BURSTI am reading a fascinating book called “The Genetic Book of the Dead” by Richard Dawkins. He writes very well about a difficult subject; i.e. how past lives have affected the composition of the gene pools of animals (including us) over millennia.
I particularly liked his discussion about cuckoo birds of many different species that occur all over the world. Cuckoos are brood parasites (kleptoparasites) that trick birds of other species into incubating and raising their offspring for them.
The cuckoo Dawkins describes in detail is the ‘common cuckoo’ (Cuculus canorus) that used to be common in Britain (but not anymore). It is the one that sings (or calls) “cuckoo.” Female common cuckoos evolved to lay their eggs in a wide variety of songbird species’ nests – just because.
Over the course of time, foster parent species have learned to recognize when an egg deposited into their nest has a different colour or pattern to their own eggs. If it is different they will tip the invasive egg out. In most cases, the cuckoo egg is slightly larger than the host’s eggs, but size does not precipitate rejection of the cuckoo egg – but the wrong colour and pattern raises alarm bells.
What is amazing is that, although common cuckoo females are all the same species, they can lay eggs in the foster nests that match the colour and patterning of the foster bird’s own eggs. Cuckoo eggs laid in meadow pipit nests are dark and speckled like those of meadow pipits, whereas cuckoo eggs laid in brambling nests are pale greenish and speckled like those of brambling for example.
How they do this is still not completely understood, but Dawkins suggests the following. Whereas human males carry one of each sex chromosome (XY) in every cell, human females have two X sex chromosomes (XX) in every cell. In birds the sex chromosomes are called Z and W. Female cuckoos carry one Z and one W (i.e. ZW – the opposite to human females) whereas males carry 2 copies of the W (i.e. are WW).
So, in cuckoos the W chromosome and its genes are only passed down through the female line. Each female will carry the same sex genes that their mother, maternal grandmother, maternal great grandmother etc. back in history also carried, and these genes seem to have a memory – a cultural memory. The female cuckoos can remember what their foster parents looked like as well as what the nest and the eggs of the foster parents looked like. They recall the details of their upbringing.
This means the female cuckoos only lay their eggs in the nest of the bird species that they themselves were reared in. Within the total population of common cuckoos there are different ‘cultural types’ – some laying their eggs only in dunnock nests, another group laying their eggs in water pipit nests and so on. In this way, the appearance of the eggs laid by each cultural type of cuckoo match the eggs of the foster parent.
Ah, but that is not the end of the story – the parasitic cuckoo chick can also recognize the host bird’s eggs (or hatched chicks) so it scoops them into its especially hollow back, edges them up the side of the nest, and out – so it gets all the food to itself.
Our North American cuckoo – the cowbird – has quite a different life story that I will talk about next week.