Hybrid animals are valuable because they can give us insight into the borders of species identification. The biological species concept defines species as interbreeding populations that can produce viable, fertile offspring. Thus, whenever we see two “separate” species hybridizing, it call into question our species classification. For example, the hybridization of hamadryas and anubis baboons suggested a re-classification as two different subspecies within the same species. However, this also brings up questions. If the five traditional classifications of baboons are subspecies, why do we see pronounced differences in behavior between hamadryas and other baboons? Do these behaviors serve as a reproductive isolating mechanism that prevents hybrids from being successful?
Bergmann and colleagues (2008) examined the behavior of males living in the hybrid zone. They found that morphology and behavior were linked. For example, hybrid males that appeared more like pure hamadryas baboons exhibited behavior similar behavior to pure hamadryas males, such as herding. This suggests that there may be a link between genes underlying morphological appearance and social behaviors. However, males that exhibited both intermediate morphology and intermediate behavioral patterns had the highest reproductive success in hybrid groups. However, hybrids had a disadvantage among social groups comprised predominantly of either hamadryas or anubis baboons. Essentially, hybrids “fit in” in social groups where their intermediate behavior is the norm. However, this study brings up a major question–why are morphology and behavior linked? What proximate mechanisms underlie the distinctive herding behavior of hamadryas baboons?
Although fertile hybridization in baboons support classification as one species with different subspecies, the situation is very different for mantled and black howler monkeys. Despite the overlap in their current ranges, A. pigra and
represent two opposite extremes of howler social organization and behavior (Treves, 2001). While most howler species remain in small groups with only one dominant male, mantled howlers tend to live in larger multi-male, multi-female groups. As Cortes-Ortiz (2007) explains, there is a large gulf between these two species in estimated divergence time. This gap of approximately three million years was enough for these two different howler species to develop along different trajectories. Nonetheless, they now overlap in Mexico and interbreed. However, these hybrids face greater limitations than the baboon hybrids. Data from mitochondrial DNA indicate that most hybrids carried mitochondrial DNA from black howlers. This suggests partial infertility in hybrids–while female offspring are viable and fertile, male offspring are not. This evidence supports classification as two separate species.
Recent genetic evidence regarding Neanderthal’s contribution to the human genome suggests a similar pattern. Genes associated with the testes and on the X chromosome of modern humans do not indicate Neanderthal ancestry in modern humans (Sankararaman et. al, 2014). This suggests that Neanderthals and humans encountered a similar pattern to the hybrid howler monkeys. While females were likely fertile, males were likely sterile. Similarly, this new evidence supports classification of modern humans and Neanderthals as two separate species. While limited interbreeding introduce Neanderthal DNA into the modern humans, they were unable to completely hybridize.
For more information about hybrid howlers:
And for Neanderthals and Modern Humans:
Bergman T, Phillips-Conroy JE, Jolly CJ. 2008.Behavioral variation and reproductive success of male baboons (Papio anubis x Papio hamadryas) in a Hybrid Social Group. American Journal of Primatology 70: 136-147.
Cortes-Ortiz L, Duda T, Canales-Espinoza D, Garcia-Arduna F, Rodriguez-Luna E, Bermingham E. 2007. Hybridization in a large-bodied New World primates. Genetics 76: 2421-2425.
Sankarararam S, Mallick S, Dannemann M, Prufer K, Kelso J, Paabo S, Patterson N, Reich D. 2014. Nature. http://dx.doi.org/10.1038/nature12961
Treves, A. 2001. Reproductive consequences of variation in the composition of howler monkeys (Aloutta spp.) Behavioral Ecology and Sociobiology 50: 61-71.