status and level of female competition

Exaggerated sexual swellings in female nonhuman primates are reliable signals of female fertility and body condition

Consequences of a male takeover on mating skew in wild Sanje mangabeys

Maternal effects on offspring stress physiology in wild chimpanzees

Sex or power? The function of male displays in rhesus macaques

Social life

Obstacles and catalysts to peaceful coexistence in chimpanzees and bonobos

Sex and friendship in a multilevel society: behavioural patterns and associations between female and male Guinea baboons

Working the crowd: sociable vervets benefit by reducing exposure to risk

Strong, equitable and long-term social bonds in the dispersing sex in Assamese macaques

Experience-independent sex differences in newborn macaques: Females are more social than males

Adaptations for social cognition in the primate brain

Social behavior shapes the chimpanzee pan-microbiome


Effects of group size and contest location on the outcome and intensity of intergroup contests in wild blue monkeys

Social power, conflict policing, and the role of subordination signals in rhesus macaque society

Ecology & conservation

Feeding habitat quality and behavioral trade-offs in chimpanzees: a case for species distribution models

Spatio-temporal complexity of chimpanzee food: How cognitive adaptations can counteract the ephemeral nature of ripe fruit

Sympatric Apes in Sacred Forests: Shared Space and Habitat Use by Humans and Endangered Javan Gibbons (Hylobates moloch)



Journals checked for papers:

  • The discipline journals: American Journal of Primatology, International Journal of Primatology, Primates, American Journal of Physical Anthropology
  • Behavioural journals: Behavioural Ecology, Animal Behaviour, Behavioral Ecology and Sociobiology, Ethology, Behaviour, Hormones and Behavior, Physiology and Behavior
  • Broader journals: Current Biology, Science Advances, Nature Communications, Science, Science Advances, Nature, Scientific Reports, Biology Letters, Proceedings B, Proceedings of the National Academy of Sciences of the USA, Open Science, eLife, PeerJ, PLOSOne, Functional Ecology, Journal of Animal Ecology, Ecology and Evolution, Evolution

New papers (9 – 16 Jan): crickets, tree frogs, and great tits

Comparing pre- and post-copulatory mate competition using social network analysis in wild crickets (open access article!)

The two types of mating competition – pre- and post-copulatory are usually seen as alternative tactics that male animals can adopt as they try to ensure they sire as many offspring as possible. This study, however, shows this not to be the case, at least in crickets. Using social network analysis, the authors found no trade-off between pre- and post-copulatory competition: males who fought each other more during pre-copulatory competition also interacted indirectly with one another later on, during the post-copulatory phase. The really interesting bit is how post-copulatory social networks were constructed. Males were linked to one another if they successfully mated (by transferring a batch of sperm in a spermatophore, a structure typical of insects) with the same female and were thus considered to be in sperm competition. Fighting off competitors was not an effective means from preventing them from copulating with particular females. The authors conclude their data support the idea that in species where males cannot monopolize mating, the evolution of alternative male phenotypes is unlikely. They also suggest that mating success can be a poor proxy for reproductive success, given that males who mate more often may lose out more paternity via sperm competition. A useful reminder for those of us, studying primate mating behaviour and mating success.

Male treefrogs in low condition resume signaling faster following simulated predator attack

Individuals that have less to loose are expected to take greater risks (especially when the reproductive payoffs of doing so may be significant). This study shows this to be true in male treefrogs as they called to attract mates. The time they took to resume calling (advertising themselves as potential mating partners to nearby females) after a simulated predator attack (grabbing the frog) was shorter in males who were in poor condition – i.e. low-quality males were willing to take greater risks in the face of potential predation. Some caveats apply (condition predicted response in only one of two years; old age did not predict shorter call resumption times, contrary to predictions) but the results provide a nice example of the how individual physical condition may affect risk-taking behavior and thus potentially affect male mating success.

To sing or not to sing: seasonal changes in singing vary with personality in wild great tits

Staying with the topic of signalling, this study adds another dimension to our understanding of what determines how much effort males put into advertising their presence to mates and competitors. Personality matters, too. Male great tits that were determined to be ‘faster explorers’ (i.e. less shy), increased their singing more during the periods their mate was most fertile and during periods of maximum maternal investment (egg laying and incubation). This contrasts with earlier ideas that faster explorers pay less attention to changes in their social environment than slow explorers. The increased singing of fast explorers during key stages of the reproductive cycle was also correlated with more fledged chicks. This finding supports the idea that greater singing activity may reflect the quality of the males or the territory they occupy but the exact mechanism that links male personality and their singing patterns to increased reproductive success remains an open question.

