Blog written by Eva Keppner & Sandra Steiger. Read the full paper here.
Parental care – investing energy and time to raise and protect offspring – can be observed in many species spread all over the animal kingdom. Drivers of the evolution of parental care seem to be, among others, a harsh environment, a structured habitat or ephemeral resources. Interestingly, it is often only the female parent who does most of the work and the males wander off in search for new mating partners. However, sometimes male and female together form a family unit for a certain amount of time and raise their offspring in a joint endeavor.
It is not quite clear what exactly leads to the participation of the male. Much of what we know about the mechanisms of biparental care came and still comes from research on birds, where males and females often share the burden of raising their offspring. Some years ago (although thoroughly described already in 1933), another organism moved into the spotlight of the study of parental care: the burying beetle. Beetles of the genus Nicrophorus show an elaborate set of parental care behaviors, both males and females are capable of preparing an adequate nursing environment, feeding and protecting their larvae, as a pair or a single parent. The nursing environment, a small vertebrate carcass, e.g. a mouse or a small bird, also conveniently serves as the sole food source for the whole family during reproduction. This detail of their biology played the main role in our study. We wanted to disentangle the benefits of two caring parents from the downsides of two parents eating from the family’s dinner table instead of only one.
Parental behaviors in burying beetles comprise pre- and post-hatching activities. During pre-hatching care, the parents bury and prepare the carcass so it is suitable for their soon-to-arrive larvae. We first compared parent pairs and single females during this approximately three-day task. Almost all of 163 experimental beetles gained weight during this time and the carcasses, which were of very similar weight at the beginning, lost more weight when prepared by two beetles instead of one. Consequently, most of the beetles have eaten from the carcass and two beetles eat more than only one. Hence, pairs have less food available in the post-hatching phase, where larvae are fed with regurgitated carrion and also self-feed from the cadaver. Quite obvious one might think. But this is where it gets interesting (for us biologists at least…). Most studies that compared the efficiency of bi- vs uniparental care in burying beetles used the same set up: a single female or a pair of beetles receives a carcass of the same size and offspring fitness is used as a measure of brood success. Within this study design, offspring fitness never differed between uni- and biparental condition. Additionally, it is known that carcass size influences brood success in burying beetles. So why do carcasses which are no longer of similar size at the time of larval hatching, due to the amount eaten by one or two parents, not lead to a noticeable difference in offspring fitness between uni- and biparental conditions, like bigger or more larvae?
In other words, why does our single female with a bigger carcass at the time of larval hatching not raise more or heavier larvae than a pair of beetles with a smaller carcass? Probably because she is missing the help of a second caring parent, the male beetle. To find out if this assumption is true, we performed another experiment. Again, we provided single female and male/female pairs with carcasses of similar sizes. However, this time we switched the carcasses after preparation by the beetles shortly before larval hatching. Now the beetle pairs had bigger carcasses than the single females (only slightly bigger – we’re talking of differences in the range of milligrams). Pairs reared heavier broods than single females. Therefore, we were able to find some support for our initial assumption that two beetles are better in caring for their offspring but this effect is often masked because a male parent also eats from the resource, which leaves the rest of the family with less food. We only found this effect for small carcass sizes when food is limited.
We tried to also align our findings within the evolution of biparental brood care in burying beetles and started some speculations. As female N. vespilloides as well as females of other species in this genus are perfectly capable of preparing the carcass and rearing the offspring alone, why did biparental care evolve in the first place? One certain benefit of a male beetle is brood protection against unwanted intruders (mainly other burying beetles). This might have been the first step. Of course, a male sitting guard at the brood chamber could hardly resist the smell of a food source – and also started eating from the carrion, leading to an imbalance again, as everything he eats is no longer available for his progeny. Males, who were also able to provide more than just safety by also feeding the offspring might have been better fathers by rearing heavier larvae which will eventually hatch into bigger adult beetles. Which led to the state of care we can now observe in burying beetles.
With our simple experimental set-up, we were able to gain a little insight into the mechanisms of family life. However, this field of research with its intertwined relations and behaviours between family members is incredibly exciting and still offers a lot to find out.