At last, dinosaur sex

25 07 2007

While the new Science paper on Triassic dinosauromorphs was definitely the “big news” of last week (and the new PLoS paper on skimming pterosaurs [or not] seems to be this week’s), another interesting paper on dinosaurs came out in Biology Letters that didn’t receive quite as much attention. The research, “Growth patterns in brooding dinosaurs reveals the timing of sexual maturity in non-avian dinosaurs and genesis of the avian condition,” non-avian dinosaur nesting sites provide some rather interesting insights into how dinosaurs grew and (perhaps) mated.

As the authors note in the abstract, the closest living relatives of dinosaurs can be found amongst crocodilians and birds (descendants of dinosaurs). While the growth rate of crocodiles slows down as they approach sexual maturity (crocodiles grow throughout their lives), birds grow to their adult size well before they start mating, giving the researchers some baselines to compare with dinosaurian growth rates. Using seven specimens (Oviraptor philoceratops; Oviraptor sp.; 2 X Citipati osmolskae, “one deinonychosaur (Troodontidae nov. sp.)” from China and Mongolia found in nesting positions, as well as Deinonychus antirrhopus and Troodon formosus found in association with eggs) the authors assumed that each of these dinosaurs was a parent (a point I’ll get back to later), although the sex of the dinosaurs could not be determined. The age of the dinosaurs, however, was determined by looking at annual growth lines in the bone as well as overall body size comparisons, allowing the researchers to get a fairly good idea as to the “stage of life” each of the dinosaurs may have been in.

After looking at the ages of the dinosaurs and determining their growth rates (as can be seen in a colorful graph in the paper, which is open access) the researchers found that the dinosaurs seemed to have distinctly non-avian growth rates and reproduction. Rather than quick growth leading to adult size, followed by reproductive maturity sometime afterwards, the dinosaurs studied seemed to show that they were brooding on nests (and therefore assumed to be mating) before they had reached full adult size. This trend is closer to that of crocodilians, where growth slows down at the time of reproductive maturity, but is not the “hurry up and wait” model that extant birds employ. Being that the dinosaurs studied are regarded to be among the closest to birds (although not ancestral to birds), it is a bit surprising to see this difference. The authors explain, however, that the difference in growth rates may not have been something inherited from dinosaurs, especially since some of the earliest known birds took at least a year to reach maturity while extant birds may do so in a matter of weeks (but this all depends on the size of the bird and other factors that it would be foolish to generalize).

I have to wonder, however, were all the dinosaurs on the nests actually parents? It would seem to be a relatively straightforward find being that the parents of potential offspring would sit on the nest, but the Pied Kingfisher (Ceryle rudis) keeps pecking at my brain when I consider this topic. For those who have not heard the story, the Pied Kingfishers employ an interesting nest-care strategy; breeding pairs will sometimes accept the assistance of a “helper,” especially when resources like food are scarce. Helpers may be related (“primary helpers” that are always tolerated) or unrelated (“secondary helpers”) to the nesting pair (Reyer, 1984), although it appears that all the helpers are males. This means that secondary males, as in the Reyer study, may be accepted or rejected as they would otherwise be in competition with the breeding male, while relatives are more generously tolerated.

The main activity of the helpers, however, is catching food to bring to the nest, thus enhancing the breeding success of the parents. While there are many bird species that have “helpers” (over 150 according to Reyer, 1979), I don’t know of any that help the parents incubate the eggs or sit on the nest. In fact, I probably wouldn’t expect to see such a behavior being that eggs are the primary investment and I wouldn’t imagine a mother giving up her care of the eggs to a male relative (and especially not an unrelated male), but I am not going to say that such a system would be impossible given my ignorance on this topic. Still, I have to wonder if non-breeding dinosaurs assisted nesting parents in any way, either by gathering food or (like I said, highly improbable) related non-reproductively-mature young sitting on the nest. This is certainly not parsimonious and there is simply no way that I can think of to tell, but I guess I continued through this post (even though I’m fairly sure that I’m wrong) to illustrate my thought process. I saw a study, thought of a related piece of information, got an idea, and tried to follow through by looking up other papers to help determine whether there are modern analogs that might give me some hint as to whether I could be right. It appears that I’m not, but I’m definitely better for the time spent, and hopefully this definitely adds to my still in-progress post about the evolution of precocial vs altricial young.

[Anne-Marie also offers her thoughts on the dino-sex paper on her blog]

Our Inner Ape OR I Like Bonobos and You Should, Too

24 07 2007

I wasn’t having a particularly good day yesterday (hence the lack of posting), but I did receive three Journal of Mammalogy issues (it’s nice that they caught me up so fast) and Franz de Waal’s Our Inner Ape. I didn’t particularly feel like going through Survival of the Sickest like I had planned, so I cracked open de Waal’s book and I’m now halfway through it. Much like many other primate-oriented books I’ve read recently, this is a popular work, treading light on science and dealing more with overall behavioral observations. Not that this is a bad thing (I think behavior, especially in captivity, is often overlooked), but if you’re looking for a rigorous treatise of ape and human evolution/relatedness, it would be best to look elsewhere.

As I read the first chapter (and bits of other chapters) aloud to my wife, she noted how enamored de Waal is with the Bonobo, Pan paniscus. While de Waal does make it clear that chimpanzee and bonobo behavior are more complex than one being “good” (the bonobo) and the other “bad” (chimpanzees), his affections and praise primarily fall upon the bonobos. This probably stems not only from de Waal’s own work with them but the overall ignorance surrounding them; as de Waal points out, everyone is familiar with chimps, but few seem to be familiar with bonobos. They also suit de Waal’s purpose in combating some evolutionary psychology/biological determinism as evidenced in books like The Selfish Gene or Demonic Males. While chimps are often used to show that we had brutal, combative ancestors, de Waal hopes the bonobo will reflect our better nature. While he does have a point, that we are not pre-programmed bundles of genes or behaviors nor are we special creations disconnected from our past, de Waal does not always make the best arguments. Case in point; in discussing homosexuality de Waal points to other cultures where homosexuality is prevalent or even accepted, but his examples primarily focus on cultures that introduce children to homosexuality. None of the examples feature a mixed society where some members are gay and others not, but rather that young men and women have to go through homosexual “rites of passage” before becoming adults in the society. I’m not enough of a moral relativist to simply say “Oh, good for them. They’re an accepting society,” and it made me think more of child abuse than of openness to diversity. I do not know enough about any of the groups de Waal mentions to make any definitive judgments, but his examples of young boys that must “live sexually” with an older male until they become men did not support his case.

Oddly enough, Jonah of The Frontal Cortex has just posted about a new New Yorker article about bonobos, focusing on how they’ve essentially been marketed as the kinder, gentler, and more feminist chimpanzees. While de Waal mentions aggression amongst bonobos in his book, the peaceful nature of the apes is what takes the forefront, and at times the book feels like a PR campaign to make us vote for bonobos as if some election for which closest living ape relative was taking place. At the same time, however, it’s easy to just repackage bonobos as another male-dominant ape species where instead of competing with females, males stand aside (de Waal scoffs at this “chivalrous” method, and I think rightly so). As in many issues, the truth of the matter probably lies somewhere in the middle of the two positions that are becoming more and more polarized, and more observation of bonobos in the wild are going to be needed if we’re really going to understand their significance to us. I don’t believe apes are wholly aggressive, nor are they wholly peaceful and loving, each species seemingly working on its own continuum to meet different needs. In fact, I think chimpanzees and bonobos are a good example of evolution working on the same animal to produce different behavioral and physical adaptations, showing that there is not some kind of evolutionary determinism (else chimps and bonobos would be the same and no distinctions could be made). For now, however, bonobos will probably remain more symbols for peace and “free love” more than anything else, and I have to wonder how many of the people that champion them (like the hippies in the New Yorker article) really know anything about them.

Update: Oh, and lest I forget, de Waal has a new article available for free through PLoS Biology, too (although I haven’t had the chance to read it yet).

Kinky Alien – Ape Sex

19 07 2007

There are lots of crackpot beliefs and ideas in the minds of people the world over. Some believe that Jesus really traveled to India, where his body rests in a tomb. Others believe that Gigantoraptor was really a magical bird called a “Fandor” that carried Adam and Eve around the Garden of Eden. Some of the most surprising and strange beliefs, though, have to do with the origin of our own species, Homo sapiens.

