Thursday, April 12, 2012

How to lay an egg

The captivating lapwings
Some years ago I learnt from a friend who rears chicken that a freshly laid egg can be put away in the fridge and when given to a broody hen a few days later, that egg would still hatch. On some study, it was clear that this was very sound empirical knowledge but I had not thought enough about its evolutionary significance. Most birds can only lay an egg a day, so if a bird laid four eggs in a clutch, they would hatch at one day intervals if they all took the same time to develop. The earliest birds at nest that caught my attention were Yellow-wattled Lapwings. Just a short walk and in plain view of the balcony of the quarters where I lived as a teenager, I could spend my summer observing them at their nest and with young birds. It was something, a great privilege, that I took for granted then. One person who actually boasted of such a privilege was J B S Haldane who wrote to Julian Huxley about how he could study the species right from the comfort of his verandah.[1]
Nesting lapwings mobbing a kite, an artistic impression
What I did know about lapwings from those teenage birdwatching years was that all the chicks hatched at around the same time. How did this happen? I now know that one aspect to the possible answer is that the birds actually begin to incubate the clutch only after the clutch is complete. Synchronous hatching, it turns out, is particularly useful if all the chicks need to move away from the nest after hatching - as is the case in nidifugous birds (ducks, fowl, lapwings and others). As long as an egg is kept below what is called "physiological zero", the metabolic process of development is not switched on. Physiological zero is about 26° C for chicken, but there appears to be no such estimate for waders (plovers and lapwings included) - and Jayakar and Spurway (1968) [Spurway was then Haldane's wife] note in their study that the soil temperature during the breeding season was about 60° C  while air temperatures were about 30-40° C. Further, they noted (emphasis mine):
"All 4 eggs have hatched in 5 clutches ; the interval between the hatching of the first chick and the removal of the last egg shell was, for pair 1, 23 hours 10 min. (b); the interval between the day of first finding a chick (often more than 1) and the nest emptying was, for four nests (2, 7, 11 and 13') 1 day, and for one (5) 2 days. However, 3 days intervened between the first and the last hatch of the 3 surviving eggs of clutch 6'. These intervals are less than the 4 days between the 1st and 4th lay, confirming b that some synchronisation of hatching exists. We have also confirmed that this synchronisation is not achieved by delaying incubation, as it seems to be by species in colder habitats."[2]
It seems that this whole idea of egg hatching was of considerable interest to the Haldane gang for I hear that they also noted how a certain wasp (or is it a leaf cutter bee?) that builds a nest in a tube with multiple cells laid outwards hatched in reverse order - that is the outermost and last laid egg leaves first. (pers. comm. K. Chandrasekhara, original source still sought).

It turns out that in some cases it is better to have eggs hatch early - a cuckoo and possibly other brood-parasites preincubate their eggs before laying them in the nest of a host, giving their own egg a headstart.[3]

There appear to be a number of works on this subject but few really comprehensive summaries of the issues involved. Several authors have tried to find the costs and benefits of having young hatching simultaneously. In turtles, it is suggested that hatching in one big bunch leads to satiating predators and diluting the individual risk of predation. Others note that the nutrient quality in eggs improves from the first to the last laid - so that the last laid eggs develop faster. That of course is teleological wording that reminds one of Haldane famous quip-  ‘Teleology is like a mistress to a biologist: he cannot live without her but he's unwilling to be seen with her in public.’ But in looking at the whole issue I came upon an even more serious philosophical question in Murray (2001), although the author suggests a comprehensive mathematical model for examining synchrony in hatching! So much for an egg ...



References
  1. Krishna R. Dronamraju (1987) On Some Aspects of the Life and Work of John Burdon Sanderson Haldane, F.R.S., in India. Notes and Records of the Royal Society of London 41(2):211-237. 
  2. Jayakar, SD; Spurway, H (1968). "The Yellow-wattled Lapwing, Vanellus malabaricus (Boddaert), a tropical dry-season nester. III. Two further seasons' breeding". J. Bombay Nat. Hist. Soc. 65 (2): 369–383.
  3. Birkhead TR; N Hemmings; CN Spottiswoode; O Mikulica; C Moskát; M Bán and  K. Schulze-Hagen (2011) Internal incubation and early hatching in brood parasitic birds. Proc. R. Soc. B 278(1708):1019-1024.
  4. Murray, BG, Jr. (2001) Are ecological and evolutionary theories scientific? Biol. Rev. 76:255–289. 
  5. http://www.stanford.edu/group/stanfordbirds/text/essays/Brood_Reduction.html

2 comments:

  1. Sensational post, Shyamal. I have been reading a lot of accounts of naturalists in history and I have often felt a distinct lack of an Indian perspective. I'm looking forward to reading more on this theme. Thanks!
    Dinesh
    (I don't know if you remember me...Koshy's.. bangalore..spiders?)

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  2. Thanks for your comments Dinesh (of course I remember!)

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