If you’ve missed me, it’s because I’ve been superbly busy with this summer’s field season in Alabama! It’s been a boom season: we caught females in April/May, they laid fast and furious at Penn State over June, and since the beginning of July I’ve been back at the Solon Dixon Forestry Education Center hatching out the babies, and putting them into experimental enclosures. I will write a full report at the end of the season (currently coming to a close), but for now, a reminder that you can read more regular updates using the #ALlizards2017 hashtag or via my profile on Twitter. You can also check out pictures from the field on my Instagram: click the picture for more!
In case you haven’t noticed, it’s been an unusually mild winter in most parts of the United States. How mild? Well, for the first time in 146 years, Chicago had no snow in January and February (38 years longer than they went without a World Series win, not that anyone was counting) – and in Miami, temperatures never once dropped below 50˚F, a first in recorded history (and they’ve been keeping records there for 121 years). Although the north west saw record cold, far more weather stations (8% in fact) recorded the HOTTEST winter on record.
To put this in perspective, the record for “warmest winter” in the Lower 48 states was held by – you guessed it, the winter immediately before this one (2015/16). And this is part of an upward trend in winter temperatures, not just a 2-year fluke:
Looking at the top graphic, the south east region of the US (where I conduct my research on eastern fence lizards) was particularly warm this winter. This area is an important hotspot for reptile and amphibian diversity, and as I’ve been working on reptiles in particular, these interesting (and worrying) trends have had me thinking about how warm winters might affect reptiles.
Reptiles are ectotherms, meaning they depend on external sources of body heat, and have limited ability to physiologically control their own body temperatures. Maintaining an optimal body temperature is therefore very environmentally-dependent in these species. This is thought to make them particularly vulnerable to the effects of climate change.
Elegant work done by Barry Sinervo and colleagues (published in Science) suggests that increased global temperatures will lead to a reduction in reptilian diversity because as temperatures increase, reptiles are limited by their upper temperature limits. Every reptile (and animal generally) has a “critical thermal maximum/minimum” – these are the upper and lower temperatures beyond which physiological processes will cease and they will die. So, if it’s hotter, reptiles can spend less time being active and foraging, and have to spend more time resting in the shade to avoid reaching critical thermal max. This will have effects on their body condition, ability to reproduce, and ultimately, on population growth, leading to increased likelihood of extinction.
So it’s pretty obvious how warmer SUMMERS could impact reptiles, by pushing them more frequently to their critical thermal maximum, and forcing them to adjust their behaviours to avoid this. But, what about warmer WINTERS?
My hunch was that warmer winters probably interfere with regular over-wintering behaviours in reptiles. But when I started to dig into this topic I found that what a “regular” over-wintering behaviour is in a reptile is highly variable, even within species. Some species, like rattlesnakes (see pic), go into hibernation much like a bear or a small mammal – though when they do this, and for how long, is highly dependent on their latitude. To generalise, reptiles at least “power down” for the winter months when food is relatively scarce: they lower their metabolism to conserve energy, and may burrow underground or move into crevices, which is why they’re usually hard to spot during this period.
I work on the eastern fence lizard, Sceloporus undulatus, which is a widely-distributed species, occuring from Mississippi to southern Pennsylvania. We don’t know much about over-wintering behaviour, but we do know that they come out when it’s warm, even if that’s in the middle of winter:
Although they are likely to display variation in over-wintering behaviour given their huge latitudinal range, seeing them out and about this early in central Arkansas is definitely unusual.
But – should we be worried to see little Scelops out in late January or February? Are warm days at this time of year going to do them any harm? Temperature increases during the winter are surely likely to fall well between critical thermal min & max, and if anything, should result in temperatures that are closer to each species optimum (the smaller range of temperatures that ectotherms will seek to achieve, at which physiological processes, such as digestion, are optimised). So, do warmer winters matter?
The short answer is yes, it probably does, and it’s probably not great news for reptiles like the fence lizard.
There are a couple of reasons why. First, becoming active in winter requires upregulation of the animal’s metabolic rate, which has been lowered for the winter period. Our metabolism, amongst other things, is the suite of processes that convert food/fuel to usable energy to allow us to be active. Metabolic rate is highly sensitive to temperature in reptiles, so as it gets warmer, they start burning through their energy stores. Remember, there’s a reason that reptiles go into a state of reduced metabolism during winter: there’s less food around! Especially for those that rely on hibernating mammals, but also those that need to eat lots of insects that are less plentiful when it’s cooler, like lizards. So, becoming active and increasing your metabolism is probably a bad thing when it’s going to burn up your stored energy, and there’s limited potential for you to recoup it by finding lots of food.
