Body size on mainlands versus islands, in whiteyes and silvereyes (Zosteropidae: Zosterops)

@thebeachcomber @lukedowney @kokhuitan @hedgehog111 @lloyd_esler @rion_c @tonyrebelo @jeremygilmore

INTRODUCTION

The genus Zosterops, in the family Zosteropidae, is remarkably widespread in Africa, Asia, Australasia, the Indian Ocean, and the western Pacific (https://www.inaturalist.org/observations?taxon_id=17439).

Furthermore, the many spp. of Zosterops are remarkably consistent in their generalised body-proportions and diminutive body size.

What this means is that, although this genus qualifies as remarkably speciose (https://www.collinsdictionary.com/submission/17774/speciose#:~:text=Of%20a%20taxon%20or%20other,many%20species%3B%20species%2Drich.), it does not seem to have undergone much adaptive radiation (https://en.wikipedia.org/wiki/Adaptive_radiation).

THE 'ISLAND RULE'

A well-known biogeographical pattern is that, among small birds and small mammals, the forms on islands tend to be relatively large-bodied (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1691042/ and https://www.jstor.org/stable/3448886 and https://nsojournals.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1600-048X.2012.05820.x and https://www.perplexity.ai/search/Various-passerine-birds-8OD79qKESu..NPBfwAcnrg).

This raises the question:
Do species/subspecies of Zosterops on islands tend to be larger-bodied than those on mainlands?

RESULTS: MAINLANDS

Africa:

Zosterops virens and Zosterops pallidus are restricted to southern Africa. Their values for body length and body mass are, respectively:

• about 12 cm and 8-15 g, and
• 12-13 cm and 8-20 g.

Zosterops abyssinicus is restricted to northeastern Africa and southern Arabia. It has body length 10-12 cm (https://en.wikipedia.org/wiki/Abyssinian_white-eye).

Zosterops senegalensis is restricted to West and central Africa. It has body length about 11.5 cm and body mass 7-14 g (https://en.wikipedia.org/wiki/Northern_yellow_white-eye).

India:

Zosterops palpebrosus has body length 8-9 cm (https://www.perplexity.ai/search/What-is-the-TubdL27WQ_CkvjY53Kzdbw). Zosterops japonicus has body length 10-12 cm, and body mass 10-13 g (https://www.perplexity.ai/search/What-are-the-1sCGnnQJSquLDTXxrGEYgw).

Australia:

Zosterops lateralis occurs on both the Australian mainland and many islands of various sizes, in the western Pacific, east of the mainland (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4607525/). On the mainland, it has body length about 12 cm and body mass about 11 g (https://www.perplexity.ai/search/What-is-the-0LPFYjq7SwyQfM7xs_3Jgg).

Overall, on mainlands:
Zosterops, regardless of species, has body length about 12 cm, and body mass about 11.5 g.

RESULTS: ISLANDS

Body mass in certain populations of Z. lateralis, inhabiting islands, is about 2 g more than on the Australian mainland (https://nsojournals.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1600-048X.2012.05820.x).

In New Zealand, body mass of Z. lateralis lateralis is about 13 g (https://www.birdsnz.org.nz/publications/diurnal-pattern-of-mass-in-an-urban-marlborough-population-of-silvereyes-zosterops-lateralis/ and https://nzbirdsonline.org.nz/species/silvereye and https://nzbirdsonline.org.nz/sites/all/files/ZOSLAT_FG.pdf and https://www.perplexity.ai/search/Which-species-of-3wYdn_FQQfW92M9qKJNDZA).

An extreme example is Zosterops lateralis chlorocephalus, restricted to the Capricorn-Bunker group of islands (https://en.wikipedia.org/wiki/Capricorn_and_Bunker_Group), off the Great Barrier Reef. This weighs 14-15 g, which is about 4 g heavier than conspecifics on the mainland.

Zosterops lateralis melanops, restricted to New Caledonia, has body mass about 14 g (https://www.perplexity.ai/search/Which-species-of-3wYdn_FQQfW92M9qKJNDZA).

On Lord Howe Island (https://en.wikipedia.org/wiki/Lord_Howe_Island), off eastern Australia, there are two indigenous spp. of Zosterops, viz.

• Zosterops tephropleurus, which has body length 13 cm and is said to be more massive than the closely-related Zosterops lateralis of the mainland, and
• the recently-extinct Zosterops strenuus, which was larger-bodied again (body length 15 cm) with a large, loosely-constructed nest (Hutton 1991, https://catalogue.nla.gov.au/catalog/531978 and https://ianhutton.info/product/birds-of-lord-howe-island-past-and-present-2/).

Turning to spp. other than Z. lateralis, on islands:

On Norfolk Island (https://en.wikipedia.org/wiki/Norfolk_Island), two spp. occur, both of which are restricted to the island and are relatively large-bodied, viz.

