How hurricane severity affects extinction probabilities for howler monkeys

Howler on Barro Colorado Is. Photo: Vince Smith

Howler monkeys are known for their long, bellowing calls that can be heard in the forests of Central and South America. There are five subspecies, the taxonomic rank below species, of howler monkeys. Alouatta palliata mexicana is a critically endangered subspecies found in parts of Mexico and Guatemala. Ameca y Juárez and colleagues had the idea to examine how hurricanes affect population numbers of this arboreal, New World primate. Using data from 1988 to 2002, the authors modeled how hurricane intensity affects extinction likelihood of local populations of this New World primate. They also wanted to know which human disturbance, hunting or habitat loss, had a greater impact when combined with different intensities of hurricanes.

Previous studies have examined how other species of howler monkeys have been impacted by hurricanes on a short-term basis, but this study is the first to look at the effects over a long period of time. The area in which A. p. mexicana inhabits is experiencing more hurricanes lately (Manson et al., 2009; Portilla-Ochoa et al., 2006), thus this study is of increasing relevance.

Studying A. p. mexicana living on Agaltepec Island in Mexico, which is isolated and predator-free (including human predators),  Ameca y Juárez and colleagues estimated predator, hunting, and habitat loss pressures from other A. p. mexicana populations in the area. They used population viability analysis (PVA) to determine the impact of different hurricane intensities on howler monkeys' quasi-extinction risk, or the probability of reaching a population size in which additional threats could cause the loss of the entire population.

The lowest degree hurricane disturbance result in a 28% quasi-extinction risk forty years later. Baseline estimates of hurricane intensity result in a 74% risk. Overall, the authors found that hurricanes have the ability to exceed the risk of human impacts. The combination of hurricanes and habitat loss have a substantially faster effect than when hurricanes and hunting are paired together.  It is the decrease in survival of adults that most affects the quasi-extinction rates, with adult males having an especially significant effect on the population.

The authors do point out that recurrent exposure to disturbances, including hurricanes, can be expected to affect the adaptability over time, thus reducing the likelihood the species will go extinct from the disturbance. Thus, the relationship between quasi-extinction risk and hurricane intensity won't be lineal in reality, as populations frequently exposed to hurricanes over time will adapt to lessen or moderate the negative effects.

As more frequent and intense hurricanes are expected in the future, studies such as this one allow us to determine how primate populations might be affected. Future modelling of hurricane intensity and frequency may incorporate other factors likely to affect population numbers, such as inbreeding.

Food for thought:
How might conservation biologists use information from this study to protect A. p. mexicana?
How might this study apply to other species of primates and other animal species in general?

Links of interest:
IUCN's page on A. palliata
How animals survive hurricanes
Why we should expect more extreme weather
Howler monkeys' call


Works cited:
Ameca y Juárez EI, Ellis EA, Rodríguez-Luna E. 2015. Quantifying the severity of hurricanes on extinction probabilities of a primate population: Insights into “Island” extirpations. American Journal of Primatology 77:786-800.

Manson RH, Jardel Pelaez E, Jimenez Espinosa M, et al. 2009. Perturbaciones y desastres naturales: impactos sobre las ecorregiones, la biodiversidad y el bienestar socioeconomico, en: Capital natural de Mexico, vol II: Estado de conservacion y tendecias de cambio. Conabio, Mexico, pp. 131-184. 

Portilla-Ochoa E, Sanchez-Herdandez AI, Hernadez-Meza D. 2006. El impacto de los huracanes en la biodiversidad del estado de Veracruz. In: Inundaciones 2005 en el estado de Veracruz. pp. 101-119. Universidad Vercruzana, Veracruz Mexico. 

A look inside an ancient primate's brain yields surprises

Fifteen million years ago the earliest known member of the subfamily cercopithecine roamed the planet. Victoriapithecus is the most ancient old world monkey scientists have discovered to date. This frugivorous primate was found on an island in Lake Victoria (Benefit, 1999). What we know about this species is a result of one fossil, a skull. A recent study by Gonzales and colleagues studied the inside of this monkey's skull (also called an endocast) to learn more about the brain structure of Victoriapithecus.

Endocast of A. sediba, Photo credit: Lee Berger

Using high-resolution X-ray imaging, researchers were able to create a 3D model of what Victoriapithecus's brain looked like. The results were surprising. At only 35.6 cm³, this monkey had a brain about the size of a plum: small when compared to the animal's body size.  Most primates this size have a brain that's about the size of an orange.

The olfactory bulb, the part of the brain in which sense of smell is processed, was three times larger than expected. It was along the lower end of what we see for strepsirrhines, a group of primates with a greater reliance on sense of smell when compared to haplorhines (new and old world monkeys, tarsiers, and apes).

Despite having a small brain and a greater reliance on smell, characteristics typically seen in the so-called lower primates, there is more to Victoriapithecus than meets the eye. Micro CT scans were able to show the wrinkles and folds in the brain.  Given that this animal had quite a small brain, the numerous ridges and folds (sulci and gyri) show that this species was complex.  Folds on the brain are generally linked to greater intelligence. The organization of Victoriapithecus's brain and the sulci and gyri are what we see in present-day cercopithecines. Thus, it looks like a having a large brain is not a prerequisite for having a complex brain.



Links of interest:
Brain size and evolution
Developmental pattern of primate brains
How snakes may have influenced primate brain evolution


Literature cited:

Benefit, B. R. Victoriapithecus: the key to Old World monkey and catarrhine origins. Evol. Anthropol. 7, 155–174 (1999). 

Gonzales, L., Benefit, B., McCrossin, M., Spoor, F. Cerebral complexity preceded enlarged brain size and reduced olfactory bulbs in Old World monkeys. Nature Communications, 2015; 6: 7580 DOI: 10.1038/ncomms8580