It’s rare to have good data on fitness of individuals in wild populations of animals. It’s even more rare to have data on fitness of hybrid individuals in natural environments. Reproductive fitness in many species of primates depends upon individuals’ behaviors in the particular environment of their social group.
A 2007 paper by Thore Bergman, Jane Phillips-Conroy, and Clifford Jolly provided fascinating observations of fitness of baboons in a hybrid zone between Papio hamadryas and Papio anubis in Awash National Park, Ethiopia: “Behavioral variation and reproductive success of male baboons (Papio anubis × Papio hamadryas) in a hybrid social group”. This paper focused on the reproduction of male individuals in the hybridizing group, while a book chapter by Jacinta Beehner and Bergman focused on reproduction of females in the same group: “Female Behavioral Strategies of Hybrid Baboons in the Awash National Park, Ethiopia”.
Today, primatologists recognize six species of baboons across Africa. This wasn’t always the case. For many years, some primatologists regarded different baboon populations as subspecies of a single polytypic species. Most today accept a more speciose taxonomy, as evidence of social and morphological distinctiveness has become stronger, and genetic evidence has substantiated the long historical duration of most regional lineages. I’ve written about the population structure of baboons previously: “Genomes and the complicated history of baboons “.
P. hamadryas and P. anubis are estimated to have diverged around 700,000–800,000 years ago. That’s approximately the same length of time since modern humans and Neandertals began to differentiate. The two baboon species have different kinds of social groups, and those differences seem to depend upon individual behavioral preferences of both male and female individuals.
The abstract of Bergman and colleagues’ paper tells the story of their research:
We take advantage of an array of hybrid baboons (Papio anubis × Papio hamadryas) living in the same social group to explore the causes and consequences of different male mating strategies. Male hamadryas hold one‐male units and exhibit a sustained, intense interest in adult females, regardless of the latter's reproductive state. Anubis baboons, by contrast, live in multi‐male, multi‐female groups where males compete for females only when the latter are estrous. These two taxa interbreed to form a hybrid zone in the Awash National Park, Ethiopia, where previous work has suggested that hybrid males have intermediate and ineffective behavior. Here, we first examine male mating strategies with respect to morphological and genetic measures of ancestry. We found significant relationships between behavioral measures and morphology; males with more hamadryas‐like morphology had more hamadryas‐like behavior. However, genetic ancestry was not related to behavior, and in both cases intermediates displayed a previously unreported level of behavioral variation. Furthermore, male behavior was unrelated to natal group. Second, we evaluated reproductive success by microsatellite‐based paternity testing. The highest reproductive success was found for individuals exhibiting intermediate behaviors. Moreover, over nine years, some genetically and morphologically intermediate males had high reproductive success. We conclude that the behavior of hybrid males is therefore unlikely to be an absolute barrier to admixture in the region.
The mating behavior of hybrid males turned out not to be “ineffective” at all. The intermediate mating behavior phenotype had the highest fitness, and there was no significant difference between individuals with different hybrid ancestry.
Constrasting somewhat with the male outcomes, Beehner and Bergman found that female individuals that sought out group situations with one dominant male had more offspring.
The harem-based social organization of hamadryas baboons has been attributed primarily to the predisposition of hamadryas males to herd females into one-male units (OMUs). Hamadryas females, by contrast, are thought to be behaviorally flexible. In this chapter, we describe and analyze female behavior in a hybrid group located in the Awash hybrid zone of Ethiopia. All individuals in the group are hamadryas–olive hybrids, but individual phenotypes range from mostly hamadryas to mostly olive. We use data across a 40-month period to assess whether or not females of different ancestry exhibit different behavioral strategies within the same mixed group. We found that females follow three distinct behavioral strategies: strict OMU, loose OMU, and non-OMU. Behaviors that suggested a hamadryas-like social organization (e.g., strong intersexual bonds) were associated with the strict-OMU females, and behaviors that suggested an olive-like social organization (e.g., strong intrasexual bonds) were associated with the non-OMU females. Furthermore, there was a significant relationship between morphological phenotype and behavior: Females that had more hamadryas-like morphological phenotypes exhibited behaviors characteristic of hamadryas baboons, while females with more olive-like morphological phenotypes exhibited behaviors like that of typical olive baboons. Loose-OMU females ranged across the spectrum both morphologically and behaviorally. Finally, strict-OMU females enjoyed higher reproductive success during this study than females in other kinds of groups. In sum, females under identical ecological pressures still exhibit particular behavioral strategies consistent with their morphological phenotype, suggesting that some aspects of female grouping behavior have a genetic basis.
The observations in these studies suggest that the mating behavior is not principally learned. Individuals have preferences or tendencies toward different social grouping patterns, and these are correlated with morphological and genetic markers that reflect the degree of ancestry from the two parental species.
Together, the fitness data do not lead to the conclusion that mating behavior of hybrid individuals in this population reduces their fitness. This is an active hybrid zone. The ecology seems to favor hamadryas-like female mating preferences, at least for the period that the researchers were observing reproductive outcomes. The ecology also seems to favor intermediate male mating behavior, a pattern more characteristic of hybrids. These are small samples, from a single setting for a limited period of time, and might be uncharacteristic of the longer-term evolutionary story. But they point to an interesting dynamic within a mixing population.