In the first chapter of Strange Creations: Aberrant Ideas of Human Origins from Ancient Astronauts to Aquatic Apes, Donna Kossy reviews many of the strange New Age/Gnostic/pesudo-religious/UFO-obsessed hypotheses that have been invoked to explain how humans came to be, some with great success that others (i.e. Erich von Däniken’s Chariots of the Gods? is the popular standard by which other idiotic UFO books seem to be judged by). While there are plenty of similar creation myths to choose from (and I could probably devote post after post to such woo), I did notice a strange misogynistic trend among the books which (with the exception of the highly eccentric Helena Blavatsky) were predominantly written by men.

The first example that caught my attention was that of George Van Tassel, author of Science and Religion Merged, and he certainly broke his own spine bending backwards to try and accommodate “science” with religion and paranormal buffoonery. Here is Kossy’s summation of Van Tassel’s mythology;

Van Tassel reveals the true meaning of evolution and the book of Genesis in a chapter of Religion and Science Merged entitled “The Missing Link.” God created Man, Van Tassel tells us – on many planets in many solar systems – before He created Earth. We didn’t evolve from “lower animals,” but were created “to serve as the instrument of God’s doing.” Adam wasn’t the first man, he was an entire race of men brought here by the Adamic Federation in a spaceship. But the colonizers forgot to bring women along – a rather large oversight for a supposedly perfect race. If they wanted to people the earth, the Adamic race would have to mate with female Earth creatures. Fortunately, they were able to breed with one of the animals races here on Earth, the race of Eve, which Van Tassel describes as “upright walking animals… the highest form of lower animal life” on Earth, but they were not apes.

The Bible story of Adam and Eve and the serpent, writes Van Tassel, is really an account of God’s displeasure with this coupling between man and beast; eating the wrong apple is a metaphor for mating with the wrong species. “This,” says Van Tassel, “is where Man became hu-man.” “Hu” represents the animal, killer nature of the tribe of Eve, while “man” is the higher nature, created by God.

Likewise, the authors of Mankind – Child of the Stars (Otto Binder and Max Flindt) tried to find “the missing link” through supposed breeding experiments between early hominids and aliens. Kossy explains;

All [the enigmas of human evolution] Flindt and Binder claimed, were solved if we realized that our species is a hybrid between “Apeman” and “Spaceman.”

Although they try very hard to look scientific, Flindt and Binder find their clearest evidence for the improbable coupling of apeman and spaceman in the Bible. From Genesis 6:2, in which the sons of God marry the daughters of men – “And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, that the sons of God saw the daughters of men that they were fair, and they took them wives of all which they chose” – they conclude that “starmen visited Earth and mated with early females (perhaps hominids) to sire the modern human race of Homo sapiens.”

Flindt and Binder conclude that not only humans – but just about every species on Earth – were originally brought here by aliens. The space people first arrived here long before hominids appeared, making countless trips to Earth, each for a different “hybridization project.” They brought with them such creatures as the coelacanth and the duck-billed platypus… The aliens also imported the first apes and then returned about 12 million years ago, in time to breed with Ramapithecus, an early hominid. Not quite 12 million years later, the offspring of this strange coupling had developed into Australopithecus, an early ancestor of Homo sapiens.

As mentioned earlier, Erich von Däniken was perhaps the most successful of the UFO nutjobs, and he borrowed from science, the Bible, and other writers like Blavatsky to come up with his brand of alien breeding project;

Von Däniken’s version of humanity as an alien breeding experiment, like those that came before, loosely follows the Bible. Like Blavatsky’s narrow-headed Lemurians and Van Tassel’s alien-beast hybrids, some of Adam’s genetically altered progeny were unable to control themselves, and continued to mate with animals. This “backsliding” represented the Fall of Man because it “impeded evolution,” and retained the bestial in man.”

These sentiments actually echoed other racist/religious arguments that preceded von Däniken, the racist Edward Long writing (sometime between the late 18th and early 19th century);

Oranguatans, he wrote, “did not seem at all inferior in the intellectual faculties to many of the negroe race; with some of whom it is credible that they have the most intimate connexion and consanguinity… the negroes themselves bear testimony, that such intercourses actually happen; and it is certain that both races agree perfectly well in lasciviousness of disposition…” Mulattoes, Long proclaimed, are infertile, like mules. They come in two types: first, the offspring who result from the coupling of a Negro and a Caucasian; and second, the offspring of a Negro and an orangutan. The latter are often the result of “the passion the male Orang-Outang has for the Negress.”

