Santas little helper

Is there any mammal species where females are on the average larger than males ? If not is there some evolutionary explanation ?

Azathoth

The naked mole rat,is one.

http://www.lpzoo.com/tour/factsheets..._mole_rat.html

http://www-personal.umich.edu/~cberg...us_glaber.html

Minnesota

never mind: forum wouldn't let me delete

BDS

Female hyenas are larger than males, and, apparently, politically dominant withing hyena clans.

http://www.hlla.com/reference/anafr-hyenas.html

"Evolutionary explanations" tend to be classic examples of a posteriori reasoning. It is a logical mistake to assume that because a trait (larger female size) has occurred, it must have some adaptive value. Although the inverse is true (if a trait has adaptive value, it will tend to spread), we cannot assume by the existance of a trait that it is beneficial. After all, some humans are born blind. Does that mean there's some sort of benefit to blindness?

Coragyps

Several of the whales, dolphins, and sea cows (dugongs) are sexually dimorphic with the female being the big'un. I have no idea what significance this has.

Demosthenes

Humans are born blind occasionally compared to the numbers of normal humans being born everyday. Also we can compare the blind humans to normal humans to know that something is not right.

However in species where females are bigger than males, all females of that species are consistently larger than the males. Those females that are born smaller than males are the exception. So we conclude that evolution must have favored larger sizes for females for some reason we haven't figured out.

There's nothing logically erroneous about that chain of reasoning.

theyeti

To the extent that mammals with sexual dimorphism tend to have the male as the larger of the two sexes, it's probably due to the greater reproductive investment of the females. The very definition of mammals tells you that they're animals with high reproductive investment (i.e. internal birth, lactation), and this burden of this falls most heavily on the females. Given this fact, the females tend to be the more "choosy" of the two when it comes to mating, whereas the males tend to strive for quantity rather than quality. This means that there is often severe competition among males for mates, and because of this they often evolve the means to fight other males, including large sizes.

The greater the reproductive variance among males, the more likely there is to be high sexual dimorphism. Gorillas, for example, have an extreme amount of sexual dimorphism, with the males two or three times larger than the females. It turns out that gorillas are polygynous, such that one male controls a harem of females while the others go without almost entirely. This puts a heavy amount of selective pressure on the male's ability to win fights with other males. Chimpanzees however do tend to have dominate males, but subordinate males are able to mate as well but at a lower frequency. Their sexual dimophism is less pronounced than with gorillas. Other animals that are monogamous tend to have little sexual dimorphism. Those that are somewhere in between, like humans, tend to have a smaller but noticeable amounts of sexual dimorphism.

There are exceptions to the usual male dominated reproductive competition in which the females are the choosier of the two. In these exceptions, where the females compete with each other for mates and the males are the choosy ones, it always turns out the the male has a higher reproductive investment than the female does. For example, in a fish species where the female lays eggs, but the male has to guard them and protect the young after they hatch, the male is the one spending more energy and time in the process of reproduction. And because of this, natural selection favors a choosy male and a prolific female. Situations like these can lead to sexual dimorphism with the females bigger than the males, though off-hand I don't know of any cases in which the females actually fight for territory. But given the nature of mammals, it's easy to see why this doesn't happen with them.

Of course it's important to remember that reproductive strategies like the ones I've mentioned are not the only factors that can influence sexual dimorphism. In many invertebrate species, the need to carry tons of eggs can make a female many times larger than a male. Undoubtedly there are additional influences at work among mammals as well.

theyeti

BDS

Demosthenes:

No doubt yeti is correct in asserting that there ARE reproductive and selective advantages to sexual dimorphism. However, my point is nonetheless also correct. It remains a logical error to assume that because a trait exists (even if it is wide spread) it MUST have selective advantages.

Species become extinct all the time, because they lacked selective advantages. Suppose, hypothetically, that competition among males became so great that the only males who ever bred were so huge that the females could not bear their young successfully (i.e. a large percentage of them died in child birth).

At a particular point in time, this is certainly a possibility, and might very well lead to the extinction of the species (if the huge males were successful enough at preventing smaller ones from breeding). This is just one of many possible examples of how the reasoning is fallacious.

It is certainly reasonable to speculate that there are advantges to sexual dimorphism, but the advantages may:

1) Be an advantage to the large male in spreading his genes, but disadvantageous to the species as a whole.

2) Be random (for example, they may be the result of an advantage in the past, which has since ceased being advantageous).

So although it is LIKELY that sexual dimorphism resulted from selective advantage, it is logically incorrect to CONCLUDE that it did, without further evidence.

1) If there is a selective advantage to sexual dimorphism, such dimorphism will thend to spread. This statement is correct (given evolutionary theory), but it is an error to conclude the inverse (i.e. 2) If a trait has spread, there must be a selective advantage to it.)

If A, then B. One may not conclude from this: "If B, then A." This is precisely the logical error I'm referring to.

Jesse

If it appears consistently across species, it's unlikely that one could find a non-adaptive explanation--if it was not adaptive genetic drift would probably wipe it out in many species, unless it was a "spandrel" that was necessary for the existence of some other universally adaptive trait (but then it is adaptive in a way, just indirectly). The idea that it's adaptive is also reinforced by "exceptions which prove the rule" like hyenas, where the sexual dimorphism is reversed and the male/female behavioral roles are reversed as well, when compared to typical mammal species (the females even have enlarged genitals which look very similar to the males').

You're right that it might not be adaptive for all members of the species, though--in theory it could be adaptive for males but not for females, or vice versa.

trillian

The largest mammal ever on earth is the female blue whale. (all baleen whale females are larger than the males)

In this case perhaps either her size allows for her enormous 8 ton baby or the male is smaller to allow him better speed and agility to mate with her. I think the latter is more likely.

trillian