When a new male gelada monkey takes control of a reproductive group, he will typically kill off the babies of his predecessor. Now, new research shows that pregnant females have an adaptive strategy to minimize their losses: They spontaneously miscarry.
In 1959, biologist Hilda Bruce first demonstrated the so-called Bruce effect in mice, where recently pregnant females miscarry after being exposed to novel males. Since then, researchers have documented the phenomena in other rodent species. However, until now, the Bruce effect seemed to be something restricted to the laboratory, as nobody had conclusively shown that it exists in wild animal populations. Moreover, studies have not shown that there is any evolutionary advantage to miscarrying when confronted with new males.
To see if the Bruce effect exists in gelada monkeys (Theropithecus gelada), Jacinta Beehner, an anthropologist at the University of Michigan, and her colleagues tracked 110 females across 21 groups of wild geladas living in the Simien Mountains National Park in Ethiopia.
“We saw that as soon as a new male came into a group, there were no births for the next six months,” Beehner told LiveScience. In fact, the researchers documented only two births in these replacement groups in the five years of the study. “We get this big gap, screaming out that something is going on — it’s statistically almost impossible to get this by chance.”
To be sure what they were seeing was indeed the Bruce effect, the researchers also took hormonal data from the fecal samples of females before and after a new male arrived. Out of the 10 cases of pregnancies the researchers looked at, eight of the females miscarried within two weeks of a new male coming on to the scene. Most surprising to the researchers, the miscarriages happened the same day the male took over.
Of the two females that didn’t miscarry, one quickly showed signs of fertility swelling and eventually mated with the new male while still pregnant. The other didn’t, and probably as a result, the male killed her infant, but didn’t kill the infant of the female with whom he mated. This behavior suggests that the males figure out which babies are theirs simply by knowing which females they mated with, Beehner said.
Females that miscarried as soon as new males arrived also became pregnant again, and the researchers saw a twofold increase in births during the seven to 12 months after new males took over. They also found that females that experienced such primate infanticide took longer to become pregnant again, suggesting these miscarriages are evolutionarily advantageous to the mama monkeys.
Peter Brennan, a physiologist at the University of Bristol in the United Kingdom who was not involved in the research, said that the study was quite convincing. “It’s a great example of pregnancy block being demonstrated quite convincingly in the wild,” said Brennan, who has studied the Bruce effect in lab mice. “And there’s good evidence that it’s adaptive in evolutionary terms.”
Brennan is curious as to exactly how the females miscarry. In lab mice, he notes, females miscarry after picking up on chemical signals put off by the new males. “The actual physiological mechanism (in geladas) may be different,” he said, adding that the miscarriages might be a response to social stress.
Beehner said that the next step is to pinpoint this mechanism, though this research cannot be conducted on a threatened wild primate like the gelada. Domestic horses may be good candidates for further research, as scientists have seen the Bruce effect in the species before, she said.
Having your mother constantly watching your back may not be a sexy trait in human males, but in some primate species, mom rules the roost and her presence may help her sons hook up with eligible females, a new study suggests.
The female-ruled society of the Muriqui Monkey (Brachyteles hypoxanthus), aka Wooly Spider Monkey, in Brazil is egalitarian and peaceful, the researchers say. The group’s reproductive success, it seems, is spread evenly across the males of the group instead of being determined by male dominance, as it is in many other species.
“The new data show who’s pulling the strings in muriqui society,” study researcher Karen Strier, of the University of Wisconsin-Madison, said in a statement. “It’s the mothers.” This means males don’t need to fight for dominance and territory, saving the muriqui from the warmongering habits of other primates…
Supposedly, if women live together their hormonal cycles start to synchronise, thanks to a pheromone. If that were true it would mean that they all have their period simultaneously. Just think about it.
This “menstrual synchrony” argument was first reported in 1971 by psychologist Martha McClintock, who noticed signs of it in her own college dorm. But it may not really exist. Studies have had mixed results, often reporting no synchrony at all.
Assamese macaques, however, have evolved an unmistakable kind of synchrony: they all have sex at the same time.
Assamese macaques live in troupes of a few dozen, including about a dozen adults of each sex, plus offspring. Although there are strong social bonds within the troupes, they are dominated by the males, who compete vigorously to mate with the females.
The mating season runs from October to January, and the males become increasingly aggressive as it goes on. The males do show some solidarity. If a female attacks a male, other males will rally to his defence. But it is the females who form close friendships with each other, while males are only loosely allied with their fellows.
The females also have ways of resisting the males’ control of the troupes, says Ines Fürtbauer of the University of Göttingen in Germany. For one thing, like human females, they do not show external signs of fertility, so males have no way of knowing whether the female they are mating with is actually able to conceive. The females mate throughout their cycles, further confusing the issue. As a result, the dominant males can’t monopolise fertile females. Instead each female mostly mates with her preferred male, regardless of how high-ranking he is – although she will mate with every male at some point.
This suggests that the females are trying to keep all the males friendly. Not knowing who fathered which baby, the males ought to refrain from killing young. In fact, Fürtbauer says, the young spend most of their time being cared for by the males.
