Hot Boys, Cool Girls, and the Fate of a Living Fossil

Imagine if the sex of your unborn offspring was determined by the climate you lived in while pregnant. Vacation in Cabo? Guaranteed son. Visiting the Northern lights? You’re having a daughter. This method of sex determination would play havoc with human sex ratios: countries like India and China, already on the verge of sex-ratio breakdown would become even more male-dominated, while Canada and the Scandinavian countries would swing towards a female majority (and given the general state of the world when men have been in charge, that’s a fairly appealing thought).

But sadly that’s not how sex determination works in humans — instead it’s a 50-50 chance based on whether the sperm fastest to the egg carries a male chromosome or a female chromosome. However, it is how sex is determined in lizards and crocodiles,[1] which might prove to be a bit of a problem as the worlds climate changes for the warmer.

When mating season comes, and reptile hormones are all in a tizzy, males donate a packet of sperm to a female, which is stored in her cloaca (how romantic). She then uses this sperm to fertilize eggs, and bury them in a nest for incubation. Lizard moms manipulate the sex ratio of their offspring by choosing where to build a nest: if they want more females, they will build a deeper nest in cooler areas — if they want sons, they build a shallow nest in warm habitats.

A doting mom.

A doting Nile crocodile mom.

Why exactly this method of sex determination has evolved is up for debate. Some research indicates that it may be the ancestral state for all amniote vertebrates (animals which lay eggs on land), dating back around 300 million years. It may continue to exist in lizards, crocodiles, and turtles because it is adaptively neutral — that is, it doesn’t necessary convey a great evolutionary advantage, but it also isn’t disadvantageous.

Other scientists argue that temperature-dependent sex determination (TSD) ensures that regardless of the climate or seasonal conditions, the sex best able to cope will hatch. For example, the spotted skink in Tasmania uses TSD: cool incubation temperatures lead to male offspring, and warm incubation temperatures lead to female offspring. In order for newborn female skinks to survive winter, they need ample amounts of time to grow during the summer. Having a brood of female skinks late in the breeding season is a bad idea: they won’t have time to grow, and will likely die over winter, meaning a wasted breeding season for mum. However, because of TSD, this doesn’t happen – females hatch only early in the summer, when temperatures are warm. As the average temperature cools down in mid- and late-summer, any egg laid hatch as males. No matter what time a clutch of eggs is laid, TSD makes sure that the sex that appears is the one best able to survive.

The Tasman spotted skink. Credit: Parks and Wildlife, Tasmania

The Tasman spotted skink. Credit: Parks and Wildlife, Tasmania

Unfortunately, this can backfire if the climate moves out of the ranges in which that behaviour has evolved to be adaptive (the evolutionary trap that also affects sea turtle behaviour — a behaviour that was previously beneficial becomes negative in light of recent, rapid changes). That’s the possible fate facing the tuatara.

The tuatara is a New Zealand reptile that looks like a lizard, but isn’t. Instead, it is the only living member of an ancient order of reptiles, the Rhynochocephalia, which reached its peak 200 million years ago. Anatomically, they are the most unspecialized amniote, and researchers think they may be good models for understanding the behaviour of dinosaurs. They don’t reach sexual maturity until they’re 20, and can live until well over 100. For millennia they were widespread across New Zealand, until the introduction of rats and cats as invasive predators led to a dramatic decline in their numbers, and eventually extirpation from the main islands. Currently, the tuatara survives in relic populations on the small islands of New Zealand which have never been colonized by predatory mammals.

Looks like a lizard...but isn't.

Looks like a lizard…but isn’t.

But even if it can survive the rats, cats, and minuscule gene pool, climate change might get it. Tuatara sex is determined by temperature — warm temperatures lead to males, cool temperatures to females. Like in other TSD species, to some degree the effect of air temperature can be mitigated by changing nest depth. Digging a deeper nest can, in theory, counter-balance increased solar radiation or air temperature to maintain a balanced sex ratio. Unfortunately, the islands of New Zealand that the tuatara inhabit don’t have a soil base deep enough to allow that sort of digging (plus, tuatara arms are pretty stubby, they’d be hard-pressed to dig a deep nest).

Which means that the tuatara might be in trouble. Researchers predict that, if global climate change proceeds according to schedule, by 2080 tuatara’s will be laying nests consisting entirely of male eggs. This might be great for a fantasy football league, but isn’t quite so good when it comes to species survival.

A tuatara, disturbed by the possibility of living only with other males.

A tuatara, disturbed by the possibility of living only with other males.

Active intervention by humans might help. Tuatara’s can be translocated to islands with cooler climates. Or, as has been done with sea turtles, volunteers can move nests — reburying them in shadier locations, or at lower depths. But without that help, one of the last living fossils could very well go extinct.

Sources

Mitchell, N. J, M. R Kearney, N. J Nelson, and W. P Porter. “Predicting the Fate of a Living Fossil: How Will Global Warming Affect Sex Determination and Hatching Phenology in Tuatara?” Proceedings of the Royal Society B: Biological Sciences 275, no. 1648 (October 7, 2008): 2185–2193. doi:10.1098/rspb.2008.0438.

