Beetle moms benefit from absentee dads

Nicrophorus vespilloides. Image via Wikipedia.

What’s good for the goose ain’t always good for the gander–until it is.

In evolutionary biology-speak, sexual selection happens when one sex benefits from something that harms the other. For example, male seed beetles use their spiky penises to transfer as much sperm as possible during mating, but as you might imagine, those spikes aren’t as popular with the ladies. When this kind of conflict occurs, the “losing” sex may evolve counter-measures to try to even the playing field (like vaginal spikes in the seed beetle).

But who are we to say what works for a beetle? Instead of fighting back, the “losing sex” may actually adapt to the sub-optimal situation–until it becomes the optimal situation. Think of it as making lemonade when life hands you lemons. And then becoming dependent on lemonade.

In the burying beetle, Nicrophorus vespilloidesparents put a lot of effort into raising their young. Often, it seems to be too much effort for the dads, and they simply take off, in search of new ladies to knock up and desert. Sounds pretty bad for the moms, right? Scientists at the University of Cambridge wanted to know just how bad this scenario is for a single beetle mom. Research carried out by Giuseppe Boncoraglio and Rebecca M. Kilner found that actually, mom’s better off without him. The study was published Feb. 15 in the journal PLoS One.

Burying beetles care for their young for about 8 days, until the larvae leave the “nest” to pupate. In this case, the nest is a rotting animal carcass. The larvae are most dependent on their parents for feeding during the first day after hatching, and become progressively more independent after that. Besides helping the young find food, the parents also protect their babies from predators–including infanticidal beetles from the same species who would like to get that rotting carcass all to themselves.

In the wild, about 85% of burying beetle broods are tended by both a mom and a dad, at least at first. Both parents want to get out of there as soon as possible, because raising a family is hard work. But dads tend to leave 2 to 5 days earlier than moms. The prevailing wisdom is that dad has more to gain by leaving early. He has more sperm to spare than mom has eggs, and even if some of the little brats die, there will be enough left over to spread his genes. And besides, he’ll be able to make more babies with the next female. Mom, on the other hand, has less chances to transmit her DNA, so she should put more effort into each offspring.

But is it really that cut and dry?

Previous work found that the larvae don’t suffer from being abandoned by their father, so this study was focused on whether the mother suffered any harm. To measure this, the researchers asked 2 questions:

  1. What are the costs to the female when she has to raise her offspring entirely on her own vs. when the male sticks around the whole time?
  2. When females are abandoned by their mate, at what point to they suffer the greatest cost?
To answer question 1, they separated beetles into pairs of males and females and gave each pair a dead mouse (yum!). The pairs were allowed to mate, and then were split into 2 groups: the happy couples and the single moms. Happy couples were kept together to raise their young until the larvae left the carcass. Single moms were abandoned after laying eggs, but before the larvae hatched, leaving them to raise their young entirely on their own. Afterwards, all moms were moved to a new carcass and allowed to mate with another male who stayed on to raise the second batch of kids together. 

Three measurements were taken: female lifespan, brood size and mass after first mating, and brood size and mass after second mating. The question: how does abandonment affect how well a mom cares for her young, how long she lives, and the quality of her later reproductive efforts?

And drum roll…moms who were abandoned lived longer! Around 5 days longer. As expected from previous research, there was no effect of father abandonment on the number or mass of offspring, but apparently keeping dad around stressed mom out so much it shortened her life.

Male abandonment also had no effect on female subsequent breeding, so the shortened lifespan was not balanced out by more total offspring. In fact, the opposite is probably true, since a shorter life means less opportunity to mate. After all, you don’t just stumble upon the right conditions for love–romantic lighting, a rotting corpse–every day. These things take time.

As for question 2: Larvae that were abandoned early by both parents didn’t fare as well as those that were cared for, but moms that left early did live slightly longer–up to a point. When they left early they did better than moms that left after 8 or 24 hours of care, but not better than moms that stayed for the full 8 days. They don’t really follow up on this point, and I have no idea what it even means (if anything).

Om nom nom. Image via Wikipedia.

Why might females live longer when abandoned by their mate? The authors suggest that maybe the male competes with her for food. The first 24 hours of child-rearing are tough on a lady, and she needs all the nourishment she can get after that to stay healthy.

But this then brings us to a bigger question about the evolution of parental care in this system: did females adapt to male abandonment, or are males trying to adapt to females chasing them away? After all, the earlier the male leaves, the less he gets to nosh on that smorgasbord of rotting mouse. Maybe, as the paper suggests, it’s not a race to see who can leave first, but a race to see who can stay longest. There is some evidence to support this–a previous paper by J. Bartlett in 1988 found that males tend to leave sooner when breeding on a smaller carcass–and in 3 instances the male was even killed by the female. Maybe she was sick of him hogging all the food. The fact that both parents tend to leave before the larvae are all grown up may have more to do with the declining quality of the carcass than the laziness of the parents.

So, is this a case of developing a lemonade addiction after being handed lemons, or was lemonade always the best thing going?

I think this illustrates a very good point when talking about evolution of sexual conflict. We can’t always assume that what works best with us (like not being abandoned by the father of your children) works for everything else.

Boncoraglio, G., & Kilner, R. (2012). Female Burying Beetles Benefit from Male Desertion: Sexual Conflict and Counter-Adaptation over Parental Investment PLoS ONE, 7 (2) DOI: 10.1371/journal.pone.0031713

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3 thoughts on “Beetle moms benefit from absentee dads

  1. Hmm… isn’t this a rather artificial construct though? Is forced partnership really equivalent to enduring partnership under natural conditions?

    Secondly, what supports the ‘counter-adaptation’ model here? Isn’t an alternative position that the females are driving the males from the carcass in order to have exclusive feeding access? Is there any data to distinguish these interpretations?

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