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On Earth, the polar bear is among the most sexually dimorphic animals, with males being on average almost twice as heavy as females (Wikipedia claims average weights of 450 kg for males versus 260 kg for females, with a significantly larger difference in the extreme cases).

There is an obvious natural limit to how large dimorphism can be sustained, namely the reproductive behavior of the species. In mammals, if the male is unable to safely mount the female, then procreation becomes much more difficult and at the extreme end impossible. That would then lead to either limit or reduce the dimorphism in the species (possibly instead leading to a subspeciation, with size as a determining factor), or lead to a change in the reproductive behavior of the species.

However, what if we are looking at it from the other end of the scale? What evolutionary factors could contribute to sexual dimorphism in animals, and what factors may contribute to supporting a large degree of sexual dimorphism in large mammals?

Bonus questions: Does the polar bear represent approximately the largest sexual dimorphism sustainable in large mammals in an Earth-like environment, and if so what might be the specific limiting factor in the case of that species? Would a larger degree of dimorphism be sustainable in an environment different from the polar bear's natural habitat, and what environmental difference might be the determining factor in that case?

For the purposes of this question, I am primarily concerned with differences in size, but if answers touch on other aspects as well I certainly am not going to downvote because of that.

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  • $\begingroup$ From the question it seems you are primarily interested in size differences between males and females. The term sexual dimorphism refers to all phenotypic differences, including morphology, colorations, and behavior. Are you only interested in gross size differences? $\endgroup$ – Mike Nichols Apr 6 '15 at 16:55
  • $\begingroup$ @MikeNichols For the purposes of my question I'm mostly interested in size differences, but if an answer can go into some detail on other phenotype differences as well, I certainly wouldn't mind that! $\endgroup$ – a CVn Apr 6 '15 at 17:23
  • $\begingroup$ "the polar bear is among the most sexually dimorphic animals" ... check out the southern elephant seal! $\endgroup$ – Rand al'Thor Apr 7 '15 at 0:45
  • $\begingroup$ @randal'thor I'm not sure what your point is. I never said the polar bear was the most dimorphic (as also pointed out by Mike Nichols, that is almost certainly going to depend on what metric you use for measuring the dimorphism), only that it was among them. $\endgroup$ – a CVn Apr 7 '15 at 7:14
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Sexual dimorphism occurs when the male and female individuals in a species come under different selective pressures. Most evolutionary pressures, be it predators, disease, or starvation affect both sexes indiscriminately. Males and females have to worry about them equally and so tend to arrive at the same optimal solutions, and so are not different themselves. You only get sexual dimorphism when the males and females need to do different things. This can be because of different behavioral roles. You can imagine a species where the males hunt and the females guard the nest and therefore need different physical traits to better adapt them to their respective roles. But by far the most frequent cause of sexual dimorphism is sexual selection.

There is an inherent asymmetry between the male and female members of a species when it comes to reproduction. In mammals, reproduction for a female is a prolonged, energy-intensive process. In some species females can spend years rearing a single offspring. Most male mammals on the other hand have virtually no limit to how many offspring they can father. This means females and males have very different reproductive strategies. Females tend to be selective in their mates, while males tend to try to copulate with as many females as possible. Of course, there are plenty of exceptions, but these are general trends. These different reproductive strategies result in different evolutionary pressures on the different sexes. Males that have bright coloration may be more visible to predators, but are also better at attracting females. Whereas the female members of the species have a reduced incentive to make themselves flashy.

To get to your specific question about size dimorphisms, male mammals tend to be larger than their female counterparts because of competition. Simply put, males compete for females, but females don't compete for males. The larger a male is the better it is able to out compete rival males to monopolize as many females as possible. The biggest downside of being too large is that you have to eat more. I don't think ability to copulate is too much of a concern, because all of these changes are gradual and females would adapt as well. Outside of various specific limitations like needing to be able to climb trees, run fast, or fit into narrow tunnels, getting enough food is definitely the limit to animal size.

The above is a decent (I hope) description of how sexual dimorphism arises, but I'd like to give you more succinct list of the most important factors for getting big males.

  1. Your species needs to be polygamous, and the more polygamous the better. In a monogamous species males have less reason to fight over females. In a polygamous species in which males control large harems of females a large fraction of males will never be allowed to mate and therefore will have strong pressures to be able to compete.

  2. Your species needs to have an abundance of available food. A large male needs more food, but also has less time to forage it as they must defend their territory and females. If food is a limiting factor then the largest males will be less successful than the smaller males and male size will be driven down.

