Yesterday’s discussion of future biological advances that will piss off the religious right had me thinking about other innovations that I expect will happen within a few decades that might just cause wingnuts to freak out. First thing to come to mind is that it will be something to do with reproduction, of course, and it will scramble gender roles and expectations…so, how about modifying men to bear children? It sounds feasible to me. Zygotes are aggressive little parasites that will implant just about anywhere in the coelom — it’s why ectopic pregnancies are a serious problem — so all we need to do there is culture a bit of highly vascularized tissue in the male abdomen that will serve as a secure home for a few months. We’ll have to play some endocrine games, too, which may effect his love life but will also prepare him to lactate post-partum. There’s the minor anatomical problem that the vagina is a unique tissue, and no, the urethra is not homologous or analogous (fortunately; we wouldn’t want to have to push an 8 pound baby through the penis, even if female hyenas can manage it) — but that’s what c-sections are for. Given money, time, and a few weird volunteers, it could be done.
The next question is, has it been done? Are there any other vertebrates that have males doing the hard work of pregnancy? There were the gastric brooding frogs, which one would think could have made the leap easily — the eggs were just swallowed and developed in the stomach — but only the mothers seemed to have done the job. They’re all extinct, anyway. Male frogs of the genus Rhinoderma brood their young in their mouths, but this is after external fertilization and development, so they’re actually simply holding larvae in a safe place — and they’re also endangered. The precedents aren’t promising.
There is an extremely interesting and successful example, though: the syngnathid fishes, sea horses and pipefish. In all 232 species, the female lays her eggs in a specialized male structure called the brood pouch, where they are fertilized and develop. It’s a true male pregnancy!
Now this is interesting stuff. Despite the fact that this is a rather unusual vertebrate group and following a unique pattern of reproduction, we can see the pattern of evolutionary history fairly clearly in its lineage. There is a great deal of reproductive diversity in the syngnathids, but there is also a detectable pattern of increasing male investment in the care of embryos. There is a clear, early split in the group by the location of the brood pouch, either abdominal or under the tail, but both groups show increasing elaboration of the structures involved in protecting the embryos.
The other mark of specialization is dedicating new tissues to the job. These brood pouches aren’t just passive pockets to shelter the young — there are layers of epithelia that regulate the environment, and they show patterns of growth in response to pregnancy. It’s nowhere near as elaborate as the placental-uterine interface in mammals, but there are dense layers of vascularized supportive tissues into which the embryos nestle, and a specific chemical environment is maintained around them. These cross sections of a between terms (left) and pregnant (right) seahorse are pretty darned cool — that’s not just a bag for holding eggs, it’s a whole uterus-like environment.
This more detailed diagram of the brood pouch shows the degree of investment. The male makes these epithelial bilayers that partially surround the embryos, with a convoluted surface to increase the area of contact and secretory cells. They pump out lectins that play an immunoprotective role, and they may also be releasing amino acids and proteins into the protein-rich fluid surrounding the embryos, which are capable of taking up amino acids from their environment. Whether the father is actually making a significant nutritional contribution to the embryo is unclear so far. It’s definitely the case that the father is providing protection and a more consistent environment for growth, but whether he is actually enhancing the growth of the embryos by input of energy beyond the yolk of the egg has not been determined.
This situation has some interesting reversals of consequences. Since the female lays unfertilized eggs in the brood pouch, which are fertilized on the spot, paternity is not in question, and males can reduce investment in sperm production. Seahorse testes only contain about 150 sperm, total, at any one time! There is no concern about sperm competition, either. Males of other species face selection for greater quantities or motility or other qualities of sperm to maximize the chance that it is their payload that actually fertilizes the egg, rather than the deposit some other male left in the oviduct. Syngnathid males, however, have sole access to the eggs and can be a little more lackadaisical at fertilization, although, of course, the tradeoff is that they have to make a much larger investment in the pregnancy itself.
Female syngnathids have no such security, though. Males can mate multiple times, and contain multiple clutches of eggs. Furthermore, about half the eggs do not develop, and are broken down or resorbed. This raises the interesting possibility of egg competition — females might be under selective pressure to increase clutch sizes to both exclude other females’ eggs and to increase the probability that it is their eggs that make it to term. Another speculative twist is that eggs represent a much greater potential nutritive contribution than a small quantity of semen, and the breakdown of eggs could provide fats and proteins that other eggs could absorb. A male could ‘feed’ his babies by breeding with females who would deliver some eggs for breakfast.
Hmmm. I started this by speculating that it would be possible for human males to play a much greater role in childrearing, with just a few technical breakthroughs and some extensive experimentation, of course. Now I’m beginning to think this could actually have some dramatic effects on human society and human biology. Science should frighten the conservatives!
Stölting KN, Wilson AB (2007) Male pregnancy in seahorses and pipefish: beyond the mammalian model. BioEssays 29:884-896.