A number of studies have found that milder over-wintering temperatures result in decreased body condition come Spring-time, and higher levels of physiological stress, as a result of this increase in metabolism. For example, painted turtles that experience just a 5˚C increase in winter temperatures use almost twice as much energy (Muir et al. 2013); and aspic vipers had reduced Spring body condition after being kept at 14˚C instead of 6˚C (Brischoux et al. 2016).
There’s another reason that “milder” temperatures are likely to be bad news for reptiles (and other animals too): they may be high enough to promote activity, but still low enough to result in impaired digestion, performance etc. Remember the optimal range of temperatures that maximise physiological performance? For Sceloporus lizards, that range centres around 30˚C. So, they have to reach this temperature, or close to it, to be able to digest food properly. In other words – mild temperatures of, say, 16-22˚C may be enough to fire up the engine, but not get it working efficiently, which can lead to problems like this:
@kirstyjean@AlongsideWild Winter heat spikes cause issues for alligators- resume feeding, gets cold again, temps 2 low to digest food->die
As well as digestion, other aspects of physiological “performance” may suffer at low-mild temperatures, such as sprint speed. As you can see in this figure, endurance (minutes on a treadmill) and especially sprint speed (centimetres moved per second) in Sceloporus lizards (both things that are likely to be important in escaping a predator, or trying to catch a food item) are maximised above 26˚C, and reduced below 20˚C.
So, to sum up my non-expert forays into thermal biology of reptiles and answer my initial question: yes, increased temperatures during the winter period most likely do matter. To summarise:
mild temperatures are likely to bring reptiles out of torpor
this necessitates/is accompanied by an increase in metabolic rate, which uses stored energy
furthermore, food availability is likely to be lower at this time
and their digestion/other physiological processes are impaired at these temperatures
with the result that they are in lower body condition when the Spring finally rolls around.I’ve really enjoyed diving into the thermal ecology literature out of interest to know how the warm winter we’ve been experiencing will affect my study species. But this is outside my realm of expertise! So, if you’re interested in this topic, here are some great Twitter accounts to follow for people who know far more about this than me:
Muir et al. 2013. Journal of Thermal Biology. Energy use and management of energy reserves in hatchling turtles (Chrysemys picta) exposed to variable winter conditions. http://www.sciencedirect.com/science/article/pii/S0306456513000557
This week my recent field work and research in Alabama is featuring on our lab website, The Lizard Log! Text and photos below: check out the post on the lab site here.
When you tell people you are going to be studying lizards in Alabama for the summer, you get used to a raised eyebrow or two. The heat! The snakes! The bugs! When I told people I was going to be deliberately seeking out (ecological) public enemy number 1, fire ants, everyone made clear what I already knew – the theme of my summer was going to be STRESS.
In fact, stress (and in particular, maternal stress during gestation) is exactly what took me to Alabama. Stress during pregnancy can alter the characteristics of the resulting offspring, from morphology to behaviour. That’s assumed to be a bad thing. But could stress experienced by mothers during gestation actually program offspring for life in a stressful environment, giving them an advantage in the long run? This was the question I set out to test during my first, recently completed, field season.
Our next step was bringing the females into the lab, and subjecting them to a highly controlled “stress” treatment – a very low dose of a stress hormone every day, the equivalent of a single fire ant sting. It’s important to note that this is NOT a pain treatment – we use the hormone corticosterone, which is released as part of a lizard’s natural stress response, and which has a number of downstream effects including helping the body’s metabolic system turn amino acids into carbohydrates for use as fuel. In short, our treatment was tricking the lizards’ system (but not the lizard) into thinking they were in a stressful situation.
The next step was waiting for the females to lay their eggs, at which point our stress treatment ceased, and females were ready to be returned to the wild. We incubated their eggs (incubation takes around 50 days) and waited for the babies to hatch to begin the next step of our experiment…
We hypothesised that if maternal stress was adaptively programming offspring to be be better suited to a stressful environment (for example, by making them more responsive to predators, or better able to cope with frequent stressors), then we should see offspring from stressed mothers surviving better in stressful environments than offspring from mothers that did not experience stress during gestation.