Utterly disgusting and despicable, and yet such notions found favor among many. While there are plenty examples of humans at their worst in trying to figure out their own origins, I picked these examples primary because they showed the overall bias towards the white male as being the most godlike. According to the racist and idiotic mythologies, men are degenerate gods/aliens while women are directly connected with bestial and stupid apes; the bias could not be more clear. Just like the classical Genesis mythology, “Eve” gets the blame for the Fall or man’s degenerate condition, and the males in these stories are always placed nearest to godliness (if they aren’t already gods).

While I would like to believe that such misogynistic and untenable beliefs no longer exist today, I would be a fool to do so. It is absolutely incomprehensible to me how, at the beginning of the 21st century, we have seemingly changed so little in our prejudices and habits. Like Kossy writes in another chapter of the book, there has never been a “Golden Age” of humanity (nor were “The Good ‘Ol Days” really that good), but still I am continually shocked and amazed by the sort of untenable nonsense that is all-too-readily accepted.

Freudian Field Day; 10 Idiotic Assertions in Psychology Today

10 07 2007

Update: Bora had kindly put together a short-list of other bloggers who have addressed the intellectual poverty of the article. Have a look here.

There’s been a little bit of a stir in the blogosphere in reaction to a recent evolutionary psychology article entitled “Ten Politically Incorrect Truths About Human Nature” that showed up in Psychology Today. I honestly am not terribly familiar with evolutionary psychology outside of how contentious it is (scientists like Niles Eldredge and Ian Tattersall, among others, regarding it as junk science), but at the moment I suppose I take something of the middle ground; our evolution certainly affects our mind today, but figuring out what has and has not changed is the trick. Case in point, I recently watched the first few installments of the BBC program The Human Face hosted by John Cleese. In the discussion of professional models, it was said that a particular look that models can give works so well because it is basically the face that women make before orgasm, so we’re naturally more attracted to such a face. I really have no idea if this is true, and overall it seems a bit too simplistic to me; everyone doesn’t just go helpless with euphoria when we see such an image (at least, I know I don’t). In any case, I’ll be going through the 10 assertions the authors of the Psychology Today list, giving my thoughts on each.

1. Men like blond bombshells (and women want to look like them)

One of the first things that strikes me about this article is that it isn’t especially tentative nor does it back up it’s sources; things are simply said to be as they are and we’re expected to believe what the authors are saying is true. Beyond that, the general hypothesis is that blond women remind men of young women, young women with large hips and a small waist being able to be better reproductively, and therefore they are more desirable sexually. On top of that, large breasts are supposed to be good age indicators, the amount of sag telling males how old the woman is. This, I must say, just sounds stupid; surely there are better indicators of age than breast size in relation to “sag”, and while I’m not an expert, isn’t there a large amount of natural variation in female chests anyway? The authors seem to be confusing correlation with causation, raising a “just-so story” to the level of fact. Likewise, their discussion of blue eyes sounds equally dubious;

Women with blue eyes should not be any different from those with green or brown eyes. Yet preference for blue eyes seems both universal and undeniable—in males as well as females. One explanation is that the human pupil dilates when an individual is exposed to something that she likes. For instance, the pupils of women and infants (but not men) spontaneously dilate when they see babies. Pupil dilation is an honest indicator of interest and attraction. And the size of the pupil is easiest to determine in blue eyes. Blue-eyed people are considered attractive as potential mates because it is easiest to determine whether they are interested in us or not.

This seems more of a reaction to modern eye color variations (which can be artificially achieved through contact lenses) than the effects of something evolutionary; I doubt there was a bias in our hominid ancestors towards blue eyes (if blue eyes were even common enough then). It also raises the question of blue eyes in populations where they nearly never occur today; is there any significant change in attraction to an individual solely because of eye color? What is people just like the color blue, or like blue eyes because they are unusual? The authors don’t put forth any alternative hypotheses.

2. Humans are naturally polygamous

The first thing that irked me about this discussion is that no distinction was made between what science may or may not tell us and morality, i.e. that whether our species was polygamous throughout our evolutionary history does not dictate whether it is good or natural today. As for the argument itself, the authors point to the mild sexual dimorphism between men and women and suggest that bigger, stronger males monopolized the females, females also preferring big and strong males. No alternate hypothesis was given for the variations, nor was it mentioned that it is unlikely that only the biggest, strongest men mated. Thinking back to what I’ve come to learn about sexual selection and the book A Primate’s Memoir, a hyper-masculine male may be highly aggressive (even abusive) in addition to being the biggest and strongest, and just because one male tries to monopolize all the females does not mean that other males never get the chance to mate (less privileged males may keep up longer term relationships with females and produce offspring surprisingly often).

