Horse Nations

Just over 10 million horses live in the USA and 6.8 million in China*. But New Zealand has only just over 56 thousand horses. You and a friend could count the horses in Malawi, Granada or Guam on your fingers and toes. Apparently, the island territories of Saint Helena, Ascension and Tristan da Cunha Islands have none – really. Big, more populous, nations have more horses, but that is not interesting. Which are the Horse Nations of the world?

Where is the horse most common because they more densely populate the landscape (horses per square kilometre) AND people are more likely to own them (horses per person)?

Horses per 100 people in the worlds nations. Statistics from http://www.fao.org for 2005 and map from http://kids.fao.org/glipha/

The Americas – north, central and south – are nations of horses. Southeast Asia and Africa are not. Central Asia might be a rival for the American crown.

Argentina, Brazil and the USA are mad with horses – places with more recent traditions of colonial expansion across vast frontiers and the cowboy ethos. Over half of the world’s horses (32 million or 54%) are American. There are 3.6 horses for every 100 Americans. The value does not sound high but it is when we consider how urbanised the Americas are. Around 82% of North and South Americans, and 72% of Central Americans, live in cities – mega-cities. Almost every American nation has at least a horse per square mile. In central America, in particular, you can’t move without being kicked or bitten by a horse. It has an extraordinary 14 horses per square kilometre of land (36 per square mile).

Horses per square kilometer in the countries of the world. Statistics from http://www.fao.org for 2005 and http://kids.fao.org/glipha/.

The five republics – the ‘stans’ – of central Asia are not far behind. There are 3.4 horses for every 100 people living in central Asia but they are spread thinly – about a horse every two square kilometres (1.3 per square mile). China and Mongolia have almost one horse (0.8) per km² (2 per square mile) but China is a populous nation and so there is just half a horse per 100 people.

Some other nations and regions of the world are dense with horses but they also are dense with people, like Europe. A smaller proportion of Europeans are horsing about. In other nations a larger proportion of people might be horsing about but they are also large countries with small populations of people and horses, like Namibia.

So which country is the world’s horse nation?

… take a guess.

* In text statistics from www.fao.org for 2011. Statistics used to construct maps from 2005 at http://kids.fao.org/glipha/.

“The Craptastic Conversations of the Black Rhinoceros”

Bradon Crocker, research assitant, viewing a dung-scrape mark by a male black rhinoceros.

My research in southern Africa, with colleagues from Germany and San Deigo, on black rhincoeros scent communication is profiled here in the The Scorpion and the Frog blogspot.

The author, Sarah Jane Alger, is a graduate student in Zoology at the University of Wisconsin – Madison, where she is researching how the male birds song and female mate preferences are regulated.

Sarah’s is a diverse and interesting blogspot. Other graduate students will likely identify with its mix of graduate student experience and research.

Linklater, W., Mayer, K., & Swaisgood, R. (2013). Chemical signals of age, sex and identity in black rhinoceros Animal Behaviour, 85 (3), 671-677.

Stallions together

It is clear from our art and literature that we romanticise the lives of wild horses. In ways that impose our human-centred ideals and prejudices – anthropomorphism – mares are imagined living in social groups that are protected and defended by a single handsome, intelligent and brave stallion. Visions of stallions standing on rocky outcrops or atop hillocks, overseeing their mares and offspring, vigilant for danger, and ready to fight are routine in children’s literature and persist into adulthood to say more about how we view ourselves and our own societies than about how horses live.

Real life, however, is seldom so quaint… … but it is more interesting.

The truth is that some mares live without the companionship of other mares. And some mares and mare-groups live and breed with more than one stallion. The breeding groups of wild horses, called bands, are reported to sometimes include just one mare. They may also include up to five stallions such that the stallions in a band sometimes outnumber the mares. In mine and colleagues’ work with feral horses of the Kaimanawa Mountains, New Zealand, we have studied bands with as few as one mare and up to four stallions.

Multi-stallion bands are not unusual either. They were reported in 15 of the 20 feral and wild horse populations described by the scientific literature [1] and comprise up to a third of bands in some populations [2]. Multi-stallion groups are also reported in zebras, although they appear less commonly [3].

It is clear that horses are polygynous breeders – one stallion can sire many offspring every breeding season but mares are limited to one offspring, and therefore sire, per season and year. Bands with just one stallion, therefore, would appear to ‘make sense’. Any additional stallions are apparently superfluous. It has long puzzled ecologists, therefore, why stallions would share a mare group.

It is especially puzzling that multiple stallions might live together in a band with just one mare. Clearly, it is not possible to share the paternity of one mare’s offspring in a season. Surely, the stallions in such bands would achieve better breeding success if they left to join bands with more mares or just lived as a bachelor, like so many other especially young and old stallions do, to sneak mating opportunities from the willing mares of many bands when their stallions are not vigilant?

Many hypotheses have been advanced for multi-stallion breeding groups. Most behavioural ecologists who have pondered the puzzle liked the idea that stallions are cooperating to breed. There are several hypotheses for cooperation. Some have applied terms like mutualism because each stallion’s breeding success might be improved when they work together to defend mares. However, mutualism is a poor concept for stallion relationships because the stallions that share bands are not equal partners – one dominates the other in all populations where the relationship has been studied. Mutualism is also a term, in modern usage, for positive relationships between species, not individuals of the same species.

Some described the relationship between stallions as reciprocal altruism whereby the dominant stallion tolerates subordinates in exchange for assistance in defending the mare group from other stallions. Imagine an ‘I’ll scratch you back if you scratch mine‘ relationship. This theory for stallion cooperation fits best with the modern characterisation of cooperation between individuals of the same species called Concesssion Theory. Dominant stallions may offer reproductive concessions – opportunities to mate – in exchange for subordinate stallion assistance in the defence of mares. In this circumstance the subordinate stallion has been refered to as a ‘helper’.

Not all favour cooperative hypotheses for multi-stallion breeding groups, however. Some have pointed out that they might just as well be explained because stallions have limited control of their mare group and other stallions. If a stallion is persistent there may be little the resident stallion can do to exclude him from their band. Some have coined the term mate parasitism to explain multi-stallion bands in this way. The additional stallions are supposedly associating with bands to take whatever breeding opportunities the dominant stallion cannot prevent.

With so many hypotheses, experiments are the only way to be certain which is the best explanation. In a recently published article in the scientific journal Behaviour Processes, colleagues and I tested some of these hypotheses by temporarily removing the subordinate stallions from multi-stallion bands. If stallion relationships are cooperative then subordinate stallion removal should require the remaining stallion to increase mare defence result in his losing mares to other stallions. The reverse is expected if the subordinate stallion is a mate-parasite. Mare defence will decrease but mares be successfully defended.

In a future post I will describe this work and the findings. There is more work to be done on this problem but the experiment has revealed where we should be looking, and not looking, for answers.

Bibliography

1 Linklater, W.L. 2000. Adaptive explanation in socio-ecology: lessons from the Equidae. Biological Reviews of the Cambridge Philosophical Society, 75: 1-20.

2 Linklater, W.L., Cameron, E.Z., Minot, E.O. and Stafford, K.J. 1999. Stallion harassment and the mating system of horses. Animal Behaviour, 58: 295-306.

3 Rubenstein, D. and Nuñez, C. 2009. Sociality and reproductive skew in horses and zebras. In: R. Hager and C. Jones (Editors), Reproductive skew in vertebrates: proximate and ultimate causes, Cambridge University Press, Cambridge.

Back in the saddle…

You noticed – thank you for saying. I have been away.

My daughter Zoerita Jacks was born in early February. During my 10 weeks parental leave, I had plans to at least keep writing and posting on this and my other blog. But the impossibility of daily routines with new borns and preparations to move the family for eight months sabbatical at UC-Berkeley drew me away. And also parental “leave” – hilarious – was punctuated by the usual university works, like post-graduate students’ thesis deadlines and research contract milestones, that sealed the fate of any aspirations for writing. So, sadly, I haven’t been in this space for many weeks.

But that is all behind me. With gorgeous smiling new daughter we arrived yesterday The Bay. I am on sabbatical at last! Zoerita woke me early and I packed her for a dawn stroll to induce her next deep sleep. This part of Oakland, with its treed streets, birds and hillside views is restful - especially at dawn while people slumber. I passed-off the sleeping babe to my wife who too is recovering sleep. I claimed a coffee from the fresh pot and woke my first daughter, Anneles, for breakfast. As I write Anneles is serving me, and her rabbit Bunny Foo Foo, imaginery tea. Its good to be back.

Though not writing, or perhaps because I have not been writing, I am congested with ideas that have abandoned polite and tidy mental queues for a mad crush at the gates of articulation. I am not sure where to start. But start soon I will and with renewed vigour. I will pick-up my presence on Twitter (@Perissodactyla) and Facebook (https://www.facebook.com/wayne.linklater.9) too from where I left off…

… soon. Anneles has asked me if we can make pink lemonade from the enormous number of lemons on Grandpa’s tree and a water melon sweet with the Californian summer…

Got Milk? – zebras stealing and sharing milk

Our species is not unique for stealing another species milk, but we certainly do it on a scale unsurpassed by any other. But milk stealing and milk sharing is common between members of the same species and the subject of very much conjecture. Why would a female tolerate another mother’s young stealing her milk?

Lactating and nursing is a major investment by mothers in their offspring. Milk is nutrient-rich and imposes extreme demands on the mother. It seems intuitive, therefore, that mothers should be very careful to make sure their offspring benefit from their milk and none is wasted.

For the same reason it seems intuitive that young should try to steal milk from other mothers. Offspring can have an insatiable appetite or their own mother might not produce enough. Either way being able to sneak some extra from a mother that is not your own would be a useful skill.

We should not be surprised, therefore, that milk stealing is common in amongst mammals. We should be surprised, however, if mothers tolerate it, and dumbfounded when they encourage it.

SURPRISE! – the mothers of many species are known to provide milk for young that are not their own and it is called allonursing. In human communities allonursing is called ‘wet nursing’ – still widely practiced and once more common in western societies.

Allonursing is also common amongst ungulate species, especially those that typically live in family groups where females live and raise their own offspring with their mothers or sisters.

To scientists that call themselves behavioural ecologists allonursing is interesting because it appears prima facie to contradict their expectation that the behaviour of animals exists because it improves the individual’s fitness – in this case of the mother. Why would a mother waste milk on another mother’s young when her own might benefit from it?

Nursing and allonursing at the same time in captive plains zebra at Dvůr Králové Zoo. Source: http://www.sciencedirect.com, Dr Jan Pluhácek et al.

Although allonursing is common in some ungulates, it is rare amongst the asses, horses and zebra. Allonursing in the equids has only been observed in a ‘handful’ of cases worldwide and only after a mother has adopted another mother’s offspring – sometimes because the first mother has died or one of two mothers’ offspring died. Allonursing has also not been described in rhinoceros and tapir – although this might just be because these species are difficult to observe in the wild and less common in zoos.

So rare is allonursing in the equids that the circumstances of its exceptional occurrence have been used to try and understand why allonursing might have evolved to be more common in other species. It seems sensible that it was once also unusual in some other species but, for reasons unknown, became more common. For example, when allonusing occurred in Kaimanawa feral horses [1] it was in the unusual circumstance that a filly and her mother dispersed into a new breeding group together – ordinarily both female, as well as male, offspring leave their mothers for a differnt breeding group. Both mother and daughter foaled around the same time but one of the foals died and both adopted the live foal. We used this unusual occurrence to conclude that allonursing might be more common in horses if females lived with their mothers and, by implication, the potential for kinship to be important for the occurrence and evolution of allonursing.

A recent study in the Journal of Zoology (London) updates our understanding of allonursing in the equids, especially zebra [2].

The authors are a new and interesting collaboration. Dr Jan Pluhácek has a suite of previous publications on the maternal behaviour of equids from his detailed measurements involving thousands of hours of observation, especially of zebra at Dvur Králové Zoo, Czech Republic, where he did his Ph.D. Dr Sarah King is known for her work on the ecology of Przewalski’s horse in Mongolia. She is now a post-doctoral researcher interested in small mammal ecology and biodiversity at the University of Colorado.The lead author Michaela Olléová is new to work with the Perissodacyla but with a growing publication list.

The authors compared the rate of allonursing by three different zebra species: plains, mountain and Grevy’s zebra, in the Dvur Králové Zoo. Although the work is on captive populations which might have higher rates of allonursing, the authors’ comparisons of the zebra species are informative.

Source: http://ziva.avcr.cz/

All the Grevy’s zebra foals attempted to suckle from another mare and most were allowed by the mothers but attempts by mountain or plains zebra foals were rare and never successful (although note allonursing in plains zebra from previous study – see image above and study). Interestingly, allonursing commonly occurred when the Grevy’s zebra mother was also nursing its own young. Although the rate at which attempts to allonurse were tolerated was low (10% cf. 59-74% of attempts for offspring) they were sufficient for one foal orphaned at six months to survive beyond weaning – remarkable.

Grevy’s zebra mares appear to be more tolerant of other foals nursing from them. When plains and mountain zebra mothers rejected an allonursing attempt they did so with a kick, whereas Grevy’s zebra mothers were just as likely not to kick but simply move away from the foal.

Grevy’s zebra are regarded as an arid-adapted equid and many have explained its different behaviour from other zebras in this way. But if adaptation to arid environments influenced allonursing then Grevy’s zebra mares should be less, not more, tolerant of allonursing – the water and nutrients of milk should be more valued and defended, not less. I agree with the authors – the arid-adapted hypothesis for allonursing is flawed [3].

The authors discuss, instead, how allonursing in Grevy’s zebra might be explained by their different social system from other zebra. Plains and mountain zebra form stable groups of mares but Grevy’s zebra do not. Thus, attempts to steal milk would be less common in Grey’s zebra and a temporary cost even if successful. If plains and mountain zebra mothers, however, tolerated allonursing they could suffer high rates of milk theft. The rationale, therefore, is that there has been selection for intolerance of allosucking in plains and mountain zebra, but not Grevy’s zebra. I like this argument because the authors have flipped the question ‘on its head’ and asked ‘why not allonursing?’, rather than ‘why?’.

One aspect of the study which leaves me unsatisfactorily guessing is the relationship between the allonursing mothers. Although the authors did not find more related mothers to allonurse more related offspring, they did find a relationship between maternal kinship and allonursing. Unfortunately, the authors do not say whether kin were more or less likely to allonurse. If kin were more likely to allonurse each other’s offspring the authors’ conclusion that the relationship of mothers is not important seems premature.

If females were more likely to allonurse the foals of their mothers, sisters and daughters it would confirm my suspicion that mother-daughter or sisters relationships are important to the occurrence and evolution of allonursing that is ordinarily prevented by female dispersal in equids. I will follow this up with the authors for clarification and perhaps discuss it in another post.

Source: http://www.rosiesworld.co.nz/

I am not convinced yet, therefore, of the authors’ conclusion that allonusing might be maternal reciprocity – a sort of ‘I’ll nurse your young if you nurse mine’ arrangement. The problem with the reciprocity hypothesis is that a mare whose foal allonurses from another mare, conveniently near by, might also have excess milk and become more tolerant of the other mare’s foal also allonursing. One does not need to add reciprocity to explain why a mare would defend her milk less when she has more. It might be uncomfortable, even painful, to have a full udder. My recollection of cows late to the milking shed was of their bellows of discomfort as they arrived and how they lowed with relief as their udders were emptied.

The authors are careful, however, to point out that they cannot dismiss other hypotheses for allonursing including that the mare may just want to get rid of her milk. Nevertheless, I agree with the authors’ summation – equine social systems are likely to have influenced the pattern of allonursing behaviour in the zebra species.

There are limitations to the sorts of maternal behaviour and investment studies like the one described because they do not actually measure the amount of milk received by the foal. It is understood that the time or frequency of nursing by offspring does not correlate reliably with the amount of milk it receives for an enormous number of reasons [4], even in equids where foal suckling is so easy to observe [5]. Sometimes suckling for longer and more often can mean the foal is getting less, not more, milk, especially if the mother is producing less milk or not ‘letting it down’. Some suckling is also just for young animals’ comfort. It remains possible that much of the allonursing in Grevy’s zebra does not yield milk, or at least not as much milk as the mothers own foal would receive.

The next step in this research should be measurements of the milk transferred – it would be fascinating and provide more direct estimates of allonursing. Unfortunately, the opportunities for more detailed work to measure milk transfer is limited in the zoo environment. Does anyone have a captive population of Grevy’s zebra females where milk transfer could be measured? I suspect the authors would like to hear from you.

Bibliography

1 A case of co-operative nursing and offspring care by mother and daughter feral horses. Cameron, E.Z., et al. (1999) Journal of Zoology 249, 486-489.

2 Effect of social system on allosuckling and adoption in zebras. Olleova, M., et al. (2012) Journal of Zoology 288, 127-134.

3 Adaptive explanation in socio-ecology: lessons from the Equidae. Linklater, W.L. (2000) Biological Reviews of the Cambridge Philosophical Society 75, 1-20.

4 Is suckling behaviour a useful predictor of milk intake? A review. Cameron, E.Z. (1998) Animal Behaviour 56, 521-532.

5 Suckling behaviour does not measure milk intake in horses, Equus caballus. Cameron, E.Z., et al. (1999) Animal Behaviour 57, 673-678.

Olléová, M., Pluháček, J., & King, S. (2012). Effect of social system on allosuckling and adoption in zebras Journal of Zoology, 288 (2), 127-134 DOI: 10.1111/j.1469-7998.2012.00930.x

Is your population open or closed?

Roosevelt Gate, Yellowstone National Park – a closed population for some species but open for others because fences, roads and ranches are not barriers.