New papers (Jan 1-8, 2016)

In an effort to keep on top of all the exciting papers I want to read (in a perfect world)  or, at least, skim this year, I am starting a weekly-ish round-up of those that catch my eye.

In a descending order, here are my top picks published online (early view) since the start of this year:

The rise and fall of a male rhesus macaque: Part I


Rhesus macaques are odd. Particularly, if you come to them from studying chimpanzees, which was true for me when, in late 2012, I went to Cayo Santiago, in Puerto Rico, to spend about 1.5 years with these monkeys.

In chimpanzees, males spend their entire lives in the group they were born into. They know each other well and some of them form strong affiliative bonds. In rhesus, males usually leave their natal group shortly after reaching puberty and immigrate into a new group where they will seek mates and reproduction. Unlike chimps, male rhesus macaques don’t seem to care about each other very much and male-male friendships are not known to exist. At best, most of them appear indifferent about their male group-mates. A stark contrast to the strong, cooperative, intense relationships I have seen among chimpanzee males. In both species males form clear linear dominance hierarchies but while among chimps there is intense aggressive competition for the alpha position, this is not the case among rhesus males. Instead, male rhesus macaques are known for an unusual system of rank-acquisition: they queue for rank rather than fight for it. It’s called succession-based rank-acquisition and each male start out at the very bottom when he moves into a new group. With time, as males above him in the hierarchy die or emigrate to other groups, the rank of the newcomer increases, until eventually he may get to the alpha spot. This was the picture I got from all the papers I read about rhesus macaques before travelling to Puerto Rico.

Cayo view sunrise.JPG
Cayo Santiago – off the coast of Puerto Rico is home to ca. 1,300 – 1,400 rhesus macaques.

I spent October and November 2012 figuring out who is who in my study group and how they interact with each other. It was the non-mating season. Most females had already given birth in the preceding months and were raising their infants. The males spent most of their time relaxing, feeding and not doing much else. Although the literature had prepared me for what I saw – I was still surprised to see how little they engaged with each other. Very little grooming, little proximity, just a lot of aloofness. If chimpanzee males were like the raucous members of a football team who, most of the time, got along splendidly, enjoyed each other’s company and did things together for the fun of it, rhesus males, on first impressions, were just a bunch of random guys hanging out at the doctors’ waiting room – respectful of each others’ space, polite but somewhat cold and indifferent to one another*.

(*Beware the anthropomorphisms! This is clearly a simplified/exaggerated description of male social relationships but it does convey my subjective impression of the differences between these two species.)

Trying to figure out the male dominance hierarchy in my rhesus study group (Group S) wasn’t as easy as I expected. There were few clear indications of who was the alpha and where each male fitted in the hierarchy. I talked to other researchers and staff of the Caribbean Primate Research Center to get their perspective and I collected data on even the subtlest agonistic (non-friendly) interactions between males that I could discern. One such subtle signal of existing dominance relations is the so-called approach-avoid interaction. One male would be walking in the general direction of another male and the other would actively avoid him by moving to the side and away – even in the absence of any specific threat from the approaching male. The one who avoids is lower down the hierarchy than the one who is approaching. Another good sign of high rank was the way an alpha would ‘carry’ himself around the group. I would have to watch the body language of my male study subjects closely. The alpha would appear confident, strutting casually around, with an air of superiority, his tail curled up in a twirl above his back. I could occasionally notice these telltale signs in the male, who everyone who knew this group of monkeys told me was the alpha – the one called 53N.

53N – the alpha in Group S at the start of my study

Yet he was surprisingly nonplussed about being alpha. None of the bravado that I came to expect from an alpha male, based on my experience with the Kanyawara chimpanzees that I studied a few years back in Uganda. He did not constantly run around the group, showing off his strength and intimidating rivals. Nor did other males acknowledge his superiority in the ranking by ritualized signals of submission such as a pant-grunt (a common and useful vocal signal for studying chimpanzee dominance relations but completely absent in rhesus). In short – 53N was a very relaxed alpha male.