3. Most women benefit from polygyny, while most men benefit from monogamy

Continuing with their assertion that it’s better for females to share a wealthy man (or, in terms of past history, one that can provide protection), the authors assert that monogamy benefits men because a “poor” wife is better than no wife at all. Again, outside of “protection” the authors make no qualifications as to what females actually benefit from sharing a dominant male, and polygynous mating structures do not necessarily guarantee the safety or well-being of the females. While it might be true, in a thought experiment, that less-masculine males would benefit more from a monogamous or polyandrous mating structure than a polygynous one, the authors provide no evidence to support their claim and the overall reasoning is rather shallow.

4. Most suicide bombers are Muslim

The title seems more like a matter of statistics (I wonder if they considered Kamikaze pilots during WWII), but the authors contend that Muslim men are engaging in terrorism to quench their sexual desires. Rather than being a product of religious brainwashing, the authors assert that Muslim suicide bombers are primarily doing it so they can receive 72 virgins when they arrive in heaven. They show nothing to support this at all, nor do they look at the motives of suicide bombers from other cultures and time periods. The authors case is highly dubious, at best.

5. Having sons reduces the likelihood of divorce

Once again, the authors don’t do much other than say “This is how it is, deal with it.” No statistics are given as to divorce rate, family makeup, etc., and they try and make the case that since fathers must pass their wealth and power onto their sons they are more likely to stay involved in the family. This model seems to assume that the family has only one child that is the “heir” to his father’s legacy, and overall it seems like it has more to do with culture than “evolutionary psychology.” Indeed, the model assume that the son actually inherits wealth and power from the father, but no qualification/quantification is made of what this would look like (i.e. making sure the son gets through college before divorcing?)

6. Beautiful people have more daughters

The authors contend that because males have historically been privileged and promiscuous, they produced more sons that went on to become privileged themselves, a kind of artificial selection for males that would cause a higher ratio of males to be born than females. Little is said of infanticide or males being favored over females in families, so once again the authors seem to connect two dots to make a line without trying to plot any more points to see if the results make sense.

7. What Bill Gates and Paul McCartney have in common with criminals

This one might have a grain of truth to it, but once again it is lost because the authors reduce everything to sex. Using Bill Gates and Paul McCartney as examples, the authors contend that much like Jackass-like feats of stupidity and crime, male “genius” tends to peak at an early age. Why? Because we all want to bad to impress women. Even if we were to argue that artists, scientists, musicians, etc. stayed active and creative all through their lives, the authors argue that the “best work” of all these men occurred early on in their lives because they were in competition to secure notoriety in order to obtain a mate. I wonder what Charles Darwin would say to them about this. I found the last few sentences of this point especially wanting;

Women often say no to men. Men have had to conquer foreign lands, win battles and wars, compose symphonies, author books, write sonnets, paint cathedral ceilings, make scientific discoveries, play in rock bands, and write new computer software in order to impress women so that they will agree to have sex with them. Men have built (and destroyed) civilization in order to impress women, so that they might say yes.

Yes, nothing gets women hotter than developing a new computer language or studying the flavors of quarks. While “male conquest” may have been significantly evidenced in the past, the authors don’t think about the societal context in which many men (especially within the past few centuries) create their “great works.” Can it really be all about sex and have nothing to do with having to establish oneself in science, art, etc. or other factors like an openness to new ideas? I don’t want to sound overly idealistic and I’m sure (especially in the realm of modern music) that sex does have a part to play here, but I would hardly argue that all of male artists, musicians, scientists, and writers were only driven by their sexual desire, their intellectual prowess declining as their desire did.

8. The midlife crisis is a myth—sort of

This point was a bit odd; men go through a mid-life crisis because they desire to replace their menopausal wives with a younger wife, once again having sex trump every other potential factor. Because the authors are so sex-obsessed, the “mid-life crisis” is said to have nothing to do with a man wanting to reclaim his own youth or get a “do-over,” but instead he just wants a woman along the lines of the one discussed in asinine assertion #1. I’m certainly not an expert on the “mid-life crisis” but reducing it to sex does a disservice to a complex issue.

