A fascinating new paper just came out in Nature Communications and I intend to blog it in the usual manner, but I thought I'd try something new first. Check it out:
The Research Question
...According to life history theory, mothers should invest in their offspring if this enhances offspring survival and fitness, and if the fitness benefit to mothers from increased offspring fitness exceeds the cost of their investment. Whether the maternal environment influences the fitness and reproductive value of sons is unknown in most mammals because male mammals usually disperse and, thus, few studies have matched maternal quality to the fitness and reproductive value of sons after dispersal. In the red deer (Cervus elaphus), a species with pronounced sexual dimorphism, in which male fitness is linked to body size and fighting ability, a privileged upbringing by a mother of high social status provides sons with fitness benefits during adulthood. Whether a privileged upbringing also provides fitness benefits to sons in multifemale multimale mammal societies in which male fitness depends on how well males conform to female mate-choice preferences rather than a male's fighting ability or body size is currently unknown.
The Main Finding
The reproductive success and evolutionary fitness of male spotted hyenas is affected by the social status of his mother.
What do you think could explain this finding?
Some important variables to consider:
- The spotted hyena (Crocuta crocuta) is a large carnivorous species with minimal sexual dimorphism. This means that the two sexes are more similar than they are different, particularly with respect to features like color, body size, and ornamental body parts like antlers, feathers, or horns.
- The spotted hyenas live in "highly structured, female-dominated social groups called clans," in which access to resources is determined on the basis of social status. For that reason, females of high status have higher reproductive value and success than low-status females.
- Hyena groups are organized on the basis of matrilines. "In these societies, females usually remain in the group in which they were born, form matrilines with related females, and create linear dominance hierarchies...daughters acquire a dominance position close to and below that of their mother."
- Early maternal investment in her offspring is quite high. For example, the lactation period lasts for a period up to two years.
- Female offspring generally remain in the clan into which they were born, and retain their social status (thanks to the status of their mothers) into adult life, as long as they retain the support of a close female relative.
- By contrast, male offspring typically leave the clan into which they were born after some time, and immigrate into another clan. "They join the new clan at the bottom of the male social hierarchy, observe strict conventions of social queuing and increase in status with increasing tenure when higher-ranking males emigrate or die."
- Male reproductive success is dependent on how well the males conform to the preferences of the females. Which is to say, females have complete control over who gets to mate with them.
Tomorrow, you will be able to find a post detailing the findings of this paper in my usual style, but I thought I'd try something different today. In the comments, share your thoughts, guesses, and hypotheses in terms of what the mechanisms might be that cause an increase in the reproductive success of male spotted hyenas on the basis of their mothers' social status.
And check back tomorrow to find the answers.
HÃ¶ner, O., Wachter, B., Hofer, H., Wilhelm, K., Thierer, D., Trillmich, F., Burke, T., & East, M. (2010). The fitness of dispersing spotted hyaena sons is influenced by maternal social status Nature Communications, 1 (5), 1-7 DOI: 10.1038/ncomms1059
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First thought: mom's elevated status and heavy maternal investment could make for larger, stronger more competitive males, who would have an advantage working their way up through the ranks of another clan.
Second thought: rockstar mom reached her high status because of her relatively better genes - genes she passed along to her offspring.
Third thought: somehow clans recognize and respect the social rank of individuals in other clans, so the young males are getting some kind of easy ride up through clan ranks. Not sure about plausible mechanisms here...
Final thought: My third idea seems unlikely, and my first two are essentially a nature vs. nurture dichotomy. I'm inclined to think my first idea is more correct, because it's more consistent with the apparent importance of maternal investment (i.e. nurture has big fitness consequences) in the species.
Well, if social status determines resource access, then it's likely that high-status cubs (like high-status human infants, incidentally) have higher-quality food, both because their mother will have better food and thus produce better milk, and because they will have access to better solid food when they start eating it.
It is also likely that they will have different social experiences. Presumably, hyena cubs learn through observation of other hyenas and through play, both individual and social. I'll bet that high-status cubs are show more dominant behavior in social play right from the beginning, because their behavioral model (their mother) will show more socially dominant behavior. In addition, because they're getting better nutrition, they'll be more physically able and win more play fights. Their initial ability to dominate in social play will then shape their behavior to be more socially successful. (Maybe - when I read it again, that's a sort of tenuous argument.)
Finally, females control mating, and they're probably not stupid about it. The dominant female most likely gets any guy she wants, and so will mate with the "best" (I don't know how exactly she picks) male. This will give her cub better-than-average genetic material that will help him succeed both materially and socially.
That was fun!
The different social experiences and better food seem likely, but unless we postulate (serial?) monogamy, the lower-status hyena females may well also mate with the "best" males in their clans. Female choice does not necessarily (or, I would guess, often) imply monogamy, especially if the species lives in groups that contain multiple adults of both sexes.
IIRC, how long a cub nurses has a lot to do with how dominant the mother is. The cubs of high-ranking mothers get to eat at kills early on because their moms protect them, so they may be weaned as early as the age of 8 months. Cubs from low-ranking moms may not get weaned for a year or more, presumably because they get short shrift at the kills.
So, perhaps male cubs of high-ranking females get more practice at competing at the kills at an earlier age? Perhaps because of their mothers' higher status, they also do so with more safety under her protection? Might this practice at dealing with competition and aggression teach them better social skills that help them succeed later in life? Or might it just be that they get to socialize more, period, at an earlier age?
Or, I just thought of another possibility: might it be a function of the genetics of dispersal and incest avoidance? The more dominant a female hyena is, the less likely she is to leave her clan to join (or found) another. So . . . maybe neighboring clans tend to be slightly more closely related to the *less* dominant female members of the birth clan than the higher-ranking ones.
Following this line of thought, maybe females prefer to mate with the sons of high-ranking females of neighboring clans, just because they can sense that such males are more distantly related to them than sons of low-ranking females might be. Of course, this assumes that neighboring hyena clans share genes in common in the first place, and that high-ranking and low-ranking females aren't usually full sisters or otherwise too closely related to make a difference.
regardless of whether or not the son of a matriarch hyena is born into royalty. Once the prince leaves his clan and joins his new one, he begins on the bottom of the clan chain.
Once the prince leaves his clan and joins his new one, he begins on the bottom of the clan chain.
This is exactly correct (from @5) - he does not "bring" his social status with him. So the question is what else might account for his increased reproductive success?
If the upper class hyenas are getting better milk from their well-fed mother, and then have better access to carcasses as juveniles due to their higher social status, they'd probably have less infant mortality than lower-status infants. Is it just this that leads to greater average reproductive success for the high-status young 'uns? They make up a higher proportion of adult hyenas.