Most of you will probably be aware of the existence in many species of sexual dimorphism, when males and females of the same species are significantly (and consistently) different in appearance. Indeed, considering that we ourselves as humans belong to one of the species showing sexual dimorphism, then if you're not aware of its existence you probably need your parents to explain some things to you. However, in some species sexual variation doesn't just stop there. Allow me to present you with the concept of male dimorphism.
Most researchers would agree that sexual selection is a major factor in the differences between sexes. Sexual selection is often referred to as something separate from and often opposed to natural selection, but as with so many popular distinctions the difference is somewhat artificial. Basically, sexual selection makes the fairly obvious point that if a certain form of male is more favoured in reproduction, then it will tend to become predominant in the population. This may lead to divergence between the sexes because males and females may be under different selective pressures. For instance, male deer grow large antlers that they use in fighting each other for access to females. The females themselves do not conduct such fights, so for them all that headgear would be little more than an annoyance, and they don't have them. Often features arising from sexual selection appear downright disadvantageous for the individuals possessing them - bright colours or loud calls in a bird may attract more females, but they also attract more predators. It has been suggested that this may not be entirely a coincidence - because sexually selected features usually act as a proxy for indicating the reproductive fitness of an individual, then development of such inconvenient characters indicates the male has fitness to spare.
Nevertheless, it is exactly this conflict between selective pressures that is thought to underly male dimorphism. In the majority of known cases, the two forms of male that are present in the population are one in which the sexually selected features are well-developed (large size, ornamentation, etc.) while the other form is less accentuated and more similar to the female.
The best-known example of male dimorphism occurs in Atlantic salmon where the population contains both large males and smaller "jacks". Salmon live most of their lives in the sea and travel back into fresh water to breed. When they reach their breeding grounds, the large males will occupy a territory and start gathering a harem of females that they defend from rival males. Jacks, on the other hand, do not defend a territory. Instead, they wander around the breeding grounds trying to find females that are not being well-defended by a large male. When the guarding male is distracted, the opportunistic jack will rush in and attempt to mate with his females while his back is turned. Males intermediate in size between large males and jacks are absent, and would probably be strongly selected against, being too small to effectively defend a harem and too large to mate in secret. One might also wonder why the two forms persist in the population rather than one entirely displacing the other - I suspect that this may be because the selective advantage of each form actually increases proportionally to the percentage of the other form in the population. Sneak-mating jacks are more favoured in the presence of a large density of harem-guarding males, as the guards will keep each other's attention engaged. Harem-guarders have an advantage over jacks in having females readily on hand rather than having to seek out unguarded females, and would be advantaged if there were less other large males with which to compete.
Minor males may have other advantages over major forms. In some species of mite, major males develop thickened and sharply terminated third legs that are used in fights between males (which are almost invariably fatal for the loser). Minor males lack these spines, and so are unable to compete directly with majors. However, minor males live for longer than majors, probably because they have not invested as much energy into developing the secondary sexual features (Radwan & Bogacz, 2000). Similarly, antlerless deer known as 'hummels' occur at low frequencies within the population and have a reputation for greater overall health than their antlered counterparts. Auld (1887) relayed the comments of a Lord Lovat on the subject:
Sometimes stags have no horns. These are called humle stags. If naturally so, and otherwise perfect, they will thrash any other stags of their own, or even considerably greater weight. We have known several of them undisputed masters of their own herds.
REFERENCES
Auld, R. C. 1887. Hornless ruminants. The American Naturalist 21 (8): 730-746.
Radwan, J., & I. Bogacz. 2000. Comparison of life-history traits of the two male morphs of the bulb mite, Rhizoglyphus robini. Experimental and Applied Acarology 24: 115-121.
Is there any objective reason to think that minor males live longer specifically because of their smaller investment in offensive apparatus? Or is that just a glib guess? E.g., do larger individuals within a population die sooner, in continuous proportion?
ReplyDeleteLifetime seems to be subject to direct genetic (i.e. selective) control in many organisms... But what would drive selection, then?
The paper I linked to did show that minor males of Rhizoglyphus mites did live on average about 20% longer than majors. Whether this is specifically related to the lack of secondary sexual features is a more open question. The authors themselves pointed out that while minor males lived longer when left to their own devices, they might be expected to suffer higher mortality in conflict with the majors, and so might not live any longer in the wild overall.
ReplyDeleteIt does seem to be a general pattern in many male-dimorphic species that minor males have greater vitality in some way than majors. Simmons & Emlen (2006) showed that Onthophagus beetles with smaller horns generally had larger testes, and they showed that the two were directly connected by showing that beetles which were experimentally prevented from growing horns also had larger testes.
If we assume that lifespan is subject to selective pressure, there are certainly many cases were a long life might not be selectively advantageous. A shorter lifespan may mean shorter time to maturity, which may mean more opportunity to reproduce. When a stable dimorphism exists in a population, it suggests that, like with the salmon example, there are conflicting selective pressures that are better served by extreme individuals optimising for one of the pressures than average individuals performing mediocrely at both.
This has always been one of my favorite subjects in sexual selection, I suppose it's "minor-male syndrome?"
ReplyDeleteAs to nathan's question, balancing the inclusive fitness ledger is always difficult but there is good evidence that growth has a cost. Delayed puberty in humans is supposed to correlate with longer life-span which is perhaps more of an allometric phenomenon but might hint at the general somatic cost of reproductive investment.
For instance, male deer grow large antlers that they use in fighting each other for access to females. The females themselves do not conduct such fights, so for them all that headgear would be little more than an annoyance, and they don't have them.
Except for Rangifer! I might have to blog about this...
I think my favorite example of intrasexual palymorphism is the transient crossdressing by cuttlefish.
Speaking of glib guesses...I wonder if "polled" (naturally hornless) domestic bovids derive from human selection of hummels?
Except for Rangifer!
ReplyDeleteD'oh! Yes, except for Rangifer. Actually, this was one part of the post were I was being a little glib, because there are reasons why female deer might have antlers even if they don't fight between themselves - for fending off predators, for instance. Perhaps the disadvantages of growing antlers for females still outweigh the advantages for most species - especially if running away is a more effective means of escaping predation overall than actually trying to stand and fight.
But then why do Rangifer females have antlers? Does the more open habitat of Rangifer compared to other deer mean that antlers are less of an encumbrance, and the advantages for females in having them come to the fore?
Speaking of glib guesses...I wonder if "polled" (naturally hornless) domestic bovids derive from human selection of hummels?
Except that the term "hummel" applies only to deer, this is exactly what happened (also in sheep and goats). Polled animals were supposed to be easier to control, and are also less likely to damage each other in scraps. Of course, most naturally-horned cattle these days have their horns removed when they start growing.
I haven't looked into the details, but the genetics of polling are somewhat complicated - it's not simply a single gene matter. Polled animals can occassionally produce horned offspring, while it is also not uncommon for polled animals to grow "scurs" - small abortive, coreless horns that generally fall off fairly easily.