9. It’s natural for politicians to risk everything for an affair (but only if they’re male)

Here the authors again make the assertion that every male would have sex with as many women as possible if it were feasible to do so, the primary reason for obtaining a political office being gaining sexual power (have a look at the Washington Monument; what are trying to say to the world?). This seems to run counter to their assertion that influential men do their most important work while young while being potentially in-agreement with or counter-to their “mid-life crisis” argument depending on the age of the man and his marriage. We should fully expect powerful men to try and have sex with as many women as possible, they say; why should we expect otherwise?

10. Men sexually harass women because they are not sexist

The last point is a bit confusing; men abuse and intimidate other men, so why should they treat women any different? Men try to achieve power through competition in the workplace and once they have that power they try to use it to get women to sleep with with in one-night-stands, women taking great offense to the despicable male behavior. The authors make no distinction between abuse/intimidating/hazing etc. amongst men in competition and they way they treat women at work (nor do they say that such behavior is unacceptable), sex being the most important factor in their view.

While the authors of this may have had a few inklings of insight, it was all lost in a flood of male sexuality that seems like an attempt to justify sexually motivated aggression on the part of males. Women are not considered outside their roles of sex objects that males strive to obtain, giving the whole article (and probably the book the article promotes by the same authors) of rather lop-sided view. If this is the best evolutionary psychology has to offer, than I would have to join others like Niles Eldredge who regard it as worthless and even dangerous. It would be foolish to say that our evolutionary history has not influenced the way we are today, but asserting that such selected behaviors dictate the whole of the human experience, that sex rules all and we cannot hope to ever escape our desires, cheapens us all. As much as I disagree with much of what the authors of this article wrote, I could have at least given them some points for mentioning that finding out reasons behind certain behaviors should not endorse immoral or otherwise despicable actions, but the authors of this study would rather tell men that they’re pre-programmed sex maniacs than try and do anything constructive.

White Rhino males start off as “mama’s boys”

3 07 2007

A few days ago I wrote about how the different needs of male and female cervids (deer) and bovids (sheep, buffalo, antelope, etc.) shape their armaments, and how antler evolution in red deer has essentially stopped due to the pattern of sexual selection that evolved in the species. The idea that selective pressures may differ so significantly in sexually dimorphic species was something relatively new to me, and now there it seems that artiodactyls aren’t the only animals to have their sex play an important role in fitness and evolution. In a new paper entitled “DIFFERENTIAL INVESTMENT IN SONS AND DAUGHTERS: DO WHITE RHINOCEROS MOTHERS FAVOR SONS?” White, et. al, describe how mother White Rhinoceros (Ceratotherium simum) seem to favor sons over daughters when nursing their babies.

White Rhino
White Rhinos seeking some shade at Disney’s Animal Kingdom

In order to understand why a mother rhinoceros would favor male offspring over female offspring, we need to understand something about the social structure of the white rhinoceros in terms of interactions/mating. Females do not have any known type of dominance hierarchy, but while several males may live in a given area, there is typically one dominant male that fights the others for mating rights. This doesn’t mean that the other males don’t get a chance to “consort” with the females now and again, but they had better be sure that the bull is not around when they get horny (sorry, couldn’t resist the pun). The dominant male does father more offspring than subordinate males, however, the subordinate males having less offspring related to them than any given female, and so it pays off (in a reproductive sense) to be a bigger and stronger male while the females have a good chance of producing offspring regardless.

In the study, 14 female white rhinos were tracked and observed over the course of three years, with 25 male and 20 female calf births recorded within the group, and the primary behavior the researchers were looking for was suckling. Suckling is merely the offspring trying to nurse, but does not always mean that they receive milk. While it might be easy to assume that only suckling that allows the offspring to drink milk would be beneficial, the mere act (regardless of outcome) is significant because it shows response to need by the mother and may have other social benefits for the young rhinoceros. Likewise, extended or increasing suckling can even slow the reproductive rate of the mother, so the longer and more intensely she suckles a baby, the less offspring she’ll be able to produce over the long run.

