As a new father of twins, I have an all too acute and intimate familiarity with some of the costs associated with reproduction (never did I imagine yearning for 4 h of sleep, for example). However, compared with my wife, I've had it, and still have it, relatively easy. After all, I didn't spend over 9 months gestating, not to mention giving birth to, two infants. Reproductive costs for all kinds of animals have been studied widely among behavioral ecologists, particularly in relation to the burden carried (literally) by pregnant females. For example, a number of studies of squamate reptiles have shown reduced sprint speed and/or endurance capacity in pregnant animals. What has remained unclear in many of these studies is the extent to which the performance decrement is due to physical or physiological causes. Is it simply the added weight of the carried offspring that underlies reduced locomotor capacity in pregnant females or might there be physiological changes that are to blame?
To help tease apart the importance of physical versusphysiological bases of so-called reproductive burdens, Peter Zani, at Lafayette College, and colleagues Ryan Neuhaus, Trevor Jones and Jonathan Milgrom studied locomotor endurance in hundreds of side-blotched lizards(Uta stansburiana) over the course of their natural breeding season for 2 years. Zani and colleagues worked on populations in eastern Oregon, USA,collecting lizards in late spring and early summer of 2005 and 2006. After collecting the lizards in the morning and taking them to a field lab, the team carried out endurance trials on a motorized treadmill. Having determined the time and distance traveled before exhaustion, the team sexed the lizards and categorized females further as non-reproductive, with enlarged follicles,yolked eggs or shelled eggs, or postreproductive. All animals were marked and released within 48 h except females with shelled eggs, which were kept until after ovipositing, when they again underwent a treadmill trial to assess how rapidly they re-gained their endurance.
In general, females exhibited lower endurance capacity than males, and as their reproductive cycle progressed, endurance progressively diminished, such that females carrying shelled eggs ran distances approximately 15–20%shorter than their non-reproductive counterparts. Further, data showed that following oviposition, animals recovered their non-reproductive endurance levels in a very short time – within a 12 h window – leading Zani and colleagues to suggest that the burden carried by females of this species is largely a physical one since physiological effects, such as altered blood-hormone levels, would probably last for substantially longer time periods. More manipulative experiments will be required to clearly elucidate the relative importance of physiology and physical load when evaluating the reproductive burden faced by female lizards. But what this study makes clear is that a burden with a strong physical component is indeed faced in this species and the nature of the burden remains quite consistent from year to year.
After explaining this work to my wife, she acknowledged a degree of empathy for these female lizards, as she too suffered a serious physical burden as her own pregnancy progressed. However, this empathy was short lived once she found out the lucky lizards recovered from their deficits in a mere 12 h...