To Twin, or Not to Twin?
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.