What the researchers found was what would generally be expected in an animal where males stood a better chance at reproductive success if they were more healthy/bigger/stronger and the females didn’t have to deal with dominance hierarchies. Males were definitely preferred for suckling when between birth and 6 months (Class A), and also still suckled when 24 months old or older (Class D) until the birth of the next calf, while Class D females did not suckle at all. The mother also seemed to be more responsive to males during the first six months than females, perhaps the critical time for growth in white rhino calves. The researchers sum up there findings this way;

Our findings indicate that sons received increased investment compared to daughters. Specifically, mothers tended to have longer interbirth intervals after the birth of sons than daughters, sons initiated significantly more suckling bouts than daughters, sons spent significantly more time suckling than daughters (particularly in the earliest age class), and sons were significantly older than daughters at time of weaning.

White Rhino
A White Rhino on a cold, January day at the Philadelphia Zoo

Despite all this effort, however, only one of the males will end up being dominant in any given area, so much of the maternal effort put into sons ends up being “wasted.” If a male does not receive such intense investment, however, there is little hope that he can compete with other males or leave many offspring, and so he will likely end up being a reproductive dead end by his lack of proper behavior. Indeed, it seems that mothers are responding to the persistence of males in the first six months of life especially, with age Classes B & C being a toss-up between males and females, and the suckling solicitations of Class D females being entirely ignored. Mothers didn’t seem to show any preference when breaking off a suckling bout, however, so it appears that in the first critical months males are favored, and they are allowed to suckle longer overall than females. In relation to the long term implications of this, males that are more greedy and are born to mothers who are more prone to stop and let them suckle will probably become the dominant males in the long run, in turn siring the most offspring, in turn fixing the greedy behavior in males and possibly the favoritism amongst later generations of females.

While we might expect to see some amount of sexual dimorphism in white rhinos according to size and the greedy suckling of the males, there doesn’t seem to be any appreciable difference between males and females. Where the difference is made, however, would be amongst males themselves, especially when males are young and prone to fighting. The weakest males may be weeded out through injuries sustained through fighting early, and so the stronger juveniles will be more likely to wind up being dominant. Little is said of size differences amongst males in the paper, but I would assume that healthier (if not larger) males would end up becoming dominant, and such males likely were favored by their mothers during the early months of their lives.

One further point about this paper; it’s apparent that the mothers are favoring the young males, but do we really know why? The young males solicit suckling more, this is true, but I doubt that the mother rhinos are hoping that their little boys will grow up big and strong so they can beat out the other male baby in the next field. If it were at all possible, it would be exceedingly interesting if there’s a different physiological/mental response in mother rhinos for when male or female newborn solicit suckling, as there has to be some reason why males are favored when the mother can’t possibly know what is going to become of her baby down the line (and probably doesn’t care).

The sex of an newborn animal may be something we take for granted, assuming that mothers will love all babies just the same. Anyone who’s grown up in a big family (I’m the oldest of five) knows that this isn’t always the case, however, and favoritism is something very real, and we should not expect that animals are “fair and balanced” in everything they do in relation to offspring.

Deer Antlers; it’s not all about sex

29 06 2007

Female White-Tailed Deer (Odocoileus virginianus), taken in Hopewell, NJ

July 18th Update: LiveScience has caught onto the Nature paper and has a short article here.

A new Nature paper entitled “Sexually antagonistic genetic variation for fitness
in red deer
” (Foerster, et al., 2007) has some very interesting implications for fitness selection; the most fit males may not produce especially fit female offspring, and the most fit females may not produce especially fit male offspring. While it seems to be “common sense” that the most powerful, impressive male will father the strongest offspring, male and female Red Deer (Cervus elaphus) behave differently, and what makes a fit male does not make a fit female. The authors put it this way in their introduction;

Males compete intensely for matings during the short annual rut but do not invest in offspring care, whereas female maternal investment extends over a long period during each reproductive event. Consequently, male and female life histories are likely to be under divergent selective pressures, and a particular genotype may have very different effects on fitness in males than in females.

Males have to carry around a good deal of weight on their heads and be physically strong (thus the most “masculine” males being favored when it comes to breeding), while females have to raise the young all by themselves, better mothers leaving more offspring. This is not a revelation, but I have to admit that I never really considered the differing selective pressures between males and females in a population.

