According to World Wildlife Fund, the bond between an orangutan mother and her young is one of the strongest in nature. Moms stay with their offspring for six to seven years, and female orangutans are known to “visit” their mothers until they reach the age of 15 or 16.
When you think of your mother, a warm memory probably pops up of her being there when you needed a hug, lending an ear when you wanted to talk, or standing up for you when you required a hero. This Sunday, when we celebrate Mother’s Day, I hope you’ll think of your mother fondly and all the other mothers you’ve known throughout your life, with all their wonderfully diverse parenting styles.
But human mothers aren’t the only ones I hope you’ll honor this weekend. The more-than-human ones out there have cared for their young, too, with a similar array of unique approaches to parenthood.
For example, turtle mothers seem to have a penchant for their daughters, while orcas demonstrate a fondness for their sons. And hyena mothers have the important job of handing down social status to both their female and male offspring—often not a promising prospect. Here’s a brief look at some of these different spins on parenting—and the bonds the “other mothers” form with their young.
A red-eared slider turtle is a medium-sized, aquatic turtle with patches of red on each side of its head. The carapace (upper shell) is olive brown with numerous black and yellow lines. The turtles live in a wide variety of aquatic habitats, but prefer mud bottoms, plenty of aquatic plants and abundant basking sites.
In high temperatures, turtle mothers produce daughters
Warmer temperatures are known to make more turtle eggs become female hatchlings, but new research out of North Carolina’s Duke University shows that those females also have a higher capacity for egg production, even before their sex is set.
Researchers have found, as published on June 23, 2023, in the journal Current Biology, that the number of “germ cells” (or reproductive cells that eventually become eggs and sperm) that an embryo carries is increased by higher incubation temperatures. In most cases, a higher number of germ cells results in the embryo committing to a female fate. So, the hotter temperatures that produce females are also the temperatures that increase germ cell numbers. That means that higher temperatures seem to affect sex determination in incremental ways through multiple cell types in the embryo.
Higher numbers of germ cells are known to control female development in fish. But to prove the point that more germ cells lead to female turtles, the Duke University scientists removed some germ cells from red-eared slider turtle embryos raised at an intermediate temperature that should have yielded 50-50 proportions. Surprisingly, they saw more males.
Red-eared slider turtles reside in both natural waters (including oxbow lakes, rivers and sloughs) and human-made waters (such as ditches, ponds and reservoirs).
Scientists have known about temperature-dependent sex development for decades and have found it in many different species, apparently because it evolved multiple times in multiple ways. But it seems like a risky strategy, especially in the context of weather variations and climate change. So, why would this system persist?
The scientists think it’s because temperature-dependent sex development creates a reproductive advantage. A female that hatches with more germ cells is presumably more reproductively fit; it increases her reproductive potential to carry more eggs.
As global temperatures continue to rise, the question becomes: what will happen to the turtles and other temperature-sensitive breeders?
A new scientific report provides a troubling glimpse of what could lie ahead for all turtles in a warming world.
To answer this question, the researchers carefully nurtured clutches of red-eared slider turtle eggs and kept them at a constant temperature in the lab. One incubator ran at 78.8 degrees Fahrenheit, producing more males. Another was at 87.8 degrees Fahrenheit, the optimum temperature for producing more females. The embryos that were incubated warmer were markedly larger and more active inside the egg.
The scientists hypothesize that there’s a temperature sweet spot. There is a short range where you get a large number of germ cells, but beyond that you start to see declines. Some eggs were incubated at 92.3 degrees Fahrenheit, only four-and-a-half degrees higher than the optimal temperature for females. Unfortunately, it created some bizarre results, state the researchers, such as cyclopes and two-headed embryos.
The lab plans to continue the temperature experiments with alligator eggs. Alligators are known to produce females at low temperatures and males at high temperatures, the opposite pattern from the red-eared slider turtles. However, the low temperature in alligators is the same as the high temperature in turtles, so both species produce females at 87.8 degrees Fahrenheit. What will be interesting, say the researchers, is whether they’ll see more germ cells in both species at that temperature.
Hyenas live in societies which bear a striking resemblance to human monarchies.
In inherited monarchies, hyena mothers determine social status
In some human monarchies, inherited power offers a ladder that can be ascended to absolute authority, whether through diplomacy, the passing of time or ruthlessness. But in hyena monarchies, inherited power is always a slippery, downward slide.