Population biologists call confined populations closed because animals cannot arrive (immigration) from other places or leave (emigration). Populations where these things are possible are called open. Our approach to the conservation or management of wildlife depends on whether the population of interest is open or closed.

The rhinoceros in a fenced African reserve surrounded by an agricultural landscape in which they would be unwelcome is a closed population. The feral horses of the Argo Basin in the Kaimanawa Ranges, New Zealand, are an open population because horses may move between it and adjacent areas also with feral horses.

Rhinoceros in fenced reserves are closed populations – animals cannot arrive or leave.

A population does not need to be fenced in to be closed. Inhospitable habitat, because it is unsuitable or modified (e.g., urban or agricultural landscapes), and impassable topography, like mountain ranges or rivers, might prevent animals arriving or leaving a population.

At the largest scales possible all populations are closed because they are confined to a continent or planet earth. But wildlife conservationists or managers can seldom act or are seldom required to act at this scale and so smaller populations are defined. Populations measured at smaller scales are more likely to be open.

Over long periods of time all populations are open because the features of the landscape that might prevent animals arriving and leaving change. Habitats change and even topographical barriers appear and disappear – rivers dry up or change course, mountain passes are opened or blocked. Populations measured over shorter time periods are more likely to be closed.

Some feral horse populations are open. They are unfenced and exchange members with other horse populations in other areas, including the capture of wild horses and release or escape of domestic horses from ranches.

Whether your population is open or closed therefore depends largely on your objective and how you need to define your population. Attempts to measure populations within political boundaries like your county, state or country will need to consider that the population is open – animals have no respect for the imaginary lines people draw on maps. A geneticist measuring changes in a population over many millennia is much more likely to be working with an open population – even rare immigration by single individuals can radically change the gene pool. An ecologist estimating a population’s growth within a forest or river basin is more likely to be working with a closed population – the rate of animals arriving and leaving might be so low that it can be ignored, especially over shorter periods of time.

In future posts I will consider the implications of populations being open or closed for wildlife conservation and management, especially for estimating or predicting population growth and decline.

Conservation or management?

The international furore over cats in New Zealand took my attention from Perissodactyla for a week to also post on my other blog but I am back now. While away I have been thinking about how to start the topic of populations and how we conserve or manage them.

Getting populations to grow is a central objective of wildlife conservation – fundamental to the rescue of rare and declining species. Larger populations are less vulnerable to unexpected catastrophes, like extreme weather events, and more robust to environmental changes, especially because they retain a greater diversity of individuals – genetic and cultural variation.

Grevy’s zebra. Only around 750 remain in the wild.

Larger populations are also more likely to sustain a harvest by another component of the ecosystem’s biodiversity, like a predator or parasite, or by people for food, clothing or recreation. It is more difficult to extinguish or over-harvest a larger population and diverse populations are more adaptable to changing circumstances. Thus, conservation is successful when it makes populations larger.

Reducing or maintaining a population at a particular size is a central objective of wildlife management. Population growth may need to be controlled or reversed to prevent a species from being at densities where it impacts other values of the landscape, like other plant and wildlife diversity, or the services that the ecosystem provides, like clean water. Some populations are pest species because they impose a social or economic cost on communities – impacting the quality of peoples’ lives. A pest population’s decline is the first step towards its eradication.The Perissodactyla are interesting because species are rare or endangered but also common. They are conserved in some ecosystems, but regarded as a pest in others.

Black rhinoceros in iMfolozi Game Reserve, South Africa. Photo: Dale Airton

The Javan and Sumatran rhinoceros are amongst the most endangered large mammal species on the planet. Black rhinoceros of southern and eastern Africa are critically endangered and illegal hunting for their horns threatens their extinction.

The world’s tapir were recently common but are in rapid decline due to deforestation in equatorial Asia and South America.

Plains zebra is amongst the most common ungulates on the African continent, but Grevy’s zebra number only around 750 individuals and they are endangered. Domestic horses are one of our most common large animals and released to re-wild or become feral on the landscape are regarded as pest species by some.

Posting under the category ‘Population growth’ I will explain how populations change and how that change can be measured and calculated. The uncertainties inherent in estimates of population growth will be discussed and the reliability of estimates for horses and rhinoceros debated. I will use characters of these species lives to calculate the maximum population growth rate that it could achieve under best conditions. I will evaluate some of the extraordinary claims about how fast these populations can grow or how slowly they are growing – hoping to inform the debates around the world about the conservation and management of wild Perissodactyla.