He had immigrated into Group S about 4.5 years ago. The two other males at the top of the hierarchy had moved into Group S at about the same time – within a month or two at most of each other. All three of them were born in the same group and they were the oldest (at just over 13 years of age) males in their current group – Group S. These three males were at the top of the tenure-based queue of succession – the mean tenure length (the duration of time they had spent in the group) of the remaining 6 immigrant males was 1.7 years (between 0.5 – 3.9 years, calculated as of 1st March 2013). The males of Group S thus presented a textbook example of male dominance relations in rhesus macaques – the males who had been in the group the longest occupied the highest ranks. The males who had joined the group just recently – were the lowest ranking ones.

The three oldest males in the group (53N, 48L, and 03N) had clearly spent most of their lives in each other’s company yet they showed surprisingly little indication of any sort of long-term bonding or friendliness (again, in comparison to the easily observable signs of such bonds among male chimpanzees). Interestingly, there were also very few clear signs of competitiveness or aggression among these three top-ranking males. It was almost as if they existed in parallel realities, barely acknowledging each other’s existence. Perhaps this was because the mating season was still a couple of months away and there was nothing to fight over just yet.

The mating season on Cayo Santiago. Both sexes develop more intense/redder coloration on their faces and around their genitals – a form of sexual signaling.

By mid-February 2013, ‘love’ was in the air. Mating begins at slightly different times in the different groups of rhesus on Cayo Santiago. I saw my first rhesus macaque copulation sometime in January in one of the groups on the lower parts of the island. Mating in our study group on the top of the hill, however, did not properly get going until the end of February/early March. It was then, at the start of mating season that something quite unexpected happened.

To be continued: Part II


Gorillas in black and white

The BBC breaks new ground (yet again) in a two-part documentary about Grauer’s gorillas and the quest of cameraman Gordon Buchanan to film them in the Democratic Republic of Congo.

For the first time, we see what these rare gorillas do at night in their nests up in the trees. Sensitive night-vision cameras reveal their activity in amazing detail. It’s a delightful programme. The night-time visuals are so good you can actually ‘see’ the breath of the dozing ape –  by the subtle, rhythmic changes in the intensity of the colour of the gorilla’s nostrils -in glorious black-and-white. I recommend watching the clip full-screen.

Gorilla Family & Me starts on BBC 1 Monday (21 December 2015) and it comes with a detailed website presenting a lot of background info on the work of the park guides, scientists, and the TV crew in Kahuzi-Biega National Park. There are also detailed profiles of the gorillas themselves. Well worth a browse.

To find out more about the behavioural ecology of Grauer’s gorillas have a look at the papers below:

Helpers at the nest buffer the oxidative costs of reproduction

A rare experiment with cooperatively breeding birds in the wild reveals high costs of reproduction but only in groups with few helpers.

The white-browed sparrow weaver (Photo: Dom Cram)

Every breath we take damages our cells a little. This inevitable cost of metabolism, oxidative damage, accumulates with time and can contribute to pathology and senescence1. The potential role of oxidative stress in explaining inter-individual differences in life history trajectories and fitness has not gone unnoticed by evolutionary ecologists. Researchers have proposed that the accumulation of oxidative damage is expected to occur at greater rates during periods of heavy reproductive investment and thus may be a key physiological mechanism mediating life history trade-offs between current and future reproduction, and between reproduction and survival2,3. Empirical studies, however, have produced conflicting results4. The failure of some to show the expected relationship between reproduction and oxidative damage has been attributed to the lack of experimental manipulation of reproductive effort or to the fact that many studies were conducted in laboratory conditions where subjects are removed from ecological constraints5.

Writing in Proceedings of the Royal Society B: Biological Sciences, Cram and colleagues6 overcome some previous methodological issues by conducting a clutch-removal experiment with a cooperatively breeding bird in the Kalahari. The white-browed sparrow weaver, Plocepasser mahali, nests in territorial groups of 2-12 individuals, in which one dominant pair monopolises within-group reproduction and the remaining subadults help raise the offspring. To assess how reproductive effort affects oxidative status, Cram and colleagues monitored 20 groups of weavers until a full clutch of eggs was laid in each nest. They then captured all birds (except the incubating dominant females), weighed them and collected blood samples. In 9 of the groups, they removed all eggs. About a month after clutch-removal, Cram and colleagues re-captured the birds in all groups and collected follow-up measurements on condition and oxidative stress to compare if the birds with chicks differed from those without.

