Alex Demeri

Evolutionary history of lemurs

Lemurs are recognized in the anthropology community as a primate group with

remarkably diverse phylogeny. Confined to the shores of Madagascar, off the south eastern cost

of Africa, these highly evolved primates have been in evolutionary isolation for over 60 million

years. It is widely thought that the development of lemurs is a textbook example of adaptive

radiation. Due to the dynamic circumstances of Madagascar’s environment, along with the

extended increase in diversification rates and niche filling of lemurs, it is likely there is more

than just adaptive radiation at work.

Today there are over one hundred living species of Lemuroidea. They are arguably the

most diverse group of primates. Since their arrival to Madagascar, Lemuroidea developed into

species ranging from 1.1 Ounces to over 100 Pounds. These primates have adopted many

different traits such as cathemerality, hibernation, bipedalism and gregorian social behavior.

Combinations of these adaptations enabled lemurs to fill niches at greater rate than other

primates. The dynamic range of adapted features is a product of Madagascar’s isolation and

fluctuating seasonal environment.

Madagascar was once part of the supercontinent Gondwana. Around 160-117 million

years ago, an enormous landform began to separate from mainland Africa. The large landmass

consisted of modern India and Madagascar. It drifted off the southeastern coast of Africa for a

distance of about 1,000 Kilometers. After the separation from Africa, a second tectonic event

occurred. At around 90 million years ago, Madagascar began to separate from India, leaving it at

its current geographic location. This last event left Madagascar to be the largest island in the

Indian ocean, and the fourth largest in the world.

Madagascar’s size, location and origin give it a diverse range of climates and

geographical features. There are multiple environments and climate types on the island, varying

from tropical, temperate and arid. The island has a central mountainous area of volcanic origin.

This makes a natural division isolating the costs, limiting mobility across the island. Great

variance in climate and rigorous geography has been proven to be a strong stimulant for

evolutionary change.

The earliest primates are estimated to have arrived on Madagascar around 50 million

years ago. Given the great distance from any mainland, there is much speculation regarding how

the first primates ended up in Madagascar. Some have speculated that Lemuriformes were able to

travel across the Mozambique channel between Africa and Madigascar. This is possible by riding

natural rafts for as long as 30 days across miles of water to the volcanic island.

Lemurs share their earliest common ancestor with lorisoids. They both belong to the

superfamily of lemuriformes, consisting of small nocturnal primates, mostly found in Africa and

Asia. Their common ancestor was around during the Eocene epoch, which is renowned as the

epoch with the greatest amount of prosimian adaptive radiation. The great diversity of these early

primates is likely due to the isolation and lack of competition from monkeys, apes and other

advanced primates which were still in early evolution.

Since the vast quantity of lemur species are speculated to be a product of adaptive

radiation, it is hypothesized that their rate of speciation should decrease over time. As an

environment reaches its carrying capacity, the speciation rate should slow significantly as

ecological niches reach capacity. Rate of speciation also is expected to slow down as a

population reaches evolutionary equilibrium. This means that a population will reach a point of

stability and achieve stasis, slowing further evolution until a drastic environmental change.

The rate of diversification of lemurs shows an early boom among initial arrival to

Madagascar. After their arrival, lemur development shows promising evidence of adaptive

radiation. Adaptive radiation reaches a terminal velocity once an environment reaches its

carrying capacity or a population achieves an evolutionary equilibrium. If all Madagascarian

lemoriphormes shared ecological opportunity, then they should have equal rates of adaptive

radiation and evolution. This school of thought is disproved by the fact that lemur speciation

rates were significantly greater than that of lorisiforms. The adaptive radiation hypothesis

explains the relatively constant rate of speciation seen in the losiroids. However, the extended

rate of lemur diversification suggests there was more than just adaptive radiation that stimulated

their speciation.

“The multidimensional ecospace. The niche breadth occupied by lemurs far exceeds that of lorisiforms (black points), but the occupied ecospace has contracted substantially because of the extinction of the giant lemurs. Variation among species in (natural log) body mass, diet and activity pattern were summarized into two axes using multidimensional scaling (MDS1 and 2, approximately 90% of variation). Each lemur family (coloured points, extinct in grey with crosses in legend) occupies distinct ecospace, within which lorisiforms fill only a small proportion. The giant extinct lemurs filled unique ecospace (dashed convex hull) beyond that of extant species (solid convex hull).”

Drastic ecological changes also influenced the

rates of speciation for lemoriphormes on the island. Since their arrival to Madagascar there was

significant change in climate and environment. During the Eocene, Madagascar continued to

drift north. This exposed the earliest inhabitants to an increase in temperature, which would

stimulate both diversification and extinction through natural selection. After adapting to the more

tropical Madagascar environment, there was a period of global cooling and drying during the

Oligocene epoch. This change would continue to challenge the adaptability of lemurs and force

them to explore different ecological niches. This ongoing change of environment coupled with

the isolation of the island was a very strong catalyst for lemur diversification.