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    $\begingroup$ Polar bears are not polygamous. But -- every season, since females only mate every three years -- three males fighting over every female. That's why polar bears are so big. $\endgroup$ – RonJohn Mar 14 '18 at 8:52
  • $\begingroup$ The Southern Elephant Seal is a modern mammal that is significantly more dimorphic than the polar bear, with males typically 5 to 6 times heavier than females (and plausibly up to 10 times). Both of the factors you mention contribute to this, as elephant seals (unlike bears) claim and defend large harems. $\endgroup$ – Bear Oct 11 '18 at 14:49
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While Polar bears represent the largest dimorphism in modern mammals, the most extreme case are some species of angler fish, where the male has become a small parasite attached to the female.

The evolutionary factors in general are what makes it more likely for the male or female to survive and pass on their genes. In polar bears, size provides advantage to the males in hunting and driving off rival males. With angler fish, becoming a parasite means the male has no rivals to pass on its genes once it is attached to the female, as well as relieving it of the need to hunt for food.

Since evolution has had 500 million years to experiment, I suspect the 2:1 ration of polar bears may be close to the limits of dimorphism possible in mammals. More extreme ratios might need adaptations of different morphologies (an anglerfish like ration might be achieved in creatures derived from marsupials, for example).

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    $\begingroup$ But why are polar bears and angler fish so dimorphic, while many frogs (for example) are almost identical across the sexes? $\endgroup$ – user243 Apr 6 '15 at 19:24
  • $\begingroup$ Frogs are dimorphic....Male frogs croak to attract their mate while female frogs (with some exceptions) generally do not croak. Croaking attracts mates, not size. This becomes selective pressure on male frogs to croak, not to be bigger. $\endgroup$ – Twelfth Apr 6 '15 at 23:35
  • $\begingroup$ I believe that sperm whales are more sexally dimorphic than polar bears. "It is among the most sexually dimorphic of all cetaceans. At birth both sexes are about the same size,[31] but mature males are typically 30% to 50% longer and three times as massive as females.[32]" en.wikipedia.org/wiki/Sperm_whale And if the extreme sizes alleged for some big males are correct, the dimorphism would be even more extreme. $\endgroup$ – M. A. Golding Oct 9 '18 at 18:00
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Polar bears might be the most extreme in the wild...but I get the feeling the breeders of the Chihuahua Husky mix might be a benchmark of the more extreme difference in partners size.

First to point out - size is one attribute that I can focus on, but it should be noted it's simply one attribute out of many. As a good example that was stated above, frogs are around the same size for a male and female, but (generally, some exceptions) only male frogs croak. Rather simple...the male frog is using croaking to attract a mate, which comes at the cost of attracting predators. Peacocks and Betta's use visual cue's with the males visual plume being it's method of attracting a mate. In these cases, the male is taking on the role of attracting the mate, often at a risk to it's own survival. The methodology of this 'attracting a mate' often becomes the basis of sexual dimorphism (this is an interesting line simply because it shows how evolution heavily contains a behavioral aspect)

Another point to consider...survival of the fittest/strongest isn't really true, on a genetic level it becomes more 'survival of those most capable of mating'. The fastest, strongest, smartest, fittest peacock around may never get the opportunity to breed if it lacks the plume. This is the point that really starts to define the differences between males and females in a species, those males possessing a trait that attracts them the mate and allows them to pass along their genetics will pass that attribute on to the next generation. This has the tendency to create some pretty extreme mating displays and some stark contrasts in female / male appearances and behavior.

If you want to highlight size between males and females in a species, then you need to focus on species that have the behavior that stress size as a mate attraction technique, usually in the form of strength. Antlered herbivores are good examples here...males fight between each other to win and defend their females. This process is usually the two stags sizing each other up before clashing antlers. However this doesn't necessarily promote sheer size...there are a lot of components of the fight such as the antlers, endurance, agility, etc...and although you will see a pronounced difference between males and females here, you won't find the biggest difference.

To get to the biggest difference, you need to find the mate attraction technique that values size above fighting skill. And you'll find that in intimidation. Intimidation is different than fighting...fighting is a fall back when the intimidation fails and a truly successful intimidater would never actually fight. Size and scariness becomes an end all here...not strength or agility, but size at all expense and it's in this category of creatures that you'll see the biggest variations in size between sexes in the same species. Polar bears fit in this category capable of using intimidation to scare other creatures such as wolves off a kill so they may claim it for themselves. Most male on male polar bear encounters end with one of the males backing off before fighting occurs...only extremely close matchups or extreme situations actually end in a fight.

There is one creature, now extinct, known as the giant short faced bear that might have seen a greater sexual dimorphism as far as size goes (my speculation). The giant short faced bear would have been a brutal opponent in battle, but it is most likely these creatures were scavengers and not killers, relying on their extremely strong sense of smell to locate a fresh kill and it's massively intimidating frame to scare off any competition for that kill (15 feet tall when reared up with overtly long arms and giant claws). Mate selection within the giant short faced bear was likely intimidation based, leaving the males most capable of intimidating the most capable of mating and a strong pressure on males to become larger and larger.

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