It’s easier for a female to keep the males onside if she mates with all of them, but the dominant males will try to monopolise her. To find out how the females get around this problem, Fürtbauer and colleagues monitored a troupe of wild Assamese macaques in Thailand over two mating seasons. As well as monitoring their behaviour, they took samples of their dung: the hormone levels in it told the researchers where each female was in her cycle.
There was no sign of the females synchronising their hormonal cycles, but they did synchronise their sexual receptiveness. On a given day, each female was more likely to mate if other females were mating. Spoilt for choice, the alpha male could only mate with some of them, ensuring that the other females could mate with someone else.
Fürtbauer thinks the females are working together to thwart the dominant males, ensuring that each female can sleep around without getting punished for it.
While it may not be as socially acceptable among humans, a female choosing to take multiple mates is a common phenomenon in the animal kingdom. But why the practice of polyandry (a female having more than one male mate at a time) is so prominent is still a mystery in most species.
Most theories predict that taking multiple mates would be risky for a female without adding benefits. However, new research finds that in gray mouse lemurs, a type of small primate from Madagascar, healthy females seek out multiple mates in the few hours of one night they are receptive to mating every year. These multiple mates must confer some kind of benefit to the females, though exactly how they benefit is unknown.
“Males get benefits from mating with multiple females, because they can impregnate multiple partners,” study researcher Elise Huchard, of the German Primate Center in Göttingen, told LiveScience. “In most species, females only have a few oocytes [eggs], so mating with multiple males will not increase the number of offspring they will have.”
During the intense few hours female lemurs mate annually, two things can happen — either different males chase one female up to 100 times an hour, with some chases ending successfully in mating, or one male monopolizes her the whole night.
The females have a choice to make: Either let these males exhaust them while hunting for food, or choose to hide from the males and miss a night of feeding. During their normal breeding season, females are typically smaller than males. To see if size guided the choice and larger females could fight off the males better, the researchers fed the females either a normal food or a reduced-calorie chow.
They then watched the females on their mating nights, in a cage with three male lemurs. They expected to see the larger females push off the unwanted, harassing suitors. Instead, the researchers saw the heavy females scurrying around their cages mating with multiple males. The skinny females were more likely to be monopolized by one male lemur, and had fewer mates overall.
“Polyandry might not respond only to sexual conflicts [harassment], but also provide benefits to females,” Huchard said. “That’s probably quite general in animal societies; it’s been found in multiple studies in invertebrates.”
There is some evidence that a type of cryptic choice between different sperm donors occurs in these gray mouse lemurs. Previous studies have found that female lemurs in the wild preferentially use the sperm from mates with certain genes that are different from hers. Researchers don’t know how, but after mating with multiple males, the females are able to choose which male fathers her baby lemurs. It’s possible she can distinguish between each mate’s sperm, and uses only that from the most compatible mates.
In other species, it seems a female’s ability to make a cryptic choice can offer benefits to her offspringl. The female can choose males that are better genetic matches, for example those that aren’t her close relatives, which would make for healthier offspring.
And so this cryptic choice in mouse lemurs could be one way that taking multiple mates can benefit females in the long run, allowing them to choose the best genetic match, the researchers said.
The study was published Tuesday (Oct. 4) in the journal Proceedings of the Royal Society B: Biological Sciences.
Capuchin monkeys, that are found across Central and South America, routinely urinate in their hands and rub the liquid around their body.
The reason for the strange habit has been a mystery to scientists for years.
Some thought the urine lowered body temperature, while others claimed it enabled the monkeys to identify particular individuals by smell.
Now the mystery has been solved. A new study, published in the American Journal of Primatology, has found the urine ‘turns on’ female monkeys.
Researchers at Trinity University in San Antonio, Texas, carried out brain scans of female tufted capuchins as they sniffed the urine. The urine of sexually mature males produced more activity than the urine of juveniles.
This suggests males wash with their urine to signal their availability and attractiveness to females.
Dr Kimberley Phillips, a primatologist, said females know which males to pursue from the smell of the urine.
“Since female capuchins [when they are most fertile] actively solicit males, we reasoned that urine washing by males might provide chemical information to the females about their sexual or social status,” she told the BBC.
“Female capuchin monkey brains react differently to the urine of adult males than to urine of juvenile males.”
“’We suggest that this is used as a form of communication to convey social and or sexual status.”
The new study by researchers at the University of St Andrews suggests that females produce copulation calls as a way of showing off high powered relationships during sexual interactions. The psychologists set out to study vocal communication in apes, in particular investigating the social use of copulation calls in female bonobos.
Bonobos, the sister species of chimpanzees and closest living primate relative to humans, are known for their extensive use of non-reproductive sex for social purposes, such as making friends within groups.
Researcher Zanna Clay commented, “During mating events, females of many primate species produce loud and distinct vocalisations known as ‘copulation calls’, which are considered to promote the caller’s reproductive success. “Female bonobos are unusual amongst the non-human apes in terms of their heightened socio-sexuality. We found that female bonobos engaged in frequent sexual interactions with both males and other females, while producing copulation calls in both contexts.