To test this, we needed to create “stressful” and “non-stressful” environments in which to put the offspring. When we weren’t catching females or incubating eggs, we were building four 20x20m outdoor enclosures for this purpose! Thankfully, life in Alabama with fire ants everywhere is stressful enough, so we didn’t need to artificially create a stressful environment. To create a “non-stressful” environment, we removed fire ant mounds from two of the four enclosures. We hypothesised that in these enclosures, offspring from unstressed mothers should do best.
Once the offspring hatched, we put them into the enclosures and monitored their survival by checking them every day (not an easy task, they are small and wily!). I also recorded their habitat use, how far they were moving within the enclosures from their release spots, and how they responded to small, short-term stressors, like being picked up to be weighed. I’m now in the process of analysing this data, and am looking forward to seeing if our hypotheses hold true. Watch this space!
So, lizards aside, how did I cope with the stresses of a summer in Alabama? The heat – loved it! The snakes – try and keep me away from them! The bugs – who cares?! The people – a whole lot of new friends. There may be a million ways to die in the South, but there sure are a million and one things to love.
It’s been a bumper summer for hihi research! I had two papers come out in the last couple of months, in which I explored the effects of a carotenoid supplementation experiment on a couple of different aspects of hihi breeding biology. Carotenoids are antioxidant pigments that birds must obtain through their diet. They are important in animal colouration, and are thought to play a role in immune health too (1, 2). The Tiritiri Matangi hihi population was supplemented with carotenoids supplied to the birds in sugar water feeders for two breeding seasons (2005-2007) in order to test whether mothers and/or offspring having access to more of these nutrients influenced their eggs, health, and provisioning behaviour. I was interested to follow up on these results by testing how carotenoid supplementation influenced:
and offspring sex ratio
First, what are brood hierarchies?
A brood hierarchy refers to size differences between chicks in a nest (brood = chicks in a nest). Brood hierarchies can arise if eggs differed in size or contents, but most often are thought to arise due to differences in when the eggs hatched, with eggs hatching first resulting in chicks that get a headstart and end up bigger than later-hatched siblings. This usually means they are more competitive, for example, being more successful in obtaining food from their parents.
Mothers can control how long their clutch (clutch = eggs in a nest) takes to hatch by adjusting when they begin sitting on their eggs to begin incubation. Birds lay one egg per day. If they begin incubation as soon as the first egg is laid, as below, the first egg will have a head start over those laid later as it will receive more incubation time and so will develop faster. This will usually mean that the first egg will hatch sooner, resulting in a brood hierachy:
If females wait to begin incubating their clutch until after the last egg is laid, as shown here, then all the eggs will develop at the same rate, and are likely to hatch at the same time:
(Another way to think about this – imagine you are boiling 3 eggs. If you put them all into boiling water at the same time, they’ll all be ready at the same time. If you put them in sequentially, a minute apart, the one you put in first will be hard-boiled before the others!)
Why are brood hierarchies important, and what can we do about them?
As strange as it seems, given that generating size differences within a brood is likely to result in later-hatched chicks being less competitive and perhaps less likely to survive, brood hierarchies are thought to be evolutionarily adaptive. For example, in uncertain environments, encouraging sibling competition within the nest may mean that at least the older, more competitive chicks are likely to survive and thrive – as opposed to having a nest of equally poor quality chicks that may all die. In a species of critical conservation concern like the hihi, however, brood hierarchies are not what we want to see, because every chick is vitally important!
I was interested to see if supplementing mothers with carotenoids while they were laying their eggs might mitigate the negative effects of hatching later. Later hatched chicks are often smaller, lighter, and less likely to survive – perhaps increased maternal access to carotenoids, a nutrient we know boosts chick health (3), might lessen some of these effects?
In our recent paper in Animal Behaviour, we show that carotenoid supplementation had no influence on the effects of brood hierarchy position in hihi. We did find strong hatch order effects – later hatched chicks were smaller, lighter, and less likely to survive as predicted (see figure below). This was an interesting result, as we found that the overall degree of hatch spread was very low relative to other species – usually all eggs hatched within 24 hours of each other, meaning that hihi are technically a “synchronously” hatching species! Perhaps we have previously underestimated hatch order effects in species where chicks all hatch relatively synchronously?