An 8-day old chick (Photo by Dom Cram)

Contrary to expectations, there was no difference in weight loss between birds in groups that had their eggs removed and birds in groups that kept their eggs but had many helpers (group size 5-8 birds). Reproductive effort seemed to be cost-free. Individuals in small groups (2-4 birds) that raised chicks and had few helpers, however, lost more weight compared to the birds that underwent clutch removal. The data on oxidative stress paralleled the findings on weight loss.

Dom Cram collecting a blood sample from one of the weavers.

Birds in small groups, rearing chicks, experienced increased levels of oxidative damage, compared to the birds whose eggs were removed. Breeding birds in large groups with chicks did not differ in oxidative damage from the birds not raising any chicks. Importantly, these effects were not due to differences in clutch size. Rather, it was the number of helpers available in large groups that acted as a buffer against the costs of increased reproductive
effort. This interpretation was further supported by the observation that among chick-raising birds, rates of provisioning were negatively associated with body mass – individual investment in reproductive effort carried significant energetic costs for the helpers. Although the level of oxidative damage was not associated with rates of provisioning, the level of total antioxidant protection was. Birds with higher provisioning rates exhibited lower antioxidant protection, making them more vulnerable to oxidative damage.

As the weaver chicks grow, the demand on all the helpers to bring them food increases (Photo: Dom Cram)

This study measured only one marker of oxidative damage, lipid peroxidation, in plasma samples. Its conclusions are thus tentative, as oxidative damage to other biomolecules and in other tissues may reveal different patterns. Nevertheless, the contribuion of this work is important – not only because it is one of the few demonstrations of the oxidative costs of reproduction in the wild but because, uniquely, it reveals the role of cooperative breeding as a mechanism for mitigating these costs.


  1. Costantini, D. Oxidative Stress and Hormesis in Evolutionary Ecology and Physiology. (Springer Berlin Heidelberg, 2014). doi:10.1007/978-3-642-54663-1
  2. Metcalfe, N. B. & Alonso-Alvarez, C. Oxidative stress as a life-history constraint: the role of reactive oxygen species in shaping phenotypes from conception to death. Functional Ecology 24, 984–996 (2010).
  3. Monaghan, P., Metcalfe, N. B. & Torres, R. Oxidative stress as a mediator of life history trade-offs: mechanisms, measurements and interpretation. Ecology Letters 12, 75–92 (2009).
  4. Blount, J. D., Vitikainen, E., Stott, I. & Cant, M. A. Oxidative shielding and the cost of reproduction. Biological Reviews DOI: 10.1111 / brv, (2015).
  5. Metcalfe, N. B. & Monaghan, P. Does reproduction cause oxidative stress? An open question. Trends in Ecology & Evolution 28, 347–350 (2013).
  6. Cram, D. L., Blount, J. D. & Young, A. J. The oxidative costs of reproduction are group-size dependent in a wild cooperative breeder. Proceedings of the Royal Society B: Biological Sciences 282, 20152031–20152031 (2015).


Copulatory plugs: males’ attempt at out-sperming their competitors

Chimpanzee sperm plug on the forest floor.

Picture the scene. You are trying to see which male is mating with this very attractive and sexually receptive female chimpanzee. She’s perched high up in the canopy above. The only place where you can get a good look at the action through all the foliage happens to be directly below the branch on which she is currently standing on. The male approaches briskly from behind. You peer through your binoculars with your neck craned up and aching. Then through the limited field of vision of your binoculars you see something falling down straight at you. It appears to have fallen out of the female at the moment that the male mated. You duck and dive and narrowly miss an unnecessarily close encounter with what, you later learn from the more experienced chimp-watchers around you, was a ‘sperm plug’. Up to that moment, sometime in the summer of 2006 in Kibale National Park, Uganda, I knew next to nothing about these structures. This was easily the most exciting personal ‘discovery’ of my first year in graduate school.

So why do chimpanzees (as opposed to any other primate or animal) have sperm plugs (more accurately called ‘copulatory plugs’) and what are they for? Were these things common in other primates, too? I was quite sure they were not part of human reproductive biology but I wasn’t too certain about the rest of the primate order.

Chimp plug 2
A female chimpanzee inspecting a plug she just removed from her vagina.