The primary trend pointed out in the paper is that females who mate with robust males typically produce daughters who do not leave as many offspring; we can’t simply say that the father has “good genes” offspring of both sexes will benefit equally from them. Likewise, it is not apparent if robust fathers leave especially productive sons; the deer are polyandrous, a few males mating with most of the females, and so many (if not most) of the males never successfully mate or leave any offspring.

An itchy Elk (Cervus canadensis) at New Jersey’s Turtle Back Zoo

Of further interest to us in this topic is another new paper by Jakob Bro-Jørgensen in the journal Evolution, “The Intensity of Sexual Selection Predicts Weapon Size In Male Bovids.” While Bovids are distinct from Cervids (deer) at the Family level, I think it’s important to compare the armaments of deer with those of bovids such as antelope, gazelle, buffalo, etc. The important distinction to make here (at least for our purposes) has to deal with the weapons the males conspicuously carry about; in deer, the males grow a new set of antlers every year, the antlers being made of bone and growing from an attachment to the skull called a pedicle. Horns, on the other hand, are typically hollow, have a covering of keratin, and do not fall off annually. All male deer have antlers, and many male bovids have horns, whether females have horns/antlers or not varying from species to species (i.e. female reindeer and caribou have antlers, but not as impressive at those of males. Even among species where antlers are not a typical female characteristic, some are commonly found to have antlers). All these details, of course, make generalization difficult, but there do seem to be some larger trends at work.

A Mhorr Gazelle (subspecies of Gazella damaAntler Size in Red Deer: Heritability and Selection but No Evolution” appeared in the journal Evolution, and the authors (Kruuk, et. al) found that directional selection for more impressive antlers is not happening because the success of the competing males depends on more than just antler size. Horns and antlers are dangerous things, and at least some weapon-wielding mammalian herbivores make at least some attempt at avoiding violent conflict. A male antelope that is obviously larger and has obviously bigger/longer horns than a rival will likely be able to drive off or otherwise force into submission a rival just because the rival knows he can’t possibly compete with such an impressive specimen, fights occurring more frequently when males are close to be equally matched (although things do not always proceed as orderly as this, some inadequate combatants giving it a go despite their less-impressive characteristics).

If a fight does occur, it’s not all about the antlers or horns; the weapons do not hold any sort of magic power that make their owner necessarily any more adept at fighting or stronger than any given opponent. Imagine, if you will, two men facing off with nearly equal length swords; how well they do while fighting does not so much depend on the blade but how they use it, strength, agility, health, and experience being more important than the weapon itself. Such appears to be the case with the antlers of red deer. While Kruuk’s study found that horns were heritable and did play an important role in sexual selection, how the competing males were doing in terms of health and nutrition mattered just as much (if not more) than antler size, a male will smaller/shorter/less impressive antlers able to beat a male with more robust antlers if he was physically stronger and it better health. Thus, nutrition and health mitigates runaway sexual selection for a dimorphic trait, essentially halting evolution. I especially liked the following quote from the conclusion;

Associations between phenotype and fitness, however appealing, will give a misleading impression of the potential for evolution in a trait if the true target of selection is unmeasured or immeasurable.

Indeed, sexual selection may have played the major role in the development of impressive antlers, but such evolution is not ongoing; there eventually comes a point where the antlers (and even horns or tusks) do not get bigger because the strength of competing individuals makes more of a difference than differences in antler size or shape. This study brought to mind the famous Irish Elk (Megaloceros giganteus), an extinct deer with perhaps the most impressive set of antlers known. There are hefty prices to pay when you have to not only carry such a huge rack about, but also to grow it. While their antlers were the size one would expect for their body size, they were still huge growths of bone, and these features may very well have contributed to its demise. While there does not appear to be any conclusive finding as yet, the size of the antlers may have prohibited the elk from moving through thick vegetation as climate and local ecology changed, or even suffered from various disorders/diseases as the nutrients in the soil needed to grow so much bone began to disappear. For now all we can do is look for more evidence to confirm or refute these hypotheses, but if either were true then natural selection would push against body form generated by sexual selection, showing us that we should be careful not to be become so enthralled with just one feature of an animal that we overlook all else.

If nothing else, the studies I’ve mentioned show us that there is certainly more to consider when it comes to sexual selection, reproduction, and evolution than the “most fit” males getting together with the “most fit” females and continuing the species; a more integrated approach is needed if we’re going to truly understand what’s going on and why the evolution of certain structures (especially those influenced by sexual selection) can come to a halt.