Recently, researchers from Germany’s Max Planck Institute of Animal Behavior (MPI-AB) examined the workings of hyena societies over three decades. They found that the process by which hyenas inherit rank from their mothers—known as “maternal inheritance”—corrodes the social status of individuals. According to the study, published in Philosophical Transactions of the Royal Society B in June 2023, every member of a hyena clan, except the highest-ranking queen, suffers downward mobility across his or her lifetime.
This paints a pretty bleak picture of hyena societies, which bear a striking resemblance to human monarchies. Hyena clans are arranged in a linear dominance hierarchy, and offspring inherit their rank below their mother through a monarchy-like process. At the top of the hierarchy is the highest-ranked female—the queen—followed by her young, and then all other females with their young. But no matter what position you inherit, the only way is down.
In a hyena’s world, demographics rather than status-seeking behaviors account for most hierarchy dynamics.
A hyena’s place in the hierarchy matters a lot. A lower rank means you have less access to food, you must travel more to hunt, you are harassed more, and you even have less time to nurse your babies. That caused the MPI-AB scientists to ask: can hyenas ever change their status? Or is the quality of a hyena’s life predestined from birth?
To find out, a database from the Mara Hyena Project, which has been studying spotted hyenas in the Maasai Mara National Reserve in Kenya since the late 1980s, was tapped. Analyzing 30 years of data on hyena behavior from four social groups, the scientists discovered that hyenas could, indeed, move up and down in the hierarchy over time, but they slid down much more often than they jumped up.
You wouldn’t think this downward mobility was happening if you were just observing the animals in the wild, say the researchers, because the process happens over many years. It’s only by taking an intergenerational view that you realize that a daughter born to the alpha queen suffers a significant downturn in status throughout the course of her life. But what causes the persistent downward trajectory?
The Maasai Mara National Reserve, located in southwestern Kenya, is known for its bountiful wildlife and breathtaking vistas. The Mara Hyena Project has been studying hyenas here since the late 1980s.
By digging into the life histories of all the individual hyenas, the scientists found that the animals descended in rank most often because another hyena had joined or left the group; in other words, through simple demographic turnover. An individual’s power is passively eroded as other clan members are born or die.
Drawing on methods used to study social mobility in human societies, computer-simulated hyena societies were created, where various aspects of their biology could be turned off. This allowed the scientists to pinpoint the specific societal rules that were driving this unusual pattern.
The simulation pointed to two sources: the monarchy-like inheritance of hyena societies and the fact that higher-ranking females also give birth to more offspring. These combined traits mean that new group members are not being added randomly but constantly being added to the top of the hierarchy, under the dominant females, which pushes all other individuals down over time. Only the queen escapes this fate of slow decline in status because it’s not possible for anyone to inherit a position above her.
No matter what hyenas do, they’re going to experience a decline in societal rank over their lifetimes. Somehow, though, they manage to thrive and survive.
The MPI-AB work demonstrates how societal features can have an outsize influence on individuals, sometimes usurping an animal’s own agency in altering the course of its life. No matter what a hyena does, he or she is going to experience a decline in rank over time. It’s hard to fathom that they navigate lives in which everybody but the queen suffers a loss in quality. Clearly, they do, though, so the next question to investigate is how.
In Washington state, orca mothers forgo future offspring for their sons
It’s not unusual for parents—especially mothers—to sacrifice their own future success for the sake of their offspring. But a new study published in the journal Current Biology on February 8, 2023, shows that orca mothers take this to a surprising extreme. They surrender their own reproductive success to care for their sons, even after those sons are full-fledged adults.
While it’s been known for more than a decade that adult male orcas rely on their mothers to keep them alive, it had never been clear whether mothers pay a cost to do so. Now, we know they do. Researchers studied a group of orcas known as the “Southern Resident population” in the coastal waters of Washington state and British Columbia, which has been monitored since 1976 by the Center for Whale Research. They wanted to learn whether the care adult whales, and especially males, receive from their mothers came at a measurable cost.
It’s not unusual for parents and especially mothers to put their young first, but orca mothers take that to an extreme level. They sacrifice their own reproductive success to care for their sons, even after those sons are full-fledged adults.
These orcas live in matriarchal social units that consist of a mother, her offspring and the offspring of her daughters. Although male orcas will outbreed with whales from other pods, both males and females stay in their unit of birth, with their mother, for life.