I wasn’t the first person to ask these questions. Taking a wide-angle view, via comparative analysis, Dixson and Anderson rated the level of seminal coagulation of the ejaculates of 40 primate species on a 4-point scale. Species with no visible signs of coagulation received a score of 1 and those whose ejaculates coagulated to the point of forming a firm plug received a score of 4. Clearly there were other primates, besides chimps that had copulatory plugs.

Copulatory plug in a female rhesus macaque: less solid than in chimpanzees
Copulatory plug in a female rhesus macaque: less solid than in chimpanzees.

If the function of sperm coagulation and plug formation was to aid males in competing indirectly for fertilizations (via post-copulatory sperm competition within the reproductive tracts of females), the authors predicted that species, in which females commonly mate with multiple males, would receive higher coagulation scores. And they did. On average, species with multiple-male mating had a coagulation score of 3.64, while those, in which females usually mated with only one male had a mean score 2.09. As a point of reference, according to this classification, the ejaculates of humans and gorillas both got a score of 2 (‘semen becomes gelatinous and remains semifluid but there is no distinct coagulum’), most macaques and baboons got a 3 (‘semen coagulates so that the coagulate forms a whitish, non-fluid, non-gelatinous mass, but not a compact plug’) and chimpanzees scored top marks, at 4 (‘there is a distinct compact, rubbery or semi-solid copulatory plug, retaining its shape and molded to the contours of the female’s tract’).

Another bit of evidence supporting the important role of sperm coagulation and plugs for the process of sperm competition comes from genetics. Dorus and colleagues looked at the relationship between the rate of evolution of a major seminal protein gene and the levels of female promiscuity among 12 primate species. They found that species with greater levels of promiscuity (e.g. chimps) were characterized by much higher rates of evolution of that particular gene. The species with the highest rates of molecular evolution also scored higher on the ‘sperm coagulation scale’ used by Dixson and Anderson. The genetic evidence thus suggested that selection for specific traits had occurred – traits that enabled males to do better when it came down to sperm competition.

These observations, however, do not provide conclusive evidence of the reproductive benefits that males who produce ‘better’ or ‘bigger’ sperm plugs might enjoy. Obtaining more solid ‘proof’ however could be tricky in species that are long-lived, slow-reproducing and, very often, endangered in their wild habitats (i.e. most primates). This is why a lot of the most revealing studies of sperm plugs have been carried out in other animals. Such as small rodents. They are fast-reproducing, amenable to clever experimental manipulations and overwhelmingly not at risk of extinction in the wild (which allows somewhat more invasive methodological approaches).

The function of copulatory plugs in house mice

In a study published in Behavioral Ecology this August, Sutter and colleagues wanted to test the hypothesis that copulatory plugs in mice had a role in the process of sperm competition. To do that they needed to document how the presence of plugs affects the subsequent sexual interactions of the female and whether variation in plug size affected the reproductive success of the males who produced those plugs.

So they rounded up some laboratory domestic mice and got them to mate in a carefully orchestrated series of experiments. Naturally, they recorded all the action on video. When males mated successively, the plug they produced the second time around was significantly smaller than the first one by about 19.2 mg. The duration between successive copulations also seemed to matter: males who had more time to recover after sex (up to 56 hours) tended to produce larger plugs during their second copulation than those who had little time to rest between matings (as little as a 2 hours). The size of the plug was affected by ejaculate depletion and thus time since last mating could be used as a proxy for plug size.

Next, the researchers wanted to examine how plug size affected paternity. Female mice produce a lot of embryos at once and multiple paternity is common (i.e. not all embryos are sired by the same male). In this study, females mated with two different males and the goal was to establish how the size of the copulatory plug deposited by the first male affected the proportion of embryos that the second male got to sire. To figure out who the sire was the females who were part of the experiment were sacrificed and their embryos extracted. Genetic testing established the paternity of the embryos. The size of the copulatory plug deposited by the first male was, indeed, important for his reproductive success: second-maters sired a greater proportion of embryos (within the same female) when the male that mated before them had recently mated himself (i.e. the plug he left inside the female would have been smaller). If the preceding male had not mated recently (i.e. was able to produce a larger plug), the proportion of embryos sired by the second-mating male was significantly lower. So there was evidence that sperm plugs have an important function in sperm competition! Larger plugs were better at preventing other males from siring more offspring.

