Across the animal kingdom, it’s tough to be a mom. Not all species exhibit parental care (or care about their young at all after the eggs are laid), but in those that do, usually the moms get stuck doing the bulk of the work. In mammals, moms not only have to carry the baby in the womb for long pregnancies, but then also have to provide babies nutrient-rich (and energy-expensive) milk for some time after birth as the baby grows. Life isn’t much easier for bird moms, even though they don’t have to give birth and lactate (though some birds, like pigeons and flamingos, do make a kind of milk!) Momma birds have to build the nest, usually with some help from dad, and then must exclusively produce and lay the eggs. Then, in many bird species, it’s mom’s job to sit on the eggs for hours at a time, keeping them warm and protecting them from predators as the chicks develop inside. In these cases, the male bird will often bring the female snacks and food to eat while she sits on the nest, but he is free to fly around (and often, get into trouble), while she is on nest-arrest during the incubation period. Once the chicks hatch, both parents usually help in caring for the babies, as most bird species exhibit biparental care. But when one tallies up the workload, even in biparental systems, it appears females get the brunt of the parental duties. Doesn’t seem fair, does it?
Some females, however, have found a way to have it all. Take, for example, the female spotted sandpiper (Actitis macularius). Upon their first breeding attempt, female spotted sandpipers seem to go the traditional route, finding a male mate and sharing the parenting jobs with him. But after this, females get different idea into their heads, and start multiple nests with multiple males (called polyandry). In fact, after first breeding, male spotted sandpipers do the majority of the parental care, including incubation of the eggs, brooding of the chicks and feeding the young. Just as if all the classically-assumed sex roles had been reversed, females also will compete with each other, often in violent fights, for males (sounds like fodder for a new reality TV show!). Once the male has chosen his female victor, and she lays a clutch of eggs and he begins incubating them, the female will start the search for her next mate, starting fights with other females again right after clutch completion (Reed et al 2013). Even in other bird species where female-female aggression in common, such as tree swallows where the females fight for prime real-estate (Rosvall 2008), this aggression usually goes down once egg laying finishes and incubation behavior begins.
This ability for females to get right back into the competition ring after laying eggs might probably make one wonder if their hormone levels are “sex-reversed” too. One might expect testosterone, often studied for its role in male behaviors like competition and courtship (but found in both males and females), to be higher in the female spotted sandpipers than the males. Surprisingly, researchers measured hormone levels in the birds’ blood and found that male sandpipers still had 10 times more testosterone than females, despite their docile behavior (Fivizanni and Oring 1986). Interestingly, however, when the researchers looked at patterns of the testosterone, rather than the absolute levels, female spotted sandpipers showed similar patterns to male songbirds that court, compete, and mate multiple times throughout the breeding season. So, while the momma sandpipers still had less testosterone than the dads, their hormone patterns were more like those of other “normal” male birds.
Additionally, female spotted sandpipers also have lower levels of prolactin than male sandpipers as well. The hormone prolactin stimulates parenting behavior in all sorts of animals, ranging from mammals to birds and even amphibians. As its name suggests, prolactin promotes lactation in mammals. Moreover, in birds, it helps them sit on the eggs and chicks, and exhibit guarding and caring behaviors towards their babies. Since female spotted sandpipers tend not to spend much, if any, time with the eggs and chicks, their prolactin levels are lower than those of the stay-home dads (Oring et al 1986). During incubation in bird species where mom incubates the eggs, prolactin is higher in females, and then the males’ prolactin increases as they help take care of the chicks once they hatch. In this sense, the sexes are “reversed” hormonally in sandpipers, at least in terms of prolactin levels.
A dad spotted sandpiper spends some quality time with his adorable kids. The reason for their “butt-bobbing” behavior is currently unknown, but may have to do with distracting predators (or might just be a nervous tic). [Source]
Overall, spotted sandpipers (and all the other strange animals that defy our idea of normal sexual and reproductive behavior) exemplify the need to study a diversity of species. Through these strange “edge cases”, we can learn so much about the wide versatility of animal behavior and physiological traits across the animal kingdom!
Fivizzani, A. J. and L. W. Oring. 1986. Plasma steroid hormones in relation to behavioral sex role reversal in the Spotted Sandpiper, Actitis macularia. Biol. Reprod. no. 35:1195-1201.
Oring, L. W., A. J. Fivizzani and M. E. El Halawani. 1986. Changes in plasma prolactin associated with laying and hatch in the Spotted Sandpiper. Auk no. 103:820-822
Reed, J. M., L. W. Oring, and E. M. Gray. 2013. Spotted Sandpiper (Actitis macularius), version 2.0. In The Birds of North America (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.289
Rosvall, K. A. 2008. Sexual selection on aggressiveness in females: evidence from an experimental test with tree swallows. Anim. Behav. 75, 1603—1610