The availability of detailed demographic data allowed the researchers to look directly at how caring for daughters and sons impacted females’ chances of further reproduction. A strong negative correlation between females’ number of surviving, weaned sons and their annual probability of producing a viable calf was found. Those costs didn’t get any smaller as their sons grew older, either.
The costs couldn’t be explained by lactation or group composition effects, which supports the hypothesis that caring for sons into adulthood is reproductively expensive. These findings, say the researchers, offer the first direct evidence for lifetime maternal investment in any animal, revealing a previously unrecognized strategy.
The orcas that live off the U.S. Pacific Coast—the Southern Residents—are critically endangered. They organize their societies along matrilineal lines in three distinct pods (J, K and L), each with its own dialect.
The scientists further state that the magnitude of the cost that females take on to care for their weaned sons was startling. The best estimate is that each additional surviving son cuts a female’s chances of having a new calf each year by more than 50%, suggesting that there are significant advantages to keeping adult sons alive and well. It’s thought that females gain evolutionary benefits when their sons can successfully reproduce—enough to outweigh the high price they pay.
The new findings may also have important conservation implications. The Southern Residents are critically endangered, with one major concern being their low reproductive rates. This study reveals not only the importance of animals’ social systems in determining demographic patterns but how special (and maybe unique) the mother-son bond in orcas is. This is of central importance both for an understanding of our world and to conserve endangered species more effectively.
In future work, the researchers hope to learn more about the nature of the costs to mother whales. They suspect mothers may not eat enough food themselves as they continue sharing with their full-grown sons. The Southern Resident orcas are very food-stressed; and, as such, a primary conservation goal for the whales is to recover the population of Chinook salmon they rely on
A primary conservation goal for the Southern Resident orcas is to recover the population of Chinook salmon that they rely on.
In whale conflicts, orca mothers protect sons from injury
Female orcas live up to 90 years in the wild, and most live an average of 22 years after menopause. Scientists have long wondered why humans and some whale species spend a significant portion of their lives not reproducing. Previous studies show that, even after having their last calf, orca mothers take care of their families by sharing the fish they catch.
Now, in a study published on July 20, 2023, in the journal Current Biology, researchers note that these mothers can also provide social support to their sons by protecting them from being injured by other orcas. The motivation for the project was to try and understand how these postreproductive females are helping their offspring. The results highlight a new pathway by which menopause is adaptive in orcas.
As in the previous Current Biology study, the research team studied Southern Resident orcas. Using data from the Center for Whale Research’s annual photographic census of the orca population, the researchers looked for evidence of scarring on each catalogued whale’s skin. Orcas have no natural predators other than humans, so any tooth-mark punctures are most likely inflicted by other orcas.
Older female orcas might be acting as mediators for their sons, preventing conflicts from occurring. To explore this further, researchers plan on observing more whale behavior by using drones.
The study found that, if a given male’s mother was still alive and no longer reproducing, that male would have fewer tooth marks than his motherless peers or his peers with a mother who was still reproducing. It was striking to see how directed the social support was. If an orca has a postreproductive mother who’s not his mother within his social group, there’s no benefit. So, the females are not performing a general policing role; they’re giving targeted support to their sons.
Researchers still can’t say for certain what kinds of social conflicts are leading to tooth marks or how older females are protecting their sons against them. They do note that postmenopausal females have the lowest incidence of tooth marks in the entire social unit, suggesting that they do not physically intervene in a conflict.
The researchers postulate that with age comes advanced social knowledge. Over time, these whale mothers might have gained a better understanding of other social groups and developed the skills to act as mediators. Given the close mother-son associations, it could be that she is present in potential conflict situations and can signal to her sons to avoid the risky behavior that they might be considering. To explore this further, the researchers plan on conducting an additional study using drones to observe whale behavior from above.
Attentive polar bear mothers usually give birth to twin cubs that stick by them for about two years to learn the necessary survival skills in their cold climate. As we celebrate our human mothers this Sunday, let’s give thanks for the rest of the mothers in the animal kingdom, too.
In our hearts, mothers make their mark
Few, I think, would argue with the fact that mothers take extraordinary steps to protect, nurture and raise their young. At different times in our lives, they act as our best friends, our most trusted confidants and our staunchest supporters.
And sometimes, let us remember, mothers have feathers or fur or scales.
Here’s to finding your true places and natural habitats,
Candy
