Sibling Rivalry at its Worst
Image Copyright Carin Bondar, 'The Nature of Human Nature'
First born offspring have it pretty good. Mom is generally healthy and well-rested (having no other children to look after during her pregnancy), and the newly emerged micro-human has the exclusive attention of both parents and quite likely some grandparents as well. The first born doesn’t have to share the resources of his/her parents like second and third children do. Being a mother of three children, I think about this quite often. I feel kind of bad for my youngest child, who gets dragged around to preschool, music lessons, play-dates and the other endeavors of his older siblings. He inevitably gets his toys snatched away or his snacks devoured before he can get to them…and really there’s not a whole lot that can be done to change that. A major difference between humans and many other animals with respect to sibling competition is this: first born offspring often attain independence before further offspring are born, leaving the parents to care for only one infant at a time. This is certainly not the case in our species, where offspring remain dependent for too long for mothers to wait for independence between them (our physiology just won’t allow for 18 years between children!). Instead, sacrifices are made and a lower amount of parental care is provided to all siblings. So why not just have one offspring and avoid the need to sacrifice any kind of parental commitment? This could be risky…if all reproductive effort is placed on a single offspring, and (for whatever reason) said offspring fails to reproduce, your biological fitness is doomed. The ‘bet-hedging’ strategy1 has been coined to account for uncertain conditions in the future: there are clear advantages to having more offspring but providing a lower amount of parental care to each.
Other (non Homo sapiens) mammals face a similar conundrum when it comes to repeat child rearing when the first offspring is not yet independent. Fur seal and sea lion females rear a single offspring at a time, and nurse it exclusively for a period of 2-3 years. However, many females give birth to another offspring during this nursing phase. A long term study of Galapagos fur seals (Arctocephalus galapagoensis) and sea lions (Zalophus wollebaeki) addressed the potential conflict between siblings competing for their mothers’ resources, and also the conflict between mothers and offspring (i.e. which offspring should she nurse? In what situations should she choose the juvenile over the newborn and vice versa?2). Between-sibling conflict was found to be especially strong when resource levels were low, and the older sibling was unable to forage effectively on its own (away from the supplementation of the mother’s milk). During El Nino years, (resulting in low ocean productivity) a high level of between sibling conflict was documented, which often resulted in the continued nursing of the older juvenile and the death (by starvation) of the newborn. However, during periods of high resource availability mothers aggressively defended their newborns against the juveniles’ attempts to nurse. You might find yourself wondering: why should a female continue to reproduce when there is a chance for her newborn to starve or when her juvenile offspring is not ‘ready’ to stop nursing? The fluctuating conditions of the Galapagos make it impossible for the adult females to predict what is going to happen, making a bet hedging strategy important. If food resources are plentiful when a new pup is born, it makes sense for the mother should defend the younger offspring from aggressive attacks from the older one (i.e. get off my boob and go forage for yourself, there’s plenty of food out there). However, if food resources are scarce, the mother is expected to allow the intimidation (and starvation) of the younger sibling since the older one has already received more of her resources and is more likely to survive to adulthood. Although the death of a pup is a high price to pay for a miscalculation in available resources, the mother still gets one healthy offspring when times are bad and has the potential for more when times are good.
The Galapagos study shows that bet hedging is an effective reproductive strategy when environmental conditions and resource availablity are unpredictable. Over the course of her reproductive lifetime, a female seal or sea lion can maximize her overall fitness by bet hedging rather than by waiting for favorable conditions. Humans on the other hand, have taken a lot of the unpredictability out of our ability to attain resources, so our need for bet hedging is reduced. While an El Nino year might affect the availability of jack tuna in the fresh seafood section of the grocery store, it really has no bearing on whether there will be enough food to go around. Although at times my youngest child has to fight a little harder for his share of resources, starvation is out of the question. Taking the nature out of foraging has its advantages.
1Mock, D.W. and Forbes, S. 1995. The evolution of parental optimism. Trends in Ecology and Evolution 7:409-413.
2Trillmich, F. and Wolf, J.B.W. 2008. Parent-offspring and sibling conflict in Galapagos fur seals and sea lions. Behavioral Ecology and Sociobiology 62:363-375.
