In biology monogamy is defined as forming a pair bond with only one mate for the length of a breeding season. Throughout the animal kingdom monogamy is rare and there is a differentiation to be made between "social monogamy" and "genetic monogamy." Many species of birds have been observed to be socially monogamous. They build a nest together, incubate their eggs together and feed their chicks together. Therefore when scientists started using genetics to determine whether all of the chick were the offspring of the two parents they were surprised to find that in most species of socially monogamous birds over 10-25% of the chicks do not belong to the father! Why would this be? One explanation is that females don't want to put all of their eggs in one basket. Females are choosy and look for the best mate based on characteristics such as bright coloration or a complex vocalization. But what if the female is wrong and the mate she has chosen has a genetic mutation that will cause her offspring to live short lives or have trouble finding a mate? For this reason it is advisable for a female bird to hedge her bets and mate with more than one male, just in case. Biologist call this "cuckoldry". It's important that the rate of cuckoldry not get too high or else it will no longer be to the advantage of the males to stick around to help feed and care for the young.
Parrotfish are broadcast spawners
What are the factors that lead to a monogamous mating system? The intensity of parental care needed to raise the young is one important factor. This point is made clear by a comparison of different marine reef-fish species. At one extreme are broadcast spawners who release their eggs and sperm into the water column where fertilization then occurs, sometimes many individuals spawn at the same time in a massive spawning event. This is a highly promiscuous mating system in which there is no sexual selection and no paternal care. Most reef-fish species employ this strategy. These fish species are basically playing a numbers game; they will release millions of eggs and/or (many species are hermaphrodites or change sex) sperm over the course of a lifetime; a percentage of these will be fertilized and become planktonic larvae. Those larvae that survive will then eventually return to a reef to settle, mature and reproduce. Along each step of this process most of the offspring will be lost by being eaten, starving, meeting up with adverse environmental conditions or failing to find a suitable habitat.
Spiny damselfish with young
Picture by John E. Randall from Fishbase
Damselfishes take a less cavalier attitude towards the care of their eggs. Most damselfishes lay eggs on a hard surface on the ocean floor and defend those eggs from predators then once the fry hatch they leave the reef and planktonic larvae before returning to the reef to settle. As part of my PhD I studied a unique species of damselfish called Acanthochromis polyacanthus or the spiny damselfish. What makes this fish so special is that unlike the vast majority of marine species the larvae of spiny damselfish fry do not disperse from their home reef. Instead the males and females form a monogamous pair bond. The female lays large eggs and when the fry hatch they stay in a group close to the parents. This is a perilous situation because there are many predators on the reef who will eat the young fish. In order for the young fish to survive both the male and the female must vigorously defend their brood by aggressively chasing away would be predators. The brothers and sisters stay together in a tight aggregation until they are old enough to disperse away from the parents. This is a labor intensive breeding strategy that involves the full time occupation of both parents but it means that the larvae are sure to wind up in a suitable environment. These fish have fewer, more developed young that both parents spend a lot of effort raising so that a larger percentage will grow up to adulthood. In order to see whether all of the offspring are brothers and a sisters we collected broods of spiny damselfish fry and looked at several genetic markers. In every group that we collected we found no evidence that there were more than 2 parents for each brood. Thus this is a rare case of both social and genetic monogamy. When raising the young takes the intense effort of both parents monogamy may be more likely because it is in the best interest of both parents to ensure that all of the offspring they are defending are actually theirs. Thus sound like any other species we know?
Miller-Sims et al (2008) Molecular Ecology 17:5036-5048
A contrasting example where genetic monogamy turns up (at least, as defined by only 2 parents per brood) is in mouth-brooding fish species such as several types of African cichlid. In these animals, the males are usually promiscuous and ensure one father per brood by vigorously defending territory. On the female's side, she will generally not have the opportunity to fertilize a mouthful of eggs more than once because other males are killed or driven away. But mouth-brooding ends up being an intensive kind of parental care (for fish). The eggs and their yolks are relatively large, allowing for a prolonged developmental period in the mother's mouth; some species even begin to feed as the mother allows some food into the mouth. By the time the fry are released, they have already reached their adult (albeit small) morphology and have no free-swimming larval period; their primary predators are then adult males of their own species. So, it's kind of like the good single-mom model.
ReplyDeleteI don't know if angelfish practice monagamy in the wild, but in captivity they form pair bonds and mate for life..and they can be quite selective. Once a pair has formed up usually neither partner will accept another. Both parents together care for the eggs and the brood.
ReplyDeleteNo, because from what you've written the only two adults around to defend the fry are the parents. Humans live in larger social units, making the mothers less dependant upon the fathers to protect their children.
ReplyDeleteas for mammals,there is also concealed ovulation theory. Species with what's called "hidden estrus" tend to me monogamous (humans, vervet monkeys and I believe some other primate species).
ReplyDeleteThanks for the comments. P. minnow I am fascinated by the diverse mating strategies of cichlid fishes! While many species of mouth brooding species are not monogamous genetic monogamy has been confirmed in two species Tropheu moorii and Eretmodus cyanostictus. Ammonid: angelfishes as group have a range of reproductive practices and social monogamy has been observed in several species but genetic monogamy has not yet been tested.
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