A few days ago I wrote about how the different needs of male and female cervids (deer) and bovids (sheep, buffalo, antelope, etc.) shape their armaments, and how antler evolution in red deer has essentially stopped due to the pattern of sexual selection that evolved in the species. The idea that selective pressures may differ so significantly in sexually dimorphic species was something relatively new to me, and now there it seems that artiodactyls aren’t the only animals to have their sex play an important role in fitness and evolution. In a new paper entitled “DIFFERENTIAL INVESTMENT IN SONS AND DAUGHTERS: DO WHITE RHINOCEROS MOTHERS FAVOR SONS?” White, et. al, describe how mother White Rhinoceros (Ceratotherium simum) seem to favor sons over daughters when nursing their babies.
White Rhinos seeking some shade at Disney’s Animal Kingdom
In order to understand why a mother rhinoceros would favor male offspring over female offspring, we need to understand something about the social structure of the white rhinoceros in terms of interactions/mating. Females do not have any known type of dominance hierarchy, but while several males may live in a given area, there is typically one dominant male that fights the others for mating rights. This doesn’t mean that the other males don’t get a chance to “consort” with the females now and again, but they had better be sure that the bull is not around when they get horny (sorry, couldn’t resist the pun). The dominant male does father more offspring than subordinate males, however, the subordinate males having less offspring related to them than any given female, and so it pays off (in a reproductive sense) to be a bigger and stronger male while the females have a good chance of producing offspring regardless.
In the study, 14 female white rhinos were tracked and observed over the course of three years, with 25 male and 20 female calf births recorded within the group, and the primary behavior the researchers were looking for was suckling. Suckling is merely the offspring trying to nurse, but does not always mean that they receive milk. While it might be easy to assume that only suckling that allows the offspring to drink milk would be beneficial, the mere act (regardless of outcome) is significant because it shows response to need by the mother and may have other social benefits for the young rhinoceros. Likewise, extended or increasing suckling can even slow the reproductive rate of the mother, so the longer and more intensely she suckles a baby, the less offspring she’ll be able to produce over the long run.
What the researchers found was what would generally be expected in an animal where males stood a better chance at reproductive success if they were more healthy/bigger/stronger and the females didn’t have to deal with dominance hierarchies. Males were definitely preferred for suckling when between birth and 6 months (Class A), and also still suckled when 24 months old or older (Class D) until the birth of the next calf, while Class D females did not suckle at all. The mother also seemed to be more responsive to males during the first six months than females, perhaps the critical time for growth in white rhino calves. The researchers sum up there findings this way;
Our findings indicate that sons received increased investment compared to daughters. Specifically, mothers tended to have longer interbirth intervals after the birth of sons than daughters, sons initiated significantly more suckling bouts than daughters, sons spent significantly more time suckling than daughters (particularly in the earliest age class), and sons were significantly older than daughters at time of weaning.
A White Rhino on a cold, January day at the Philadelphia Zoo
Despite all this effort, however, only one of the males will end up being dominant in any given area, so much of the maternal effort put into sons ends up being “wasted.” If a male does not receive such intense investment, however, there is little hope that he can compete with other males or leave many offspring, and so he will likely end up being a reproductive dead end by his lack of proper behavior. Indeed, it seems that mothers are responding to the persistence of males in the first six months of life especially, with age Classes B & C being a toss-up between males and females, and the suckling solicitations of Class D females being entirely ignored. Mothers didn’t seem to show any preference when breaking off a suckling bout, however, so it appears that in the first critical months males are favored, and they are allowed to suckle longer overall than females. In relation to the long term implications of this, males that are more greedy and are born to mothers who are more prone to stop and let them suckle will probably become the dominant males in the long run, in turn siring the most offspring, in turn fixing the greedy behavior in males and possibly the favoritism amongst later generations of females.
While we might expect to see some amount of sexual dimorphism in white rhinos according to size and the greedy suckling of the males, there doesn’t seem to be any appreciable difference between males and females. Where the difference is made, however, would be amongst males themselves, especially when males are young and prone to fighting. The weakest males may be weeded out through injuries sustained through fighting early, and so the stronger juveniles will be more likely to wind up being dominant. Little is said of size differences amongst males in the paper, but I would assume that healthier (if not larger) males would end up becoming dominant, and such males likely were favored by their mothers during the early months of their lives.
One further point about this paper; it’s apparent that the mothers are favoring the young males, but do we really know why? The young males solicit suckling more, this is true, but I doubt that the mother rhinos are hoping that their little boys will grow up big and strong so they can beat out the other male baby in the next field. If it were at all possible, it would be exceedingly interesting if there’s a different physiological/mental response in mother rhinos for when male or female newborn solicit suckling, as there has to be some reason why males are favored when the mother can’t possibly know what is going to become of her baby down the line (and probably doesn’t care).
The sex of an newborn animal may be something we take for granted, assuming that mothers will love all babies just the same. Anyone who’s grown up in a big family (I’m the oldest of five) knows that this isn’t always the case, however, and favoritism is something very real, and we should not expect that animals are “fair and balanced” in everything they do in relation to offspring.