Parents who play favorites – all’s fair in the animal kingdom…
Image Copyright Carin Bondar 'The Nature of Human Nature'
As parents, most of us vow to keep things equal between our offspring. In theory there should be an equal amount of time investment into Suzie’s piano lessons as there is into Bobby’s tennis practice. However, in reality these things are never quite equal are they? Bobby’s tennis practice is across town, making the investment into his achievements larger than when Suzie walks over to the lady down the street for her piano lesson. Next, the inevitable: Suzie hates piano but Bobby looks as though he’s heading straight to the Olympic trials with his perfected backswing. Resource allocation into the two offspring is suddenly a far cry from 50:50. As humans we may say that we make equal investment into each of our offspring, this is an almost impossible task in the animal kingdom. An additional complicating factor is this: what about offspring with different fathers? Perhaps Bobby was destined to be a champion based on the fact that his biological father is an admirable member of society and a physically fit athlete himself, but poor little Suzie was the outcome of a one night stand after a few too many cocktails with a character whose name you’d rather erase from memory. Life history theory predicts that maternal investment into offspring should reflect the likelihood of said offspring to contribute to the fitness of said mother…which could be bad news for Suzie.
Many species in the animal kingdom mate with different partners in different mating seasons. Based on a number of environmental, physical and random factors a female may not always end up with the mate that she most desires. However, offspring are the inevitable result and so an interesting conundrum presents itself: invest equally in all offspring despite the sub-optimal paternity of some? Biologically speaking the answer should be a resounding no! According to the ‘differential allocation hypothesis1’, females of many species do not contribute equally to the health and well being of offspring sired by fathers of different quality. In experimental mating research on mallard ducks (Anas platyrhynchos), researchers paired individual females with both high ranking and low ranking males in order to investigate whether they would invest differently in their offspring based on the identity of the father2. Egg size is entirely determined by the female, and is a critical trait influencing fitness in birds: larger eggs produce larger chicks that have an increased chance of surviving to adulthood. Sure enough, it was demonstrated that eggs are significantly larger when females mate with a high ranking male. Invest equally in all offspring? I think not!
Blue footed booby (Sula nebouxii) males have a clear signal to show females that they are highly fit and ready to sire the next generation: their feet. Female boobies prefer males with bright blue-green feet, and will actively discriminate against males without this characteristic. Once copulation has occurred, the female generally lays two eggs, the second approximately 4 days after the first. Both parents incubate the clutch and raise the young together until they are ready to leave the nest. In an experiment designed to assess differential allocation hypothesis, researchers painted the feet of the male partner to a dull (unattractive) color just after the first egg was laid3. As a result of this change in the males’ foot morphology, the second egg laid by the female partner had a lower volume as well as a lower hormonal content. So perceptive is the female blue footed booby that within a few days of her mate quality being ‘lowered’, she reacts accordingly in her egg investment!
The effects of partner ‘attractiveness’ do not end at the physiological stage of egg-laying. It has been shown for Zebra finches (Taeniopygia guttata) that the amount of post-natal parental care (by both mothers and fathers) varies with partner quality as well. Parental care activities like nest maintenance, watching over young offspring, and time spent brooding, feeding and grooming them are generally performed by both Zebra finch parents; however, levels of all aforementioned activities are lower when a partner has an unattractive mate4. It’s as if parents are reluctant to place all of their reproductive resources into a set of offspring that may have mediocre genes…tough luck for said offspring!
I suppose most human parents would argue against application of the differential allocation hypothesis in our species….and in many cases this would be with just cause. Most Homo sapiens do an admirable job of investing equally in their offspring. There are many parents out there raising children that are not only born of ‘unattractive’ parents, they are being raised by parents that are not even biologically involved (adoption/foster parenting). Despite the fact that at times Bobby’s tennis talents may overshadow the accomplishments of his sister, as a human offspring Suzie is quite likely to be well provided for. Perhaps the animal kingdom could learn a thing or two about the importance of the nurture part of ‘nature vs nurture’ argument.
1 Burley N. 1986. Sexual selection for aesthetic traits in species with biparental care. American Naturalist 127(4):415–445
2Cunningham, E.J.A. and Russell, A.F. 2000. Egg investment is influenced by male attractiveness in the mallard. Nature 404: 74-77.
3Dentressangle, F., Boeck, L. and Torres, R. 2008. Maternal investment in eggs is affected by male feet color and breeding conditions in the blue-footed booby, Sula nebouxii. Behavioral Ecology and Sociobiology 62: 1899-1908.