Although this second experiment relied on the sexual restedness of the first male (i.e. the number of days since his last copulation) as a proxy indicator of plug size, rather than directly measuring the size of the plug (the plug had to remain in place to assess its function) – in combination with the previous experiment, which showed the relationship of sexual restedness to plug size – this study does demonstrates the adaptive function of sperm plugs in species with multiple mating.

Statistically speaking, however, the effect of inferred sperm plug size (sexual restedness of the first male) on the proportion of offspring sired by a second male was reduced (still negative, but non-significant) when another variable was included in the same analysis – the ease of mating by the second male. Second-mating males who mated with greater ‘ease’ (those who were quicker to ejaculate and achieved more ejaculations) secured a greater share of the embryos that were conceived. The study also showed that ease of mating for the second male was negatively affected by the sexual restedness of the first-mating male (as a proxy of plug size). This effect, however, was small and non-significant. So not entirely an open-and-shut case.

The relatively smaller effect of sperm plug size on the paternity share of the second male and its confounding by ease of mating of the second male is not too surprising as the sample size used in the analysis of paternity distribution was not very large. There were a total of 14 pregnant females who copulated with both the first and the second male (5 other females got pregnant but when they mated with the second male he failed to ejaculate). These females produced 99 embryos (between 5 and 9 per mother) and multiple paternities were observed in 57% of all cases (only 6 females had embryos, which were all sired by the same male, in 4 cases – by the first-mating male).

The mechanism behind the finding that males who were more sexually rested managed to sire more of the embryos in a female after she mated with another male is not entirely clear but could be one of two things. First, males who were more sexually rested were also able to produce more sperm (which we known from previous studies in this and other mammals to be true) so they were more able to outcompete the sperm of the second-maters. And second, these males produced larger plugs (which this study showed), which in combination with greater sperm numbers – might have helped to reduce the proportion of embryos sired by second-maters. Either way, the evidence from this and other studies shows that sperm plugs are a product of indirect male-male competition (sperm competition, to be precise) and their role in mediating male reproductive success can be significant.

What about primates?

The mouse study of Sutter and colleagues alerted me to a large body of fascinating literature that people have produced on the topic of copulatory plugs in mice and other non-primate species. Primatologists, by comparison, are at a disadvantage when it comes to investigating the functional effects of plugs on the sexual behaviour and the reproductive success of their study subjects, for both logistical and ethical reasons. Nevertheless, detailed observational studies are feasible and would make a large contribution to our understanding of this relatively under-studied aspect of indirect sexual competition in primate males.

One of the few non-comparative studies relying on careful observation comes from the work of Joyce Parga and colleagues, who collected 9 plugs that fell out of ring-tailed lemur females when males displaced the plug left by the females’ previous sexual partners with their penis. Parga et al. used high-resolution X-ray computed tomography scanning to measure their volume and surface area. They then related the behaviour observed in the field to these measures. One question about the role of the plugs in sperm competition was whether larger ones were more difficult to displace by the next male. The data, however, showed they were not. The difficulties of collecting copulatory plugs in the field limited the statistical power of this lemur study to test an important functional hypothesis. There is still a lot of work left if we really want to reach the level of understanding of mating plugs that researchers working with rodents have.


Sutter, A., Simmons, L. W., Lindholm, A. K., & Firman, R. C. (2015). Function of copulatory plugs in house mice: mating behavior and paternity outcomes of rival males. Behavioral Ecology, arv138–11

Dixson, A. F., & Anderson, M. J. (2002). Sexual Selection, Seminal Coagulation and Copulatory Plug Formation in Primates. Folia Primatologica73(2-3), 63–69.

Dorus, S., Evans, P. D., Wyckoff, G. J., Choi, S. S., & Lahn, B. T. (2004). Rate of molecular evolution of the seminal protein gene SEMG2 correlates with levels of female promiscuity. Nat Genet36(12), 1326–1329.

Parga, J. A., Maga, M., & Overdorff, D. J. (2006). High-resolution X-ray computed tomography scanning of primate copulatory plugs. American Journal of Physical Anthropology, 129(4), 567–576.

Parga, J. A. (2003). Copulatory plug displacement evidences sperm competition in Lemur catta. International Journal of Primatology, 24, 889–899.

More on the Internet:

Sperm competition primer:

Mating plugs:

On sperm competition in humans:

More about plugs:

Plugs in snakes: