I couldn’t find this topic covered previously with the search feature so I apologize if it’s been done before and would greatly appreciate a link.
I’m curious about egg bearing versus live birth, in particular in reptiles.
My interest was sparked the other day after watching a show on Animal Planet about King Cobras. The females, after their interior incubation period, build a nest of sorts and lay their eggs within. The nest has to maintain a certain temperature and humidity range, a common requisite for egg laying species it seems, and she jealously guards the nest for the two months or so that it takes the eggs to hatch. On instinct, however, she abandons the nest just before the eggs hatch because, according to the narrator, cobras are snake eaters.
From what I know, crocodiles do the same, only they do not abandon their young, even though they are cannibalistic as well and sea turtles lay their eggs and leave them completely unattended.
I also know that, though which species exactly escapes me at the moment, certain species of snakes give birth to live young.
My questions then are: what, if any, are the advantages to egg laying over live birthing? Why do some species abandon their young after incubating and protecting the eggs, while others do not? Why have some species evolved to give live birth while others still lay eggs, and why haven’t the rest of the egg layers evolved thusly or live birthers (de?)volved to lay eggs?
I understand the geographical location and environmental factors play an important role in local species’ evolutionary processes, but as a whole why is there noticeable diversity in reptilian birthing processes that are unheard of in mammals or amphibians or avian species, etc. beyond a rare exception or two?
I think the chief advantage of egg laying is that it uses less energy on the part of the mother. There’s no need for an extensive internal ‘life-support’ system so there’s a distinct saving in how much food is required to produce young. I’d imagine it also allows for larger litters.
As for the wider diversity of reproductive techniques in reptiles, perhaps this is a reflection of their being around longer to eveolve along differnet paths?
No doubt a zoologist/biologist will be along soon with a more definitive answer.
I think I remember reading that one of the very few lizards we have here in the UK is viviparous at these latitudes, but the same species lays eggs in the southern part of its range.
FWIW: although both types of reptile are faced with the trade-offs described above, all reptiles have eggs.
Some lay eggs, these are oviporous. Some carry eggs until the babies are ready to hatch and therefore appear to give birth to live babies. These are ovoviviporous, not truly viviparous.
Being ovoviviporous is sort of the opposite of being marsupial. Marsupials literally give birth after a very short gestation, but then the baby goes into a pouch which is, effectly, another womb and must go through a great deal of development before it is truly ready to be “born.”
Some species of rattlesnakes actually have the eggs implant into the cloacal wall. A network of blood vessels form a pseudo-placenta to feed and oxygenate the developing offspring.
Definitions are variable, but both “true viviparity” and ovoviviparity are found in reptiles. In truth it is a slightly hazy continuum at times and some don’t make any differentiation at all between the two. However, even if we accept that there is a important difference, there are still some snakes and lizards that even have chorioallentoic placental viviparity, like mammals ( for example the night lizards in the family Xantusidae ). For that matter there are truly viviparous fishes ( sea perches in the family Embiotocidae, for one ).
As for why it evolved in reptiles ( and it has multiple times, in multiple lineages ), there are several non-mutually exclusive hypotheses, including greater payoff in terms of parental investment, in particular in terms of a ‘cold climate’ hypothesis that makers it an attracive option for poikilotherimc reptiles in colder, otherwise extreme, or highly unpredictable climes. Whether a given taxon is likely to develop a viviparous ( in the broadest sense ) reproductive mode will depend on the relative value of the trade-off between decreased mobility and greater energetic investment vs. the potential of higher reproductive success. Since it has evolved multiple times in multiple lineages, apparently that is pretty frequently the case.
Here’s a paper that discusses the above in light of the evidence provided by a species of skink which has a ‘bimodal’ reproductive model: