April 2006

When the kakapo, a critically endangered parrot, makes the papers, it’s generally not good news. In 1995, just 51 of these large flightless birds waddled around the forests of island sanctuaries in their native New Zealand. The kakapo used to be more widespread, but having evolved with few natural predators on the islands, the birds were poorly adapted for the modern world — and the rat and stoat invasions that came along with it. Already slow-moving and largely defenseless, the kakapo made even easier prey for those mammals by virtue of their conspicuous smell. Biologists became so concerned about the species’ extinction that, beginning in 1982, they gathered up all the remaining wild kakapo and moved them to remote islands that had been cleared of all mammals. Nevertheless, even there under the fastidious care and regular feeding provided by biologists, the kakapo did not flourish — there are now just 86 of them alive. Now we may understand why. In January 2006, biologists revealed how the kakapo’s problems might largely come down to sex — and how a simple diet could help save them from extinction.

Where's the evolution?

So what does sex have to with it, and how does this all relate to evolution? It turns out that the kakapo population has become male-biased — the birds lay more male eggs than female eggs, and now there are around three males for every two females. This is a problem for population growth. One male could fertilize many females resulting in many eggs, but one female (even if fertilized by many males) can only produce a few eggs — so in terms of population growth, females are more important than males. Without enough females, the population cannot grow. Evolutionary theory has helped us figure out how to reverse this trend in the kakapo.

For many years, biologists didn’t know why the kakapo laid too many male eggs. The kakapo on the island sanctuaries seemed to be healthy — they were safe from predators, getting enough food, and breeding, but for some reason, they produced more male chicks than female chicks. Then in 2001, Spanish biologist José Tella suggested that evolution might explain why the kakapo seemed to favor sons.

Evolution shows that, as much as possible, organisms will evolve to optimize their fitness — the number of descendents they produce for future generations relative to other organisms. So based on evolutionary theory, we would expect organisms to evolve adaptations that would allow them to have more children, grand children, great-grandchildren, and so on.

One branch of evolutionary theory known as sex allocation theory suggests that in some situations, an organism might maximize its fitness by producing all-girl offspring — and in other situations it might make more sense to produce boys. Male kakapos grow larger and faster than females and require more resources from their mothers; however, since male kakapos fight for opportunities to mate with females, larger male kakapos are more likely to mate and reproduce. In terms of sex allocation theory, this means that if a kakapo mother has few resources to provide her offspring (see Scenario #1 below), she might be better off producing a girl — which are smaller, take fewer resources from the mother, and even if small, have a fairly consistent chance of mating and producing a grandchild. In that situation, producing a son (who would probably be weak and small) would not be a good bet — since it would be very difficult for him to attract a mate and produce any grandkids.

However, if the situation were different and the mother had plenty of resources (see Scenario #2 below), she might be better off using them to produce a big healthy son, who could out-compete other males, mate with lots of females, and generate many grandchildren. In this second scenario where the mother has lots of resources, producing a daughter wouldn’t be optimal, since the daughter (even if she is big and healthy) can only produce a few grandkids. Of course, the birds are probably not consciously “choosing” one gender over another, but nevertheless, their bodies are likely adapted to sense the level of resources available and trigger a “son-making mode” or a “daughter-making mode.” This ability to shift offspring sex ratios represents an evolutionary adaptation for getting more offspring into the next generation (and the generation after that).

José Tella pointed out that the extra food that biologists provided for the kakapo gave them plenty of resources and increased their body weights. According to sex allocation theory, this might cause the females to switch into “son-making mode.” In order to test this hypothesis, the New Zealand biologists working on kakapo conservation performed an experiment. They continued to feed the kakapo, but only the smaller females were allowed to eat as much as they wanted. This scheme gave the kakapo enough food to stay healthy and breed, but not so much as to gain a lot of weight. Then they tallied the sexes of the chicks produced by these birds. Sure enough, under the new feeding scheme the chicks were about 50/50 male and female — the male-bias had been removed!

There is still much work to be done to rescue the kakapo from extinction, but nevertheless, this discovery represents an important step in ensuring that the conservation program delivers results. Biologists estimate that the simple trick of manipulating sex ratios could shave 12 to 109 years off the time it takes the kakapo population to recover. In this case, evolutionary theory was critical in making the right conservation decision. Without a deep understanding of evolution, biologists might still be scratching their heads over the rarity of daughters in the kakapo — and male kakapo might be facing a long stretch of lonely nights.