Carotenoids and offspring sex ratios – different benefits to males and females
Hihi are sexually dimorphic – males have distinctive yellow wing bars and white “eyebrow tufts”, both of which play a role in mate attraction and territory acquisition (4). Previous work has shown that dietary carotenoids are important in male yellow plumage (5), the colour of which influences the likelihood of being cuckolded (4). In other words – the availability of carotenoids in a male hihi’s early life can have big effects on his reproductive success as an adult. So, if mothers have a lot of carotenoids in their diet, in theory they should benefit most from producing lots of sons that will go on to have many offspring and not be cuckolded. So, does this actually happen?
Unfortunately, it’s hard to accurately test this question in many systems because often chicks die in the nest before researchers can identify their sexes, so commonly we know the sex ratio of a brood when they are ready to leave the nest, but not the sex ratio that was actually laid. Luckily, I was able to use tissue samples from chicks that died in the nest to obtain full sex ratios for each brood. You can read more about this lab work and what it entailed here!
We found (published recently in Behavioral Ecology) that despite our predictions that, because males apparently benefit more from having more carotenoids in early life in terms of their long-term reproductive success, supplementing mothers with carotenoids did not alter offspring sex ratios (see graph below). Although this did not match our hypothesis, this is an important result – carotenoids have been shown to influence offspring sex ratios in other species (e.g. 6), and it’s equally important to report where this result is not replicated so we can better understand the role of carotenoids.
Why might this be? A few possibilities: perhaps carotenoids are actually also important to female long-term success in ways that we don’t know about yet, meaning that it’s less beneficial for supplemented mothers to invest more in males. For example, if the yellow colouration of their beaks is important in competition with other females, as is the case in goldfinches (7). It’s also possible that our carotenoid supplementation wasn’t a sufficient cue of carotenoid availability in the environment – not enough to have the big effects on male colouration that would result in important differences. Plenty to think about for the future!
Thanks to hihi collaborators!
On a personal note, it’s wonderful to see these hihi papers out, as they were great fun to work on, and the result of a great collaboration between Cambridge and John and Patricia at ZSL. Our funders for these projects are listed below.
As always, please do let me know if you would like to read any of the papers I’ve talked about and you are unable to access them. You can find my email in my contact details tab above.
Blount JD. 2004. Carotenoids and life-history evolution in animals. Arch Biochem Biophys. 430:10–15.
Svensson PA, Wong BBM. 2011. Carotenoid-based signals in behavioural ecology: a review. Behaviour. 148:131–189.
Ewen JG, Thorogood R, Brekke P, Cassey P, Karadas F, Armstrong DP. 2009. Maternally invested carotenoids compensate costly ectoparasitism in the hihi. Proc Natl Acad Sci USA. 106:12798–12802.
Walker LK, Thorogood R, Karadas F, Raubenheimer D, Kilner RM, Ewen JG. 2014. Foraging for carotenoids: do colorful male hihi target carotenoid-rich foods in the wild? Behav Ecol. 25:1048–1057.
Walker LK, Stevens M, Karadaş F, Kilner RM, Ewen JG. 2013. A window on the past: male ornamental plumage reveals the quality of their early life environment. Proc Roy Soc Lond B Biol Sci. 280:20122852.
McGraw KJ, Adkins-Regan E, Parker RS. 2005. Maternally derived carotenoid pigments affect offspring survival, sex ratio, and sexual attractiveness in a colorful songbird. Die Naturwissenschaften. 92:375–380.
Murphy TG, Rosenthal MF, Montgomerie R, Tarvin K. 2009. Female American goldfinches use carotenoid-based bill coloration to signal status.Behav Ecol. 20:1348–1355.
I’ve recently returned to Penn State from a lengthy fieldwork stint in Alabama and a very productive summer. Lots of data were collected (more on this later), and two hihi papers came out! (You can check these out here and here – also more on these later). I’ll have lots to write about once I’ve settled back in to office and Pennsylvania life, but for now, a quick reminder that you can check out pictures from my fieldwork on my Instagram page! Sneak preview below…
I’ve just finished a manic week of curating the RealScientists excellent Twitter account (@realscientists). We talked meerkats, hihi, fairy wrens, and fence lizards – and also fitted in some excellent, supportive conversations about work-life balance, academic relationships, and science communication. And we decided that ‘Cake by the Ocean’ is the best field song of this year… obviously.
All my tweets from this week are archived here. Thanks for having me, RealScientists!