• Zosterops albogularis, now extinct, with body length 13-14 cm and body mass possibly about 30 g (https://en.wikipedia.org/wiki/White-chested_white-eye), and
• Zosterops tenuirostris, with body length 13-15 cm and body mass 18 g (https://www.perplexity.ai/search/What-is-the-komfaXE9S4.0FUcX49nyXw).

In Vanuatu (https://en.wikipedia.org/wiki/Vanuatu), Zosterops flavifrons has body length 11-12 cm (https://en.wikipedia.org/wiki/Vanuatu_white-eye).

On the islands beyond the coastal shelf in Indonesia:

• Zosterops citrinellus, restricted to the Lesser Sunda islands (https://en.wikipedia.org/wiki/Lesser_Sunda_Islands) plus various small islands in and near Torres Strait (https://en.wikipedia.org/wiki/Torres_Strait), does not differ in body size from Z. lateralis; however, it has a much larger and bulkier nest; and
• Zosterops chloris has body length 11-12 cm (https://en.wikipedia.org/wiki/Lemon-bellied_white-eye).

Sulawesi: https://www.perplexity.ai/search/What-are-the-9QxPwtDlR_yGxgVjekolvw.

Zosterops natalis, restricted to Christmas Island, has body length 12-13.5 cm and body mass about 11 g.

On Lifou island (https://en.wikipedia.org/wiki/Lifou), New Caledonia, the largest- and smallest-bodied members of genus Zosterops coexist. Body mass in Zosterops inornatus is 22 g, whereas that in Zosterops minutus is 9 g (https://www.perplexity.ai/search/Body-size-of-GfBALZ1lSpOjYyiQGLoOPQ).

Zosterops conspicillatus, restricted to the Northern Mariana islands, has body length 12 cm.

Islands between Madagascar and Africa:

• Zosterops maderaspatensis, of Madagascar, the Comoros, and the Seychelles, has body length 10 cm,
• Zosterops mayottensis and Zosterops kirki (https://en.wikipedia.org/wiki/Kirk%27s_white-eye), both restricted to the Comoro Islands, have body lengths respectively 11 cm and 10-12 cm,
• Zosterops modestus, restricted to the Seychelles, has body length 10-11 cm and body mass 11.5-13 g (https://www.perplexity.ai/search/Body-size-of-GfBALZ1lSpOjYyiQGLoOPQ), and
• Zosterops semiflavus, an extinct species restricted to an island in the Seychelles, is said to have had body size 10 cm.

Mascarene Islands (Mauritius and Reunion):

• Zosterops mauritianus has body mass about 8 g (https://www.perplexity.ai/search/What-is-the-qm0vorqfRjaUsE75MidaFw),
• Zosterops chloronothos has body length up to 10 cm (https://en.wikipedia.org/wiki/Mauritius_olive_white-eye).
• Zosterops borbonicus has body length 9.5 cm (https://en.wikipedia.org/wiki/R%C3%A9union_grey_white-eye), and
• Zosterops olivaceus has body length 11-12 cm, and body mass 9-11 g (https://www.perplexity.ai/search/Body-size-of-bbhUmoTpQL6SyvtgwRdtqg).

DISCUSSION

The evidence for the 'island rule' in Zosterops is ambivalent.

On one hand, several species and subspecies, occurring on islands in the western Pacific Ocean, do show enlargement in body size.

The prime example: in New Caledonia there is nominally an adaptive radiation, consisting of two sympatric spp. differing more than two-fold in body mass.

On the other hand, this effect is not apparent on islands in the Indian Ocean. On the contrary, all four spp. in the Mascarene Islands, for example, are smaller-bodied than is typical on mainlands.

Furthermore, even in the western Pacific,

• the body mass of the largest-bodied species of Zosterops is only double that typical on mainlands (https://www.perplexity.ai/search/What-is-the-lzC5_idzQeSCBXAS6WrXJg), and
• New Caledonia hardly confirms the 'island rule', because enlargement in one species is balanced by diminution in the sympatric species (https://upload.wikimedia.org/wikipedia/commons/0/08/ZosteropsKeulemans1879.jpg).

Therefore, I find overall that Zosterops remains remarkable for its diminutive body size, whether on mainlands or on islands.

Its consistently small, thin beak and brush-tipped tongue may constitute an unique combination among the avian genera on Earth (https://www.inaturalist.org/journal/milewski/95051-a-new-interpretation-of-the-evolutionary-and-ecological-strategy-of-whiteyes-and-silvereyes-zosterops#).

This may help to explain why adaptive radiation may be largely irrelevant to Zosterops, even on landmasses too remote to have been reached by other birds.

Publicado el mayo 31, 2024 04:39 TARDE por milewski