Evolution by natural selection is often incorrectly thought of as an unbounded path to novelty. I was reminded, when writing about coelacanths and manta rays, that there are some traits that are curiously absent from some lineages. In particular, the birth of live offspring (viviparity) has independently evolved many times in a diverse range of taxa, but not in birds or, probably, any other dinosaur (birds are theropod dinosaurs). Many adaptive explanations have been proposed to explain why this might be the case.
There are many ways that live birth can occur and many ways that the mother can provide nourishment to her offspring. The extant coelacanths, for instance, are what's known as 'oviviviparous', because females retain their eggs within their body until their offspring hatch, at which point the mother gives birth. Manta rays too, are oviviviparous, but mothers supply extensive additional nutrition after their offspring hatch.
In oviviviparous animals, the yolk allocated to the egg can be the only nourishment that offspring receive. But, there is a continuum from total reliance on the yolk, through various amounts of additional nutrition supplied by the mother (matrotrophy), to examples where all nutrition is supplied by the mother without yolk. Matrotrophy can be achieved with or without a placenta, and sometimes in alien ways, such as the cannibalism of siblings and unfertilised eggs within the uterus. Animals that are considered 'truly' viviparous are often only those where matrotrophy is complete.
After the ray-finned fishes, birds are the most species-rich lineage of vertebrates, yet not one species produces live young. The available evidence suggests that all dinosaurs were also egg laying. This is curious because all other major vertebrate groups, with the exception of the agnatha, have multiple examples (at least 120) of the independent evolution of live birth. And, unlike the agnatha, birds have several of the traits thought to be required for viviparity, like internal fertilisation.
Many hypotheses have been proposed to explain the lack of viviparity among birds; flight and the hard-shelled egg are the two explanations that seem to best fit the data. Pterosaurs, like birds, flew and seem to have all been egg laying. And bird's eggs are relatively impermeable to gas exchange when compared to those lineages of reptiles that have evolved viviparity. Indeed, turtles and crocodilians have similar hard-shelled eggs and have also not evolved viviparity.
However, these two hypotheses aren't completely satisfying. The 'flight-as-a-constraint' hypothesis is contradicted by live birth in bats and doesn't explain why none of the many flightless birds and dinosaurs evolved viviparity. The 'egg-as-a-constraint' hypothesis hinges on oxygen being a limitation and the assumption that the eggshell cannot be eliminated through evolution. But birds lay their eggs well before oxygen limitation would become a problem for embryos. And the evolution of viviparity in other lineages seems to begin with a lengthening of the period that eggs are retained before they are laid, only later does the eggshell thin to increase oxygen availability.
Some authors have suggested that viviparity has not evolved because directional selection for egg retention must be absent in birds, or that there is selection against it. The argument behind this hypothesis is that many of the advantages that viviparity may confer are solved by other means in birds and viviparity may have negative effects on mothers. For instance, retaining eggs internally may reduce the predation risk for offspring by decreasing the amount of time they're highly vulnerable to predators. But conversely, it may increase the risk of predation for mothers because they must carry an extra weight that could reduce their ability to escape.
Here too, counter examples can be found. For instance, there may be a selective advantage of egg retention in some species where eggs are a small fraction of maternal body mass and external incubation is costly. Many seabirds travel long distances between nesting sites and feeding grounds and they are also often large relative to the size of the egg. This suggests that carrying the extra weight of an egg is a minor cost and that there may be an energetic advantage to reducing the period of external incubation.
Other authors have suggested that the reason egg retention is short in birds is that their body temperature is too high for developing embryos. The resting temperature of birds is nearly universally 40 - 41 °C, while the optimal incubation temperatures are between 34 °C and 38 °C. Body temperature may, therefore, be a physiological constraint on egg retention in birds. But, correlation is not causation. Optimal incubation temperatures may have evolved because they're the temperatures that eggs are normally incubated at, not because they're the maximum temperatures that embryos can tolerate.
To me, the body temperature hypothesis probably carries the most weight as an adaptive explanation, but it remains untested. And, although the lack of viviparity in birds cries out for an adaptive explanation, we should not assume that the presence of absence of particular traits is the result of adaptive mechanisms. Nevertheless, the lack of viviparity in birds and other dinosaurs is an interesting and unresolved evolutionary question.
References:
Blackburn, D. G., & Evans, H. E. (1986). Why are there no viviparous birds? The American Naturalist, 128 (2), 165-190 DOI: 10.1086/284552 However, these two hypotheses aren't completely satisfying. The 'flight-as-a-constraint' hypothesis is contradicted by live birth in bats and doesn't explain why none of the many flightless birds and dinosaurs evolved viviparity. The 'egg-as-a-constraint' hypothesis hinges on oxygen being a limitation and the assumption that the eggshell cannot be eliminated through evolution. But birds lay their eggs well before oxygen limitation would become a problem for embryos. And the evolution of viviparity in other lineages seems to begin with a lengthening of the period that eggs are retained before they are laid, only later does the eggshell thin to increase oxygen availability.
Some authors have suggested that viviparity has not evolved because directional selection for egg retention must be absent in birds, or that there is selection against it. The argument behind this hypothesis is that many of the advantages that viviparity may confer are solved by other means in birds and viviparity may have negative effects on mothers. For instance, retaining eggs internally may reduce the predation risk for offspring by decreasing the amount of time they're highly vulnerable to predators. But conversely, it may increase the risk of predation for mothers because they must carry an extra weight that could reduce their ability to escape.
Here too, counter examples can be found. For instance, there may be a selective advantage of egg retention in some species where eggs are a small fraction of maternal body mass and external incubation is costly. Many seabirds travel long distances between nesting sites and feeding grounds and they are also often large relative to the size of the egg. This suggests that carrying the extra weight of an egg is a minor cost and that there may be an energetic advantage to reducing the period of external incubation.
Other authors have suggested that the reason egg retention is short in birds is that their body temperature is too high for developing embryos. The resting temperature of birds is nearly universally 40 - 41 °C, while the optimal incubation temperatures are between 34 °C and 38 °C. Body temperature may, therefore, be a physiological constraint on egg retention in birds. But, correlation is not causation. Optimal incubation temperatures may have evolved because they're the temperatures that eggs are normally incubated at, not because they're the maximum temperatures that embryos can tolerate.
To me, the body temperature hypothesis probably carries the most weight as an adaptive explanation, but it remains untested. And, although the lack of viviparity in birds cries out for an adaptive explanation, we should not assume that the presence of absence of particular traits is the result of adaptive mechanisms. Nevertheless, the lack of viviparity in birds and other dinosaurs is an interesting and unresolved evolutionary question.
References:
Anderson, D. J., Stoyan, N. C., & Ricklefs, R. E. (1987). Why are there no viviparous birds? A comment. The American Naturalist, 130 (6), 941-947 DOI: 10.1086/284757
This question makes me question how scientists come up with questions: it's a little like asking why there are no fish that drive cars. I don't mean that the question is not worth asking, but how can we expect to get anywhere toward solving such a question?
ReplyDeletewell, I guess it is abou finding patterns to explain or investigate principles.
DeleteAs written in the article: how come, that there is *no* bird, that does viviparity, although its such a huge group among the vertebrates, yet there is so much variety among the rest of the vertebrates?
It would be interesting to investigate why fish cant drive cars, if all other marine animals could.
As I tried to convey in the title of the post, the question about why birds do not exhibit viviparity is whimsical, not important. It's odd that such a large and ecologically diverse group of animals does not do something that virtually every other related lineage does. But, in thinking about this question you can't avoid considering things that are extremely important questions in evolutionary biology, such as the role of phylogenetic constraint and evolutionary contingency.
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