My interest in kākāpō began way back in my honours year at uni: a guest speaker told us that as far as anybody knew, the last remaining birds were a few males, somewhere in Fiordland. I remember feeling that that sounded really sad – those lonely males booming for females who never came.
Shortly after that, a relatively healthy population was discovered on Stewart Island** (where “relatively” = less than 200). Alas, cats found them too, and so now the species is found on a small number of offshore islands, free of mammalian predators, and carefully managed to maximise their breeding success. (2019 seems to have been a bumper breeding season, and you can follow progress via Twitter (for example) – where I learned that baby kākāpō have big feet.***)
The 2017 Schol Bio paper had a really interesting question about kākāpō, and I thought it would be useful to unpack it here, since many students will be starting to think about how to prepare for the end-of-year scholarship exams. (You can find the paper, and other resources, at this link.)
The paper provided quite a lot of resource material about the birds’ biology and environment, and then asked candidates to:
analyse the information provided in the resource material and integrate it with your biological knowledge to discuss:
how the reproductive behaviour AND genetic factors fo the kākāpō account for its critically endangerd status
how genome sequencing and analysis may be used in managing kākāpō populations to try to ensure that kākāpō do not become extinct.
Now, the fact that these large, cryptically-coloured, flightless parrots evolved in the absence of mammalian predators explains a lot about their current endangered status. (Humans’ propensity for habitat destruction didn’t help.) Their nests are usually in hollow trees or under logs; that, plus the fact that both nests and birds are quite smelly, means that they’re pretty easy for rats, mustelids, cats and dogs to find. Females do all the incubation and parental care, which makes them more vulnerable than males at that stage of the life cycle. (This also means that the chicks are vulnerable to both chilling and predation while the female has left the nest to feed.) However, the birds’ lek breeding habits, which see males booming for hours from the ‘bowls’ they’ve dug out, makes the males vulnerable during the courtship period.
Prior to human arrival, kākāpō’s main predators were other birds. Eagles and hawks hunt by sight, and the parrots’ habit of ‘freezing’ when alarmed is a pretty good defence in that context. So is the fact that they are nocturnal. Sadly, neither behaviour protects against those mammals, where scent is important in finding prey.
Kākāpō’s breeding cycle is unusual – they only breed in good ‘masting’ years, when canopy trees produce large quantities of seed, rather than on an annual basis. That, plus the fact that clutch sizes are small (just 1-4 eggs), means that the birds’ reproductive rate is low and population growth is slow.
In addition, the gender of offspring is dependent on the females’ weight, which can result in a skewed sex ratio – there are roughly three males for every two females. having a disproportionate number of males is worse for the population’s future than having a higher proportion of females, because while males can fertilise many eggs, each individual female can produce only a few eggs. So the rate of populaton growth is limited by the number of fertile females of reproductive age.
It’s important to remember, though, that the lek breeding system means that usually relatively few, dominant males will actually mate with females anyway. This reduces the diversity in the next generation’s gene pool – and if any of those males happen to have any harmful alleles, then those are likely to be passed on to their offspring.
So, we have a population that’s gone through a bottleneck (in 1995 there were only 51 kākāpō alive), where only some males may be contributing to the gene pool, and with the potential for inbreeding depression. The consequent reduced genetic diversity (and probable increase in homozygosity) may well have had a number of negative effects:
- an increased risk of harmful alleles being passed on, for example: researchers are interested in looking at the MHC alleles in the kākāpō gene pool, as harmful variants may affect the birds’ ability to fight off pathogens;
- a greater risk of disease, such as the systemic fungal infection that recently killed the young male called Rooster;
- and the fact that a very high proportion (40-50%) of eggs are infertile.
What to do about this? This is where students needed to apply their knowledge of biotechnology to a discussion of managing the kākāpō population in a sustainable way.
For example, an ongoing crowd-funded project aims to sequence the genomes of all living kākāpō (plus some recently-dead birds). This sort of genetic information may allow the identification of birds carrying beneficial (or harmful) alleles, and also how much individual birds are related to each other. Managers could then use this information to ensure that birds with harmful alleles don’t breed (if possible), and similarly that closely-related kākāpō don’t breed with each other. For example, analysing the genome of birds, like Rooster, that died of infections might help identify any harmful MHC alleles. It might also be possible to identify any genetic causes of infertility and, again, use that information when deciding on breeding-focused translocations between islands.
Some students showed real insight in realising that the birds sourced from Fiordland might well have different genetic variants than the population from Stewart Island, which should also be taken into account in managing kākāpō breeding e.g. ensuring that birds with a Fiordland whakapapa contribute to breeding on each of the sanctuary islands. (There are currently very few birds with that Fiordland lineage, which will make that project challenging.)
And some candidates also recognised the potential for using gene-editing techniques such as CRISPR – although I’d expect that actually doing that in the zygote stage would be a lot more complex in birds than in mammals!
** My husband was privileged to be part of the team of volunteers who worked with DOC to study and conserve that Stewart Island population.
*** This immediately reminded me of Rachmaninov, which probably says something about how my mind works!)