4Burley N. 1988. The differential allocation hypothesis: an experimental test. American Naturalist 132:611–628
The Chastity Belt: Alive and Well in the Animal Kingdom
Image copyright Carin Bondar 'The Nature of Human Nature'
It’s a tough world out there. For the average Homo sapiens, having security in your relationship is a critical part to feeling successful. If someone out there loves you enough to be faithful to you and to forsake the advances of all others, it re-enforces our self confidence and thereby makes us happier (and more productive) beings. In the real world however, not all partnerships involve parties that adhere quite as strictly to the faithfulness that I mentioned above…what happens if you are feeling less than secure about the fidelity of your partner? In the 15th century humans invented a nifty contraption that provided a guarantee to insecure males that their partners would not accept genetic donations in his absence: the chastity belt. Made out of tough material (usually a combination of steel and leather), the belt covered her sensitive areas and prevented any kind of sexual interaction. In my opinion it seems a rather drastic measure to take, but as I mentioned above it’s a tough world out there. It turns out that the ‘chastity belt’ method of preventing your mate from copulating with any others isn’t limited to our species.
A ‘mating plug’ is defined as a structure that is utilized to block the female genital reproductive tract. Males of many species (including insects, crustaceans, reptiles and even mammals) utilize such plugs to prevent further mating by a female once they have deposited their DNA into her reproductive tract. In species where there is intense sperm competition, biologists have described some sophisticated mating plugs. For example, the spermatophore of ground beetles (Oedothorax retusus) contains sticky substances that allow it to act as a mating plug by physically blocking the females’ reproductive opening. In addition to its physical blockage, the seminal fluid of ground beetles induces refractory behavior in mated females (i.e. it effects a change in her behavior that makes her avoid further copulation events with other males)1. Dwarf spider (Leptocarabus procerulus) males utilize glandular secretions that harden once they are deposited inside a female2. There is an extremely high level of sperm competition in dwarf spiders, as the sperm from a single mating event can remain fertile in a females’ sperm storage organ for several months. The glandular-produced plugs utilized by the dwarf spiders are highly effective at preventing further mating of females: an experimental study showed that a large plug (produced by an uninterrupted copulatory event) prevented remating of the female 93% of the time2.
If you think that the ‘chastity belt’ strategies utilized by male ground beetles and dwarf spiders is shocking, read on. In arachnid species where sexual cannibalism is demonstrated (these are species where the female kills and ingests the male subsequent to copulation), males go to an even greater extent to ensure paternity. This makes sense. If he’s going to die anyway, he might as well do whatever it takes to make sure that his DNA is the prize winning seed for the next generation…even if this means breaking off his copulatory device inside the female genital opening (the human equivalent of a male breaking off his penis inside a females’ vagina in order to prevent future copulations). Males of the orb-web spider Argiope lobata and the white widow spider Latrodectus pallidus have been shown to do just that3,4. Plugging up the female with his copulatory apparatus has the effect of blocking the opening from use by other males subsequent to the sperm transfer, and it has been experimentally shown that such plugging is effective at reducing the paternity share of males that attempt to mate once a plug is in place3. Further investigation on sexual behavior in this species demonstrated that males that were cannibalized on their first copulation attempt had a much higher probability of damaging their pedipalps (sexual sperm transfer appendages) than males that escaped (74% vs 15%)3. Since this self-induced damage has negative consequences for future reproductive bouts (no kidding!), it makes the most sense biologically for a male to undertake it if his chances at securing paternity are high.
Although males of several species in the animal kingdom have developed some foolproof ways in which to ensure the fidelity of their sexual partners (i.e. in order to guarantee paternity of the resultant offspring and maximize fitness), I am much relieved that in our species such methods are almost entirely lacking. For the most part the chastity belt is a thing of the past, having been replaced by a simple trust in one’s partner or the taking of ceremonial vows in order to maintain a sexually monogamous partnership. I think that both males and females of our species would agree that such methods are a much more acceptable compromise than either losing your copulatory organ or having your opening plugged up indefinitely. It’s a tough world out there.
1Takami, Y., Sasabe, M., Nagata,N., and Sota, T. 2008. Dual function of seminal substances for mate guarding in a ground beetle. Behavioral Ecology 19: 1173-1178.
2Uhl, G. and Busch, M. 2009. Securing paternity: mating plugs in the dwarf spider Oedothorax retusus (Araneae: Erigoniae). Biological Journal of the Linnean Society 96: 574-583.
3Nessler, S.H., Uhl, G. and Schneider, J.M. 2009. Sexual cannibalism facilitates genital damage in Argiope lobata (Araneae: Araneidae). Behavioral Ecology and Sociobiology 63: 355-362.
4Segoli, M., Lubin, Y., and Harari, A.R. 2008. Frequency and consequences of damage to male copulatory organs in a widow spider. The Journal of Arachnology 36: 533-537.