Refsnider, J. M., B. L. Bodensteiner, J. L. Reneker, and F. J. Janzen. “Nest Depth May Not Compensate for Sex Ratio Skews Caused by Climate Change in Turtles: Nest Depth and Turtle Sex Ratios.” Animal Conservation 16, no. 5 (October 2013): 481–490. doi:10.1111/acv.12034.


[1] Also turtles, but they’ve got it flipped the other way: high temperatures lead to females, and low temperatures to males. Just to confuse biologists even further, some species have found a third way. Temperature extremes (high or low) lead to female dominated nests, while mid-range temperatures lead to male dominated nests.

A brown anole, auditioning for a role in the Terminator movies

Brown Anoles and the Dead Man’s Hand

On a sunny August 2nd, 1876, the Union scout, lawman, and gunfighter Wild Bill Hickok was shot dead in a tavern in the town of Deadwood, in the Dakota Territory. He had followed the gold rush to the Black Hills of Dakota, hoping to strike it rich. But he spent most of his time drinking and playing poker in the frontier town’s saloons. On that day, he arrived late to the bar, and the only available seat at the poker table left his back to the door. On another day, he might have left, but this day, he chose to stay. That turned out to be a poor choice.

Wild Bill Hickok, looking dapper.

Wild Bill Hickok, looking dapper.

Hickok was a talented poker player. After a degenerative eye disease robbed him of his marksmanship, he relied on cards to make a living. He brought to the poker table all of the focus and cunning he had previously used as a scout and lawman. He could read people easily, and few men could bluff him for long. But all that attention came with a cost – his focus narrowed on the poker table, on watching the men with him, he failed to notice the door open behind him.

The last words he heard were “Damn you! Take that!”, before a bullet entered the back of his head and exited through his cheek. The buffalo hunter Jack McCall, enraged at being embarrassed by Hickok in a previous poker game, had sneaked up on the distracted Hickok and murdered him. On the floor lay the last hand Hickok was ever dealt: a pair of aces and a pair of eights, all black – since then known as the dead man’s hand.

The dead man's hand.

The dead man’s hand.

Animals live in groups for many reasons: to control territory or access to resources, to form alliances and, of course, they live in groups because that makes it easier to find a mate. But living in groups comes with costs – including the possibility that all of those wonderful friends surrounding you are making it more likely that you’re going to be eaten by a predator.

Like humans, animals have a limited capacity for attention. They can only pay attention to so many things at one time – trying to do too many things at once causes everything to suffer. (Study-after-study shows that multi-tasking, the Millenial’s favourite pastime, just results in doing five things poorly). For solitary animals, that means their attention is divided between two things: finding food, and avoiding predators. That’s pretty straightforward.

multitasking

But social animals have to divide their attention among more activities. They have to find food, look around for predators, look around to see if other individuals have located predators, and also pay attention to their needy friends that require constant attention. Heaven-forbid you’re a mother with babies to take care of too. A study published last week in the journal Ethology experimentally considered the cost of companions.

Jennifer Yee and her colleagues studied the brown anole, a ubiquitous lizard found in the Caribbean. Like all small critters, the brown anole is a favourite meal of just about everything bigger than it – but particularly snakes. Anoles rely on their keen eyesight to avoid becoming a snack – if the bushes rustle in a suspicious way they dart to safety. Yee tested the reaction time of anoles by harassing them with a rubber snake. She moved the snake to within a metre of an anole, and then recorded how much time it took for the anoles to notice. Solitary anoles noticed the snake quickly, and ran away. Good for them.

A brown anole, auditioning for a role in the Terminator movies

A brown anole, auditioning for a role in the Terminator movies. Credit: Neil Losin

 

But, anoles live at a fairly high population density, and are regularly distracted by other anoles. They may just be passing through, or they may be angling to steal your territory, or your mates, or your food. Regardless, they bear close watching. So the anoles all keep a  tense eye on one another – the same way Hickok might’ve been keeping a close eye on his fellow poker players. That sort of attention comes at a cost. Yee found that when anoles were distracted by the presence of other individuals, it took them twice as long to notice the presence of a predator – and the predator could be moved closer before the anole found it.

A green anole doing a poor job of paying attention.

A green anole doing a poor job of paying attention. Credit: USGS

That’s bad news for pre-occupied anoles. It was also bad news for Wild Bill Hickok. Distracted by the other poker players, he didn’t notice Jack McCall arriving at the bar behind him. Next time you’re at the bar, take the seat with your back to the wall.

Neil Griffin

Literature Cited

Yee J, et al. 2013. The Costs of Conspecifics: Are Social Distractions or Environmental Distractions More Salient? Ethology 119: 480-488.

Turner, T. 2001. Wild Bill Hickok: Deadwood City – End of Trail. Universal Publishers.