Another Spectrum

Personal ramblings and rants of a somewhat twisted mind

Honorary mammal

Leave a comment

The Kiwi comes from Aotearoa New Zealand and its succulent green or yellow flesh is delicious on top of a pavlova.

No! Wait! That’s a kiwifruit, not a mammal, real or honorary. They originally came from China in the late 19th or early 20th century and were known as Chinese gooseberries until it became an export commodity, at which time the name proved to something of a hindrance. Having both “Chinese” and “berry” in the name proved to be too much for the sensibilities of Americans at the time so a new name was coined.

Sorry. The Kiwi is a two legged mammal that…
Umm. I thought Kiwis were real mammals, not honorary ones.

Now you’re referring to human beings who consider Aotearoa to be their home. Yes they are real mammals. I’m referring to another 2 legged creature.

Oh! You mean the bird?

Precisely. One of the peculiarities of Aotearoa New Zealand is that prior to the arrival of humans around 800 years ago, the only terrestrial mammals inhabiting these islands were three species of bats – flying mammals. On the other hand, there was a vast range of flightless bird species.

Kind of back to front compared to the rest of the world

Back to front?

Flying mammals and flightless birds.

I suppose that’s one way of looking at it. But back to the kiwi. It’s often referred to as an honorary mammal because it exhibits some characteristics that are very mammal-like and rarely, if ever, seen in birds. Here’s a few examples:

The power of flight is very costly in terms of energy requirements and birds have made a number of evolutionary adaptations in order to minimise this cost. Think weight reduction.

Bone structure: Birds typically have lightweight bones filled with air sacs to reduce weight. Flightless birds may have slightly denser bone formation but still retain air sacs. The kiwi on the other hand, has heavy marrow-filled bones just like mammals.

Reproduction: Female birds typically have a single ovary, again an adaptation to reduce weight. Kiwi have two ovaries, and just like mammals, ovulation occurs alternately in each.

Leg muscles: Birds have considerably less leg muscle mass compared to mammals of equivalent size. Again an adaptation to reduce weight. However, the kiwi has gone to the other extreme. The legs make up one third of their total weight.

To put that into perspective, a typical kiwi is about the same size as a fowl (chicken) and weighs in at slightly over 3 Kg (around 7 lb). If you were able to buy kiwi meat from the supermarket, a single leg would weight around 600 grams or 1.3 lb. That’s some leg! Kiwi put those legs to good use.

Adult kiwi can easily outrun humans, and their legs and sharp claws can be used to fend off introduced predators such as cats, stoats, rats and possums. However, dogs can wipe out entire kiwi population in single area almost overnight.

Skin: Kiwi have tough leathery skin that is more mammal like than bird like.

Vision: Birds rely on good eyesight for survival. In the case of nocturnal birds, eyes become oversized to ensure adequate light reaches the retina. However, Kiwi imitate many mammals that occupy a similar ecological niche in other parts of the world. Eyes are small and eyesight is poor. Kiwi are very long sighted. So much so that even objects on the horizon have a focal point well behind the eye. Kiwi have almost no binocular vision (where the visual fields of both eyes overlap). They cannot even see their own beak! Rare for birds, they cannot see colour.

Sense of smell: On the other hand, their sense of smell is highly developed. In the bird world, only the condor has a better sense of smell. Birds typically have a bony ridge between the eye sockets. In the kiwi, this space is occupied by a nasal cavity, just like mammals.

Birds typically have their nostrils at the base of their beak. Not so the kiwi. It’s the only bird with nostrils at the tip of the beak. This is used to help locate underground grubs.

Sense of touch: The nostrils are surrounded with mechanoreceptors (pressure-sensitive receptors). It was once thought that kiwi located prey entirely by smell, but new research indicates that the mechanoreceptors might allow the kiwi to detect prey up to 10 cm (4″) underground.

Hearing: Few birds have highly developed sense of hearing. Kiwis are an exception. As with the areas of brain related to the sense of smell, that area related to hearing is larger in size and more complex than other birds of similar size. The external ear openings are large and can easily be seen

Whiskers: Cats and many nocturnal mammals, have whiskers to aid in the detection of objects in close proximity in the dark. Kiwi have whiskers on their face and base of their beak that serve the purpose.

Burrows: Like badgers, kiwi build burrows where they hide out when not foraging and for nesting. Kiwi sleep standing up.

No preen gland or tail: Other birds have a preen gland near the tail that supplies oil used to keep feathers weatherproof, waterproof and in good condition. The kiwi has neither.

No wings: Well not quite, even though the genus name Apteryx means wingless. Flightless birds typically have rudimentary wings equipped with flight feathers. The wings retain some purpose such as for display in attracting a mate or fending off opponents, to assist in balance, and to allow safe descents from a height. Not so the kiwi. A human in freefall with outstretched arms has greater powers of flight than a kiwi. It’s vestigial wings are barely 3 cm (about an inch) long, are not feathered and remain hidden under the kiwi’s plumage.

Hair-like plumage: Kiwi look soft and fluffy. Most birds have feathers equipped with hooks and barbs that keep the feathers in the neatly arranged vanes. Kiwi do not. The feathers hang loose, and in appearance look like fluffy hair or fur.

Body temperature: Birds typically have a body temperature of between 39ºC – 42ºC. Kiwi have a body temperature similar to mammals: 37ºC – 38ºC.

Other facts about kiwi.

Species: Up until the 1980s it was thought that there were only 3 species of kiwi. However, with the aid of DNA technology, it’s now known that there’s at least 5 species. What is unusual is the fact that there is very little difference in physical characteristics between species and DNA testing is necessary to identify them. The mechanism whereby five distinct species with almost identical characteristics could evolve in a relatively small geographical area is not fully understood, but one theory is that New Zealand was repeatedly fragmented by glaciers during Middle and Late Pleistocene. Fragments remained isolated long enough for speciation to occur but there was nothing to drive differentiation. It’s now known that there were many more species prior to the arrival of humans.

Huge egg: We Kiwi like to claim that the kiwi produces the largest egg in comparison to its body size, but the claim can also be made of the bee hummingbird. Both species produce an egg that’s up to 25% of the weight of the mother. A kiwi lays an egg that is about six times heavier than a chicken egg.

Birds’ eggs are typically 35% – 40% yolk. Kiwi eggs are 65% yolk.

In two kiwi species, incubation is shared equally by both parents, but in the other species the female takes no part in the incubation. It’s a male only task. Given the size of the egg she has to lay, I’m surprised any female kiwi would want to have anything to do with it.

Mate for life: For the most part Kiwi form lifelong monogamous partnerships. I say for the most part because while males always remain faithful, female kiwi are known to abandon their mate if she comes across a male she finds more attractive.

Declining numbers: Kiwi take around five years to reach sexual maturity, and in most species the female produces only one or two eggs each year. Mainly due to mammalian predation, only about 5% of kiwi chicks survive into adulthood. It needs to be around 20% to maintain the population. In areas with extensive (and expensive) predator control kiwi populations can double approximately every ten years. Nationwide, the kiwi population is declining at around 2% per year.

Author: Barry

A post war baby boomer from Aotearoa New Zealand who has lived with migraines for as long as I can remember and was diagnosed as being autistic aged sixty. I blog because in real life I'm somewhat backwards about coming forward with my opinions.

Leave a Reply

Please log in using one of these methods to post your comment:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s