To Twin, or Not to Twin?

31 May 2009 by Anne-Marie Hodge, posted in Uncategorized

A few weeks ago, we discussed how ecological factors influence offspring sex-ratio in elephant seal populations. Mammals (among other vertebrates) can optimize their reproductive investment by producing either males or females under difference circumstances, but there is another aspect to managing investment also: how many? Superficially, it seems that, of course, producing more babies is more advantageous. The more youngsters produced, the bigger chance of parental genes being sustained in the population. But, wait, offspring are expensive. So if more energy is concentrated into fewer offspring, you can invest more per capita and set them up for a higher chance of survival. So what is a mammal to do?

As with most things, the answer is “it depends.” Different species require different strategies. This should all be a review, if not then refer to information on K- vs r-selected life history patterns.

We already know that different species employ different methods, based on their ecological and evolutionary history. It is interesting, therefore, to zoom in on individual species that can manipulate litter size, to examine the relative advantages or disadvantages of offspring number. In a study recently published in the Journal of Mammalogy, a team of researchers has focused on the results of twinning in mule deer (Odocoileus hemionus), with some interesting results (Johnstone-Yellin et al. 2009).

The researchers tracked litter size and fawn survival rates in a population of mule deer in eastern Washington. In a nutshell, they found that singleton fawns had a significantly higher survival rate than twins. In addition, previous work has shown that it takes 1.6 times more energy for a female deer to raise twins than if she only produced a single fawn per season (Mauget et al. 1999).

At first glance, all of this seems fairly predictable and mundane. We have to wonder: if twins are less likely to survive, why do so many of these females produce them in the first place? Almost half (14/30) of the fawns in the study were twins. Surely there would have been selection pressure against such a prevalence? That leads us to the interesting part: the data suggests that females producing twins actually increased their reproductive success, despite the higher fawn mortality and increased energy expenditure involved in raising twins.

How does this work? Over half of the offspring in this study (16/30) died by the end of the summer. Singletons were more likely to survive (they had a 62.5% chance of surviving, versus only 40%) for twins, and among the fawns that died, singletons lived longer (69 days versus 16 days for twins).

Despite the depressing outlook for twins, the numbers also showed that mothers of twins produced an average of 0.92 fawns, while mothers of singletons averaged only 0.75. The mortality rate for the offspring is so high, that there is a very strong chance a female’s fawn will die regardless of litter size. Thus, having a back-up baby ends up being advantageous in the end, which is why twinning (some females actually produce triplets, also) has been sustained in the population over time. For many animals, especially prey species such as deer, the choice is sometimes not win or lose, but lose or lose less. It may sound grim, but it is an effective strategy: Johnstone-Yellin et al. determined that a the population in which all females produced twins would have a 4% higher growth rate than it would have if all females produced singletons.

Studies such as these are important for more than just clarification of life history information for a given species. This reinforces the importance of looking at the proverbial “big picture” when addressing topics with evolution, ecology, and behavior. You have to extrapolate across seasons, years, and populations in order to see the true significance of many survival and reproductive strategies.

Further research is needed to determine how twinning affects a female’s lifetime reproductive success. Will twins in one year be a detriment to her performance as a mother in following years? There has been much work done on manipulation of clutch size in birds, but studies such as the Johnstone-Yellin et al. paper are slowly shining light on how the issue is handled within different mammalian species.

Johnstone-Yellin, T., L. Shipley, w. Myers, and H. Robinson. 2009. To twin or not to twin? Trade-offs in litter size and fawn survival in mule deer. Journal of Mammalogy 90: 453-460.
Mauget, C., R. Mauget, and A. Sempere. 1999. Energy expenditure in European roe deer fawns during the suckling period and its relationship with maternal reproductive cost. Canadian Journal of Zoology 77: 389-396.

(Image credit).


7 Responses to “To Twin, or Not to Twin?”

  1. Heather Etchevers Reply | Permalink

    Twinning is fascinating for the developmental biologist, as well.

    This article may interest you, in that control of the insulin-like growth factor system is also involved in fat storage and longevity, also parameters of mammalian survival and fitness in a particular ecological niche. Epigenetic factors mark the chromatin of the embryo in a mother subjected to a dietary deprivation around the Igf2 locus (e.g.), among others, and have repercussions for the offspring.

  2. Anne-Marie Hodge Reply | Permalink

    Thanks for the links! The authors of this study also looked at female condition body condition, and the probability of twinning increased with body fat but decreased with body mass, which is also interesting. I have read about Igf in relation to fat metabolism and the effect on offspring’s metabolism, but had never contemplated the trickle-down effect on offspring sex ratio. Very interesting!

  3. Diggitt McLaughlin Reply | Permalink

    I never thought of this question before reading your post, and I’m not sure who would explore this. Is there any idea of whether singletons have been evolved towards or away from in mammals, or does it relate to relative physical size?

  4. Heather Etchevers Reply | Permalink

    Also, I think this would be prime material for Researchblogging.org, which would give the post (and others) more visibility. And so far, anyone looking for information with the keyword “twins” only comes on a series of articles on different subjects, so it would complement nicely.

  5. Mike Fowler Reply | Permalink

    Any idea if they’ve found or are looking for a genetic basis behind this?

    There’s been a lot of study on red deer offspring production in europe, for example Tim Clutton Brock’s work on sex ratio bias and selective resorption of foetuses (foetii?) under differing environmental conditions – mothers invest in sons under good environmental (and own body) conditions, while harsh conditions lead to more daughters being produced — male foetuses are resorbed and energy reinvested in females where possible. This is because sons require better initial conditions to have a good chance in life than daughters – especially important in a polygynous mating system.

    This means the females have a plastic response to the environment. Are the mule deer twinning only in response to their own condition (which is determined in part by the changing environment) or is it purely an inherited trait?

    The more study that goes into the mechanisms behind these phenomena, the more interesting the natural world around us gets!

  6. Heather Etchevers Reply | Permalink

    Thanks, Jon! I had been wracking my brains as I wanted to put a link to his studies and couldn’t remember who was conducting them…

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