“However, during same-sex mating, calls were always given by the lower-ranking partner, while the likelihood of calling increased with the partner’s rank, regardless of the partner’s gender.” The study, published in the latest edition of science journal Biology Letters, suggests that the increase in calls is a sign of signifying powerful friendships as well as pleasure.
The researcher concluded, “Our results highlight the social significance of sex in this species and suggest that copulation calls in bonobos have undergone an evolutionary transition from a purely reproductive function to a more general social function. “Like humans, sex among bonobos is not only used for reproduction, but it is also important in friendships and bonding, and keeping close to those in power.”
By now, I know of few people who haven’t seen the movie, Mean Girls. But in case you haven’t, here’s what you should know about it: the story is essentially an explanation of social cliques and aggressive teenage girl behavior. As a study recently published in Behavioral Ecology suggests, this agonistic behavior between females in cliques is not exclusive to human primates, but is found in our non-human primate kin as well.
Over 18 months and 1027 interactions, Huchard and Cowlishaw (2011) discovered a correlation between sexually receptive female baboons and female-female aggression in groups. In female baboons, sexual receptiveness is typically a period in which individuals are estrous, or “in heat.” A way in which a female can display this information is through sexual swellings, or a swelling of the perineal skin, which indicates ovulation. Additionally, females with wider sexual swellings are perceived as “sexy,” as they attained sexual maturity earlier and generally have more offspring that survive (Domb and Pagel 2001).
With that in mind, enter female-female competition. Female-female competition is thought to occur more often under circumstances where resources for success in reproductive factors might be limited: for example, yielded access to food resources inhibits successful gestation or production of milk or helpful mates that provide more access to resources through social rank.
In the study performed by Huchard and Cowlishaw, sexual receptiveness was perceived to be the driver of aggressive behaviors as sexually receptive females received the most aggression, while lactating mothers received the least. It is thought this might be a tactic to delay conception; thus, females who have already conceived or have offspring would be more likely to receive access to resources and thereof prevents competition. Females who eat less (or would have limited access to food resources) also tend to have less reproductive success (Altmann and Alberts 2003). In addition, it is also possible by inflicting the cost of aggression onto sexually receptive females, the stress may make it more difficult to conceive or support a pregnancy (Beehner et al. 2006). Therefore, by being aggressive to these sexually receptive females, pregnant females or females who have offspring are conserving their resources and limiting the competition.
While no reports of any baboons getting thrown in front of buses have been reported yet, if it does happen—be sure to check the sexual swelling for the baboon version of Regina George.
The authors of a study called High Frequency of Postcoital Penis Cleaning in Budongo Chimpanzees do not beat about the bush. “We report on postcoital penis cleaning in chimpanzees,” they write. “In penis cleaning, leaves are employed as ‘napkins’ to wipe clean the penis after sex. Alternatively, the same cleaning motion can be done without leaves, simply using the fingers. Not all chimpanzee communities studied across Africa clean their penes and, where documented, the behaviour is rare. By contrast, we identify postcoital penis cleaning in Budongo Forest, Uganda, as customary.”
Sean O’Hara, a Durham University anthropologist (who has since moved to the University of Salford), and Phyllis Lee, a psychology professor at the University of Stirling, published their monograph in the journal Folia Primatologica, in 2006.
They list the few instances in which humans had documented the practice. Jane Goodall “mentions it in the Gombe chimpanzees, Tanzania, and leaf napkin use in Kibale forest, Uganda, is known … and in 25 years of observation at Taï Forest, Côte d’Ivoire, ‘leaf-wipe’ has been recorded just once”.
O’Hara and a field assistant named Monday Gideon did the Budongo detecting “between January and September 2003 and were able to verify ‘cleaning’ or ‘not cleaning’ for 116 copulations. Penis cleaning occurred in 34.5% of copulations (9.5% with leaf napkins and 25% without use of a tool)”.
The team expresses wonder that this particular form of tool use varies so starkly in popularity. “For penis wiping to be common in some locations while rare or absent elsewhere presents a puzzle,” they say.
They point out that many kinds of animals use one or another type of tool. They cite reports about New Caledonian crows, bottlenose dolphins, parasitoid wasps, capuchin monkeys, and other species. O’Hara and Lee explain that most of these tool-using practices are “cultural behaviours” – that is, learned from fellow dolphins, wasps, monkeys, or whatever.
What’s especially notable here, they say, is that “few material cultural behaviours are conducted in asocial contexts … Postcoital penis cleaning is one such activity. Although the copulatory act is, by definition, a social event encompassing more than one individual, the penis wiping that follows is solitary and self-directed”.
They note the existence of hypotheses that the cleaning serves some important, particular function. The males do it to check for signs of sexually transmitted disease [STD], perhaps, or maybe to monitor some reproductive aspect of the females with whom they consort.
But O’Hara and Lee keep a disciplined focus on the main question: culture.
“Whatever the motivation or function”, they write, “Budongo males appear more fastidious in penis hygiene than elsewhere. We found no proclivity for the use of specific leaf types; leaves appeared to be plucked non-systematically … While the functional or STD context remains unclear, we suggest that using leaf napkins is a cultural trait in chimpanzees.”