It is possible lemurs survived the drastic climatic changes by having an especially

adaptable phenotype, however that is nearly impossible to prove. The exposure to new

environments would greatly stimulate the rate of evolution and allow for lemurs to fill further

ecological niches. For example, all prosimians around the world are nocturnal, with the

exception of life on Madagascar. The only difference between the nocturnal prosimians and

Madagascarian prosimians is their location and evolutionary history. Having an advanced ability

to adapt combined with unique ecological circumstances would explain the lack of any decline in

the speciation of lemurs.

There is little evidence of predatory competition aside from human arrival, which drove

the giant lemurs to extinction. The giant lemurs showed identifiable characteristics aside from

size that made them different than some of their smaller kin. They relied on terrestrial

locomotion, slow climbing, daytime activity and a diet more reliant on plants and seeds. Their

extinction could be considered a population bottleneck, but it does not directly explain the

further development of lemur species less affected by human activity. This event created a

vacancy of ecological niches left behind by the now extinct giant lemurs. Even after humans

arrived, the lemurs continued to diversify, showing a decrease body mass and increase in

cathemerality. This divergence from nocturnal life suggests lemurs could still be in the early

process of filling the niches left behind by their large ancestors. It is possible the continued

speciation is partially a product of new niche opportunity.

Without hard evidence of any other significant extinction events, it’s unlikely that any

other lemur species experienced a bottleneck. Madagascar’s environment is incredibly harsh but

there is not enough proof to suggest it produced further extinction. In fact, Madagascar’s tough

environment may have given lemurs better evolutionary opportunity, rather than kill them off.

For example, the Golden Bamboo Lemur developed a strong resistance to cyanide. They eat

enough bamboo containing enough cyanide to kill most animals their size. This adaptation

enabled them to take advantage of an untapped food source, filling a vacant ecological niche.

The resilient lemur population has proven the island’s ecosystem it is not too extreme and

actually may be the perfect atmosphere for their extensive differentiation.

Although lemurs showed significant evolution upon their initial isolation, the adaptive

radiation hypothesis does not explain the continuous speciation after their early burst of

differentiation. The island’s dynamic ecological history gave lemurs the opportunity for further

speciation even after the initial boom of differentiation. Since the adaptive radiation hypothesis

does not explain their further diversification, it is inapplicable to lemurs.

Madagascar’s especially active and isolated environment continues to change and foster

new life. The ongoing changes in climate are drastic, yet timid enough for life to flourish.

Typical island dwellers, with the exception of Madagascar, are rarely exposed to the changes in

environment persevered by lemurs. This makes it unlike anywhere else on Earth.

Since Madagascar’s environment and circumstances are so unique from the rest of the

world, it may be the first and only example of a rare evolutionary circumstance. The early

phylogeny of lemurs shows the expected pattern of adaptive radiation. After their initial boom of

differentiation, the reason behind lemurs’ further development is uncertain. Madagascar’s

unusual combination of evolutionary stimulants may be the recipe for an accelerated type of

evolution. This evolutionary theory is an alteration of adaptive radiation. An initial spike in

speciation occurs and if an isolated population experiences enough continuous environmental

change, the species will continue to diversify for a greater period of time than adaptive radiation

predicts.

Work Cited:

1. Lemurs in Madagascar : Surviving on an Island of Change / American Museum of

Natural History. [Video]

Publication Information: New York, N.Y. : Films Media Group, [2012], c2006.

Publication Date: 2006

Link: http://fod.infobase.com/p_ViewVideo.aspx?xtid=47539

2. Phylogeographic analysis of the true lemurs (genus Eulemur) underlines the role of river

catchments for the evolution of micro-endemism in Madagascar. [Text]

Authors: Hogg, Russell T.1, Godfrey, Laurie R.2, Schwartz, Gary T.3, Dirks, Wendy,

Bromage, Timothy G.5

Link: http://eds.a.ebscohost.com/eds/pdfviewer/pdfviewer?

vid=7&sid=a652c5db-54ba-4138-a9d7-473b59573166%40sessionmgr4008

3. Testing the adaptive radiation hypothesis for the lemurs of Madagascar [Text]

Author: James P. Herrera

Publication date: 1/18/2017

Link: http://rsos.royalsocietypublishing.org/content/4/1/161014#sec-11

I got to take a few photos of some lemurs and other primates during my visit at the

Nashville zoo. Great experience there, would definitely recommend taking the trip.